LIFE

http://beyondearthlyskies.blogspot.com/

2012-02-05T19:16:00.001+08:00

This blog has shifted to: http://beyondearthlyskies.blogspot.com/

Flares from a Supermassive Black Hole

2011-11-09T22:36:00.001+08:00

Supermassive black holes are known to exist in the cores of many galaxies. On average, each supermassive black hole in a typical galaxy disrupts a passing star every 10,000 years or so. An event like this generates a spectacularly bright flare that lasts for months as the disrupted star forms an accretion disc around the supermassive black hole. In the Milky Way galaxy, a supermassive black hole named Sagittarius A* sits in its center. Sagittarius A* is estimated to have over 4 million times the mass of the Sun. On a daily basis, Sagittarius A* is observed to emit tiny flares that lasts for only a few hours each time. These flares are billions of times smaller in amplitude when compared to a flare produced by the disruption of a star.

In a recently published paper by Kastytis Zubovas, et al. 2011, it is postulated that the tiny flares produced by Sagittarius A* on a daily basis are caused by the tidal disruption of asteroids rather than stars. There are vastly more asteroids than stars and asteroids are also much smaller than stars. This explains why the flares produced by Sagittarius A* are much more frequent but are much less luminous than a flare that is produced by the disruption of a star.

The total luminosity of Sagittarius A* in its quiescent state is approximately 300 times the luminosity of the Sun. This remarkably low luminosity is believed to be powered by a very tenuous quasi-spherical accretion flow of gas into the supermassive black hole. The quiescent state of Sagittarius A* is punctured a few times each day by tiny flares. These flares have luminosities ranging from 3 to 100 times greater that the quiescent state of Sagittarius A* in both X-rays and near infrared.

The minimum size of an asteroid that is necessary to produce an observable flare from Sagittarius A* is estimated to be around 10 kilometres. An asteroid gets tidally disrupted in the vicinity of Sagittarius A* when it passes close enough to the supermassive black hole such that the asteroid’s own gravity becomes unable to hold the asteroid together. This causes the asteroid to break up into smaller fragments that are bound by chemical forces rather than by gravity where the maximum size for such a fragment is probably less than 1 kilometre.

In order for an asteroid to be tidally disrupted, it has to come within 1 AU of Sagittarius A* where 1 AU is basically the average distance of the Earth from the Sun. Sagittarius A* is a supermassive black hole and any object orbiting it within 1 AU will be travelling at an immense velocity of over 60,000 kilometres per second. Since Sagittarius A* is surrounded in its immediate vicinity by a very tenuous gaseous accretion flow, the fragments of a tidally disrupted asteroid will get vaporised by friction with the surrounding gas as they plough through at such incredible speeds. The energy released from the vaporisation of these asteroid fragments is sufficient to produce the observable flares from Sagittarius A*. As the tenuous gaseous accretion flow around Sagittarius A* extends beyond 1 AU, an asteroid passing Sagittarius A* beyond 1 AU and remains intact will still have its surface layers vaporised.

The disruption of a planet around Sagittarius A* is expected to occur much less frequently, on the order of one every thousand years or so. Since a planet is much more massive than an asteroid, a flare produced from the vaporised fragments of a tidally disrupted planet is expected to be millions of times more luminous than from an asteroid. In fact, an observed X-ray echo from a giant molecular cloud that is located a few hundred light years away from Sagittarius A* points towards the possibility that a large flare may have been produced from the disruption and subsequent vaporisation of a planet, occurring approximately 300 years ago.

A Pulsar’s Diamond Planet

2011-09-04T21:28:00.002+08:00

A pulsar is a rapidly spinning and strongly magnetized neutron star which emits an intense bipolar beam of electromagnetic radiation that generally does not coincide with the spin axis of the pulsar. This results in the ‘lighthouse effect’ when the beam of emission happens to be orientated towards the Earth; leading to an apparent pulsed nature as the beam of emission periodically sweeps past the Earth. Hence, the pulsar derives its namesake from this observable behaviour. For some pulsars, the periodicity of its pulsed nature can be as precise as an atomic clock. A pulsar forms out from the ultra-compressed core of a massive star during a supernova explosion and a typical pulsar contains an entire Sun’s worth mass of matter packed into an incredibly small volume with a diameter of no more than a few tens of kilometres. On average, each cubic centimetre of a pulsar’s material holds on the order of a few hundred billion metric tons of mass.

PSR J1719-1438 is a pulsar with a spin period of 5.7 milliseconds, which means that it spins 175 times each second. The very rapid spin rate of PSR J1719-1438 means that it is categorized under a unique group of pulsars called millisecond pulsars. The high spin rate of a millisecond pulsar is believed to be cause by the spinning-up of a pulsar by the accretion of matter from a binary companion which transfers angular momentum to the pulsar. A recent paper by M. Bailes et al that is titled “Transformation of a Star into a Planet in a Millisecond Pulsar Binary” describes the discovery of a Jupiter-mass companion in a 2.2 hour orbit around the pulsar PSR J1719-1438. The paper also investigates the possibility that PSR J1719-1438 was once an ultra compact low-mass X-ray binary (UC LMXB), whereby matter was accreted by the pulsar from a binary companion star. Almost all of the material from the binary companion star was accreted by the pulsar, leaving behind a Jupiter-mass remnant of what was once the companion star.

The Jupiter-mass companion orbiting around PSR J1719-1438 was detected from slight pulse-timing variations observed in the pulsar’s extremely regular pulses. These pulse-timing variations are caused by the gravitational tugging of the pulsar by its Jupiter-mass companion. The Jupiter-mass companion is orbiting so close to PSR J1719-1438 that it cannot be anywhere as large in size as Jupiter because at that size, its own gravity will not be sufficiently strong enough to prevent it from being gravitationally torn apart by the nearby pulsar. For this reason, the Jupiter-mass companion must be much more compact in nature to avoid being gravitationally torn apart. A lower limit of 23 grams per cubic centimetre is required for the mean density of the Jupiter-mass companion such that its overall physical size is compact enough for its own gravity to be sufficiently strong to prevent it from being gravitationally torn apart. In comparison, the mean density of Jupiter is less than 2 grams per cubic centimetre.

Such a high density means that the Jupiter-mass companion orbiting around PSR J1719-1438 cannot be a gas-giant planet like Jupiter as it is too dense to be made up of just hydrogen and helium like Jupiter. Instead, the Jupiter-mass companion is theorized to be what remains of the degenerate core of the companion star whose material was stripped away and accreted by the pulsar. In this scenario, the Jupiter-mass companion was once a white dwarf star in a tight orbit around PSR J1719-1438. The white dwarf star was basically what remained of a star like our Sun after it had extinguished its hydrogen and helium in its core through fusion of these elements into carbon. So, if the Jupiter-mass companion is what remains of the core of the white dwarf star, it is expected be comprised of heavier elements such as carbon. This will allow it to have a mean density of 23 grams per cubic centimetre or more, making it compact enough such that it will not be gravitationally torn apart by the nearby pulsar.

The Jupiter-mass companion around PSR J1719-1438 is a very unique case because most white dwarfs stars tend not to survive the transfer of matter to their companion pulsars. A typical white dwarf star tends to be too close to its companion pulsar by the time it starts transferring material to the pulsar. This leads to the complete destruction of the white dwarf star, possibly leaving behind a disk of remnant material orbiting around the pulsar. Such a disk of material is expected to coalescence into a planetary system of Earth-mass planets rather than a Jupiter-mass object. Therefore, a Jupiter-mass companion around a pulsar will require a low mass white dwarf star so that the transfer of matter to its companion pulsar occurs at a further distance away. This can allow the transfer of matter to cease just in time before the complete destruction of the white dwarf star, leaving behind the remnant core as a Jupiter-mass object.

Of all the pulsars discovered so far, only a handful of them have planetary-mass companions. This means that the formation of planetary-mass companions around pulsars is the exception rather than the rule. Furthermore, the companion around PSR J1719-1438 is the only known Jupiter-mass object around a pulsar. All previously discovered planetary-mass objects around pulsars are around the mass of the Earth. The case for PSR J1719-1438 requires a very unusual combination of white dwarf mass and composition. This unique combination allows PSR J1719-1438 to transform its companion into very rare and exotic type of planet. The Jupiter-mass companion around PSR J1719-1438 is likely to be entirely composed of crystallized carbon, which is also known on Earth as diamond.

Land Planets and Ocean Planets

2011-07-30T10:10:00.000+08:00

The region around a star where an Earth-like planet can maintain liquid water on its surface is known as the habitable zone or the ‘Goldilocks’ zone. Previous studies of Earth-like planets in the habitable zones of stars generally assume ocean covered planets that resemble the present Earth. If such an ocean planet is too far from its star, it leads to an ice-albedo feedback which ends in the complete freezing of the planet. If the same planet is too close to its star, a runaway greenhouse effect occurs which ends in the complete evaporation of the planet’s oceans. Now imagine another kind of habitable planet whose surface is predominantly land, with only small areas of surface water. A planet like this is can be called a land planet.

Although a land planet is probably covered by vast deserts, it can support localized regions with abundant water and such regions can exist for example, near the poles of the planet. In our own solar system, the closest analogy to a land planet is Saturn’s moon Titan. Titan has lakes of methane on both its poles and between the poles of Titan is a vast desert that spans the tropics and temperate zones. The surface of Titan is far too cold for liquid to exist, resulting in liquid methane playing its role on Titan as water does on Earth. A rather engaging paper by Abe et al. 2011 that is entitled “Habitable Zone Limits for Dry Planets” studies the possibilities that land planets can have wider habitable zones than ocean planets. This means that a land planet can be nearer or further from its parent star than an ocean planet and still be capable of supporting habitable Earth-like surface conditions.

In this article, the comparison between an ocean planet and a land planet assumes that each planet orbits a Sun-like star that is identical to ours. The complete freezing of an Earth-like ocean planet occurs when the Sun is dimmed to 90 percent of its present luminosity while the complete freezing of a land planet only occurs when the Sun is dimmed to 77 percent of its present luminosity. In other words, a land planet has a greater resistance to complete freezing than an ocean planet. This is due to the fact that a land planet will tend to be less reflective than an ocean planet. One reason for this is that a land planet has fewer clouds than an ocean planet because it is less humid. The other reason is that less snow accumulates on a land planet than on an ocean planet because the atmosphere is drier and the daytime temperatures are higher for a land planet. Fewer clouds and less snow cover make a land planet less reflective than an ocean planet to incoming insolation from its parent star. A less reflective planet means a higher surface temperature. For this reason, a land planet can be further than an ocean planet from its parent star before complete freezing occurs. Therefore, the outer boundary of the habitable zone of a land planet is larger than for an ocean planet.

Moving now to the inner boundary of the habitable zone, liquid water can remain stable on an ocean planet until the Sun is brightened to 135 percent or more of its present luminosity. For a land planet, liquid water can remain stable on its surface until the Sun is brightened to 170 percent or more of its present luminosity. This means that the inner boundary of the habitable zone of a land planet is closer in to its parent star than for an ocean planet since a land planet can be nearer to its parent star than an ocean planet before a runaway greenhouse effect occurs. For an ocean planet, a runaway greenhouse effect occurs when there is enough water vapour in the atmosphere such that the atmosphere becomes optically thick to outgoing thermal radiation. This causes the ocean planet to absorb more energy from its parent star than it can radiate away, eventually causing the surface of the planet to become sterilizingly hot.

Compared to an ocean planet, the case for a land planet is rather different. The low latitude region of a land planet is expected to have an extremely low humidity and effectively no surface water. This allows the low latitude region of the land planet to absorb more energy from its parent star than it can radiate away, without leading to a runaway greenhouse effect. Furthermore, for an extremely dry land planet, all its surface water can evaporate without a significant contribution of water vapour into the atmosphere to trigger a runaway greenhouse effect. Since a runaway greenhouse effect may not occur for a land planet, the equivalent runaway greenhouse effect threshold can be defined as the maximum insolation the land planet can receive, beyond which all surface water and surface ice completely evaporate, including even those at the poles.

A land planet with no permanent surface water can still sustain a hydrated layer of surface soil by the deposition and subsequent melting of frost. At night, it may be cold enough for frost to form, especially within the pore spaces of the surface soil. During the day, the frost can melt into liquid water and moisturize the surface soil. This mechanism is particularly effective for a land planet with a thin atmosphere since a thin atmosphere is much less effective at damping daily temperature fluctuations than a thick atmosphere. Nights on a land planet with a thin atmosphere can get exceptionally cold, thereby creating an environment that is very conducive for the formation of frost. Additionally, a thinner atmosphere will reduce the rate of energy transport from the equator to the poles of a land planet. This stabilizes any polar ice caps against evaporation and reduces the input of water vapour into the planet’s atmosphere which further prevents the onset of a runaway greenhouse effect.

To conclude, the habitable zone for a land planet around its parent star is considerably larger than it is for an ocean planet around the same star. One key consideration that can be of importance is that the presence of clouds creates a major uncertainty as to the true limits of the habitable zone of a planet around its parent star. Clouds can warm or cool a planet, whereby high clouds have a warming effect and low clouds have a cooling effect. Reducing the coverage of high clouds and increasing the coverage of low clouds pushes the inner limit of a planet’s habitable zone closer to its parent star. Alternatively, increasing the coverage of high clouds and reducing the coverage of low clouds pushes the outer limit of a planet’s habitable zone further from its parent star.

The Sun’s luminosity increases at a rate of about 9 percent per billion years. As the Sun brightens, it might be possible that an ocean planet like the Earth can lose most of its water and become a land planet without passing through a sterilizing runaway greenhouse effect. However, this depends on how much water an ocean planet like the Earth can lose before it reaches the threshold for a sterilizing runaway greenhouse effect. Still, even if an ocean planet can successfully evolve into a land planet, the surface temperature of the planet during the transition phase can reach up to between 300 to 400 degrees Kelvin. Such conditions are marginally habitable as only thermophilic microbial life on Earth can exploit such conditions. Nevertheless, the possibility of the Earth becoming a land planet in the far future adds an extra billion years or so to the continuous habitability of the Earth even as the Sun evolves to a higher luminosity.

Tight Stellar Binary

2011-07-15T08:52:00.001+08:00

The discovery of a detached pair of white dwarfs with a 12.75 minutes orbital period has been published by Warren R. Brown et al. 2011 in a paper entitled: “A 12 minute Orbital Period Detached White Dwarf Eclipsing Binary”. This stellar system is designated SDSS J065133.33+284423.3 or just J0651, and it is the tightest white dwarf binary system yet discovered. J0651 is located at a distance of over 3000 light years from the Sun. Both white dwarfs are racing around each other at over 600 kilometers per second. The visible primary is a 0.25 solar mass tidally distorted helium white dwarf while the unseen secondary is a 0.55 solar mass carbon-oxygen white dwarf.

Credit: David A. Aguilar (CfA)

Both white dwarfs are separated by a mean distance of less than one-third the separation between our Earth and the Moon, and they are on the brink of a merger. The two white dwarfs are expected to merge in 900 thousand years from the loss of energy and angular momentum via the emission of gravitational wave radiation. This will eventually lead to a massive rapidly spinning white dwarf, the formation of a stable interacting binary, or possibly an explosion as an underluminous type Ia supernova. The orientation of the orbits of both white dwarfs in the binary system is such that eclipses of each white dwarf by the other are observable and this allows accurate measurements of the orbital parameters, masses and radii of the white dwarfs.

The eclipse of one white dwarf by the other occurs like clockwork, at a very predictable rate. Observers on a hypothetical planet which orbits around this star system will see one of their two suns disappear every 6 minutes or so. The shrinking of the orbits of both white dwarfs via the emission of gravitational wave radiation is expected to be measurable from observing changes in the eclipse timings. This provides a remarkable opportunity to test for the existence of gravitational waves that are predicted by Einstein’s general theory of relativity.

Faces of Iapetus

2011-06-25T15:42:00.001+08:00

Iapetus is the third largest moon of Saturn and this moon is best known for the remarkable two-tone colouration between its leading and trailing hemispheres, whereby the former is significantly darker than the latter. This enigmatic dichotomy has been debated for decades and radar and imaging observations by the Cassini spacecraft over the past few years has manage to paint a clearer picture of this two-faced moon. Iapetus has a mean diameter of 1470 kilometres and it orbits Saturn at a distance of 3.561 million kilometres, taking 79.32 Earth days to complete one orbit. Iapetus is the outermost of the regular satellites of Saturn and it is tidally locked such that one hemisphere permanently faces the direction of the moon’s motion around Saturn.

Credit: NASA/JPL/Space Science Institute

The difference in coloration between the two hemispheres of Iapetus is striking. The leading hemisphere of Iapetus appears dark with a slight reddish-brown tone, while the trailing hemisphere and the poles appear bright. Iapetus also has a massive equatorial ridge that runs precisely along the equator of the moon’s dark leading hemisphere and parts of the ridge tower more than 20 kilometres above the surrounding plains. This prominent equatorial ridge gives Iapetus a walnut-like appearance, as can be seen from images taken by the Cassini spacecraft. In this article, the focus will be on the remarkable two-tone colouration of Iapetus’ two hemispheres.

The leading hypothesis explaining the two-tone colouration of Iapetus is that the moon is continuously ploughing through a cloud of dark dust particles as it orbits around Saturn. Located far beyond the orbit of Iapetus is an irregular potato-shaped moon that is named Phoebe and these dust particles are believed to have originated from micrometeoroid impacts on Phoebe. In fact, all these dust particles from Phoebe form an enormous but extremely tenuous and virtually invisible ring of material around Saturn. The dust particles of this ring gradually migrate inwards towards Saturn. Phoebe has a retrograde orbit around Saturn and this means that it orbits Saturn in a direction that is opposite to that of Iapetus. Hence, the dust particles kicked off Phoebe are expected to collide with Iapetus head-on, at high velocities of approximately 7 kilometres per second.

The high velocity of the incoming dust particles explains why only the leading hemisphere of Iapetus gets coated by the dust particles as gravitational focusing by the gravity of Iapetus is insignificant. Furthermore, the dark region covering most of the leading hemisphere of Iapetus is centred precisely on the moon’s apex of motion. However, the result of dust deposition on the leading hemisphere of Iapetus cannot alone explain the extremely sharp boundaries between the regions of bright and dark material on the surface of Iapetus. Hence, a process of runaway ice sublimation has to occur for these sharp boundaries to exist. In such a process, regions darkened by dust absorbs more heat in the day, causing more ice to sublimate which in turn causes further surface darkening and heat absorption until no more surface ice is left.

The process of runaway ice sublimation removes ice from the darker regions and deposits them on the bright areas and at the frigid poles. Images of Iapetus taken by the Cassini spacecraft also show that ice removed from the darker regions can also be deposited on the cooler pole facing slopes of craters on the surface of Iapetus. This explains why the polar regions of Iapetus appear bright even though they extend into the leading hemisphere of Iapetus. The process of dust deposition and runaway ice sublimation both work together to give Iapetus its striking two-tone colouration.

Credit: NASA/JPL/Space Science Institute

Besides Iapetus, the moons Titan and Hyperion are also able to intercept the dust particles kicked off from Phoebe by micrometeoroid impacts. The orbits of Titan and Hyperion around Saturn are interior to the orbit of Iapetus. Hyperion is a small and irregularly shaped satellite of Saturn and it orbits Saturn between Titan and Iapetus. What is important about Hyperion is that it has a chaotic rotation whereby its orientation in space is unpredictable and changes all the time. Because of this, Hyperion does not have a two-tone colouration like Iapetus and its surface is instead more uniformly coated throughout by the dust particles.

A paper by Daniel Tamayo et al. 2011 entitled “Finding the Trigger to Iapetus' odd Global Albedo Pattern: Dynamics of Dust from Saturn's Irregular Satellites” investigates the capture of inward migrating dust particles by Iapetus, Hyperion and Titan. In this study, dust particles 10 micrometers or larger in size almost certainly strike Iapetus while a majority fraction of the dust particles ranging from 5 to 10 micrometers in size strike Titan. However, only a very small fraction of the inward migrating dust particles from Phoebe strike Hyperion due to the small physical size of Hyperion. Dust particles smaller than 5 micrometers in size migrate over a much shorter timescale as compared to the larger ones and most strike Saturn or completely escape the Saturn system.

Of the dust particles ranging from 5 to 10 micrometers in size, a majority fraction of them strike Titan as they migrate inward towards Saturn. Radiation pressure from sunlight significantly alters the trajectories of dust particles in this size regime. For these particles, the eccentricities of their orbits become large enough such that their orbits begin to cross the orbit of Titan before their probabilities of striking Iapetus approach certainty. There are two additional reasons that make Titan very efficient at intercepting dust particles. Firstly, Titan’s sheer size gives it a geometrical cross section that is over an order of magnitude larger than Iapetus’. Secondly, the relative velocities between the dust particles and Titan are substantially higher than for Iapetus, giving Titan a higher dust particle collision rate per unit frontal area. Like Iapetus, dust particles will strike Titan on its leading hemisphere. The thick atmosphere of Titan will fragment the incoming dust particles and globally distribute the materials that once make up the dust particles.

For dust particles larger than 10 micrometers in size, their slow inward migration gives Iapetus enough time to capture them before their orbits start to cross the orbit of Titan. Dust particles from other retrograde outer irregular satellites of Saturn can have comparable probabilities of striking Iapetus as the dust particles from Phoebe. A fraction of the surface material on Iapetus’ dark leading hemisphere may have originated from some of these retrograde outer irregular satellites of Saturn and this could explain the observed spectra differences between the surface material of Phoebe and Iapetus. Nevertheless, the amount of dust generated by Phoebe relative to the other retrograde outer irregular satellites of Saturn remains uncertain.

Exploding Black Holes

2011-06-03T10:23:00.000+08:00

During the first few moments after the Big Bang, the enormous temperatures and pressures allow simple fluctuations in the density of matter to form localized regions that are sufficiently dense for the creation of primordial black holes. At the present 13.7 billion year age of the universe, primordial black holes that are less than approximately half a billion metric tons in mass would have already evaporated via the emission of Hawking radiation. The amount of Hawking radiation emitted by evaporating black holes depends on the mass of the black hole and small black holes are expected to emit vastly more Hawking radiation than more massive ones. In the final fraction of a second before a black hole completely evaporates, it emits such an incredible amount of energy that it could well serve as a progenitor for a gamma ray burst.

Gamma ray bursts are the most luminous electromagnetic events known to occur in the universe and they are so luminous that they can easily be detected across distance of billions of light years. Gamma ray bursts are generally divided into three classes according to their durations: long gamma ray bursts (LGRBs) have durations of over 2 seconds, short gamma ray bursts (SGRBs) have durations of between 0.1 to 2 seconds and very short gamma ray bursts (VSGRBs) have durations of less than 0.1 seconds. A recent paper by David B. Cline et al. (2011) entitled “Does Very Short Gamma Ray Bursts originate from Primordial Black Holes?” presents the case that the evaporation of primordial black holes could account for the detection of very short duration gamma ray bursts.

LGRBs are generally associated with the collapse of massive stars while SGRBs are generally associated with the mergers of compact objects in binary systems (neutron star - neutron star mergers or black hole - neutron star mergers). As the most fleeting of gamma ray bursts, VSGRBs form a distinct group with durations of less than 0.1 seconds. NASA’s Swift satellite is a multi-wavelength space-based observatory dedicated to the study of gamma-ray bursts. In Swift’s VSGRB sample, 25 percent of the bursts have afterglows. This is in remarkable contrast with Swift’s SGRB sample whereby 78 percent of the bursts have afterglows. The afterglows can be attributed to post merger processes of compact objects in binary systems. In this case, 25 percent of the VSGRB sample can form the tail of the basic SGRB distribution. This leaves 75 percent of the VSGRB sample that do not have afterglows consistent with the evaporation of primordial black holes.

Detections of VSGRBs have shown that they have an anisotropic distribution which seem to point towards a local origin within the Milky Way galaxy. The rest of the gamma ray bursts show no anisotropy in their distribution and this suggests that they are of cosmological origin, occurring well beyond the Milky Way galaxy. All these suggest that VSGRBs are indeed a new class of gamma ray burst and the majority of the cases for VSGRBs can be the result of the explosive evaporation of primordial black holes. If the majority of VSGRBs are indeed the demise of primordial black holes, then knowing the spatial distribution of these exotic objects will help cosmologists place constraints on the spectrum of density fluctuations in the early universe.

The Runaway Giant

2011-05-18T06:59:00.000+08:00

The Large Magellanic Cloud is a nearby irregular galaxy that is located about 160 thousand light years away and it is also a satellite galaxy of the Milky Way. Extremely massive stars will up to 300 times the mass of our Sun are known to exist in a massive star cluster called R136 which is located near the center of the Tarantula Nebula, in the Large Magellanic Cloud. Residing in R136 is a star called R136a1 and this star is currently on record as the most massive star known, with a colossal mass that is estimated to be 265 times the mass of our Sun. Just after birth, R136a1 is estimated to have 320 times the mass of our Sun, having lost 50 solar masses over the past million years! R136a1 also hold the record for the most luminous star known as it blazes with 10 million times the luminosity of our Sun.

Located at a projected distance of 95 light years from the massive star cluster R136 in the Tarantula Nebula of the Large Magellanic Cloud is a very massive star called VFTS 682. Spectroscopic observations have revealed VFTS 682 to be a hydrogen-rich Wolf-Rayet star. Wolf-Rayet stars are massive stars which lose mass rapidly by emitting very strong stellar winds at speed of up to a couple of thousand kilometers per second. What makes VFTS 682 perplexing is that this star is one of the most massive stars found in isolation. Very massive stars generally reside in the centers of massive star clusters since the formation of such objects are generally known to occur in the dense environments found in the centers of massive star clusters. The presence of such an extremely massive star outside the massive star cluster R136 presents the question of whether it was ejected from R136 or did it form in isolation instead.

The physical properties of VFTS 682 are impressive as VFTS 682 is estimated to have over 3 million times the luminosity of our Sun and a mass on the order of 150 times the mass of our Sun. VFTS 682 is a single isolated star as it shows no signs of binarity. Spectroscopic observations have shown that in terms of spectral appearance, VFTS 682 is almost identical to another very massive star called R136a3 which is located in the core of the massive star cluster R136. From velocity measurements, VFTS 682 is estimated to have a true velocity of 40 kilometers per second with respect to R136, placing it in the lower range of velocities for runaway stars. If VFTS 682 is indeed a runaway star, it will be the most massive one known to date and a bow shock might even be observable around VFTS 682 as it is surrounded by dust clouds.

Very massive stars are know to form in dense cluster environments where they are generally found because the very short lifespans of very massive stars mean that they have insufficient time to travel far from where they were born. VFTS 682 is indeed a very massive star in isolation and this creates an interesting challenge for dynamical ejection scenarios and massive star formation theory. The paper detailing this discovery is by Joachim M. Bestenlehner et al and it is entitled “The VLT-FLAMES Tarantula Survey III: A very massive star in apparent isolation from the massive cluster R136”.

Lava-Ocean Planets

2011-05-11T06:43:00.001+08:00

CoRoT-7b is the first characterised rocky super-Earth exoplanet and it orbits extremely close to its parent star, at a distance of only 2.56 million kilometres which translates to just 4.48 stellar radii of its parent star. CoRoT-7b is located so close to its parent star that the length of one year on this planet is a fleeting 20 hours and 29 minutes. The spin and orbit of CoRoT-7b are likely synchronized, resulting in a hemisphere of continuous daylight and a hemisphere of continuous night. CoRoT-7b is measured to have 1.58 times the diameter and 6.9 times the mass of the Earth.

CoRoT-7b is not expected to have any appreciable atmosphere as the scorching environment on the planet does not support the presence of significant amounts of volatiles that can make up an atmosphere. Any atmosphere on CoRoT-7b is expected to be extremely rarefied. Hence, the transport of heat by any planetary scale winds on CoRoT-7b will be unable to significantly change the temperature distribution on the dayside or provide heat to the nightside, leading to very low surface temperatures on the nightside of the planet. This enables a huge surface temperature difference between the dayside and nightside of the planet to be maintained.

At the sub-stellar point on the dayside hemisphere of CoRoT-7b, the estimated temperature is a roasting 2470 degrees Kelvin. The sub-stellar point on the surface of CoRoT-7b has a zenith angle of zero and on this spot the host star of CoRot-7b is always directly overhead, making the sub-stellar point the hottest spot on the surface of the planet. An ocean of molten rocks is believed to be present on the extremely hot star-facing hemisphere of CoRoT-7b. High temperatures of well over 2000 degrees Kelvin on most of the dayside hemisphere of CoRoT-7b mean that the viscosity of the molten rocks that make up the lava ocean is probably much closer to that of water that to that of Earth’s lavas.

In order to compute the extent of coverage of the lava ocean on CoRoT-7b, certain assumptions have to be made. If Coriolis forces are negligible, such a lava ocean will have radial symmetry around the sub-stellar point which enables its extent to be characterized solely by the zenith angle of the ocean’s shore from the sub-stellar point. The ocean’s shore is basically the location on the planet’s surface where the solidification of molten rocks begins to occur.

If the circulation within the lava ocean is extremely efficient in transporting heat, it could lead to an ocean with a uniform temperature. Assuming that the lowest possible temperature of such a lava ocean is 2150 degrees Kelvin, the zenith angle of the lava ocean’s shore will be about 75 degrees from the sub-stellar point. This corresponds to 37 percent of the planet’s surface area being covered by the lava ocean. This estimate of the ocean’s size is probably a maximum and it can be seen that lava ocean is limited to just the dayside of CoRoT-7b. This means that circulation within the lava ocean cannot carry any heat from the dayside to the nightside of the planet.

If heat transport within the lava ocean via circulation is not present, then the physical extent of the lava ocean on CoRoT-7b will be smaller. In this case, assuming that the solidification of molten rocks begins to occur at 2200 degrees Kelvin, the zenith angle of the lava ocean’s shore will be about 52 degrees from the sub-stellar point. This corresponds to 19 percent of the planet’s surface area being covered by the lava ocean.

Along the shores of the lava ocean, crystallization and condensation of molten rock can occur to create pieces of rocks that sink back to the ocean floor. Also, along the shores of the lava ocean, condensation of molten rock material onto the continental edges can cause the loaded continental edges to progressively sink as it base dissolves into the mantle of the planet. The transport of silicates from the melted base of the continental edges back to the ocean floor can close the circulation of materials. Compared to the Earth’s oceans, any form of wind driven waves on the lava ocean of CoRoT-7b will be very small due to the extremely rarefied atmosphere, the higher viscosity of lava as compared to water and the higher surface gravity of CoRoT-7b as compared to the Earth.

The nightside of CoRoT-7b will be extremely cold due to the lack of any form of mechanism that can efficiently transport heat from the dayside to the nightside of the planet. The only form of heating on the nightside of CoRoT-7b will be geothermal heating from the decay of radioisotopes within the planet. This leads to a surface temperature of between 50 to 75 degrees Kelvin on the frigid nightside of CoRoT-7b. The paper detailing this study is by Alain Leger et al (2011) and it is entitled “The extreme physical properties of the CoRoT-7b super-Earth”.

The existence of a lava ocean on CoRoT-7b should also be common to many small and very hot rocky planets that orbit extremely close to their host stars. A recently discovered planet called Kepler-10b has a lot of resemblance with CoRoT-7b, but its properties are expected to be even more extreme as it has a higher temperature at its sub-stellar point and possibly a larger lava ocean. To conclude, a new class of planets termed “lava-ocean planets” may be prevalent amongst small and very hot rocky worlds with ‘star-hugging’ orbits.

Ultra-Hot Super-Earth

2011-05-04T19:25:00.002+08:00

55 Cancri is a yellow dwarf star that is located just 41 light years away from Earth in the direction of the constellation of Cancer. This star has a slightly lower mass and a slightly lower luminosity as compared to our Sun. As of 2010, five extrasolar planets are known to orbit 55 Cancri. The innermost planet is a terrestrial super-Earth planet with a few times the mass of our Earth while the outer 4 planets are gas giant planets with masses similar to Jupiter. A recent paper by Winn et al. (2011) that is entitled “A Super-Earth Transiting a Naked-Eye Star” describes the detection of transits of the innermost planet which orbits 55 Cancri. The innermost planet is designated 55 Cancri e and it was previously discovered in 2004 from radial velocity measurements.

55 Cancri e was formerly reported to have an orbital period of 2.808 days, but this value has since been revised down to just 0.7365 days or 17 hours and 41 minutes. “You could set dates on this world by your wristwatch, not a calendar,” study co-author Jaymie Matthews of the University of British Columbia said in a statement. This revision to the planet’s orbital period increased the likelihood that the planet could transit its host star from an initial probability of 13 percent to 33 percent. Observations by the Microvariability and Oscillations of STars telescope (MOST) lead to the discovery of the transits of 55 Cancri e in front of its host star. Each transit of 55 Cancri e lasts just over 100 minutes in duration and during each transit, 55 Cancri e blocks just 0.018 percent of the light from its host star.

From the amount of dimming imposed by the transit of 55 Cancri e in front of its host star, the diameter of 55 Cancri e is estimated to be 20800 kilometres, making this planet 63 percent larger than the Earth in diameter. Radial velocity measurements have also shown that 55 Cancri e has 8.57 times the mass of the Earth. With the size and mass of the planet known, the mean volumetric density of 55 Cancri e is estimated to be 10.9 grams per cubic centimetre, making this planet twice as dense as the Earth and the densest solid planet found anywhere so far. This suggests a rock-iron composition that is similar to the Earth under significantly more gravitational compression.

The amazingly short orbital period of 55 Cancri e means that this planet is located only 1.5 million kilometres from the fiery surface of its host star. In this extreme infernal environment, the temperature at the substellar point of 55 Cancri e could approach 3000 degrees Kelvin if the planet is tidally locked and if the incoming heat remains on the dayside. However, if the heat is distributed over the entire surface of the planet and if the planet has an albedo of zero, the temperature will be a lower but still blistering 2100 degrees Kelvin.

It is unlikely that 55 Cancri e can hold on to an atmosphere that is comprised of gases with low molecular weights. However, volcanic activity on 55 Cancri e can sustain a thin atmosphere with gases of high molecular weights. The presence of an atmospheric wind on 55 Cancri e could shift the hot spot away from the planet’s substellar point. On the surface of 55 Cancri e, any object will weigh 3 times heavier than it does on Earth. During the day, the host star of 55 Cancri e will appear thousands of times brighter and tens of times larger than our Sun appears from the Earth.

Extrasolar Carbon Planets

2011-04-27T15:31:00.000+08:00

In the inner solar system, the terrestrial planets - Venus, Earth and Mars are silicate planets as the bulk of their mass is primarily composed of silicon-oxygen compounds. These planets were formed from the coalescencing of planetesimals which condensed out of a protoplanetary disk of material orbiting the young Sun at around five billion years ago. In the case for the inner region of our solar system, the condensation of silicon-oxygen compounds to form silicate planets is the domineering process because the carbon to oxygen ratio of the protoplanetary disk in this region is only around 0.5, making oxygen the dominant component. In our region of the solar system, iron-peak elements condensed at the highest temperatures, followed by silicates at slight lower temperatures, water at 180 degrees Kelvin and eventually other volatiles such as ammonia and methane at lower temperatures. Hence, the Earth is comprised of an iron-nickel core within a large silicate mantle and topped on the exterior surface by water and other volatiles.

The condensation sequence of the material in a protoplanetary disk can be dramatically different if the carbon to oxygen ratio is above 0.98 whereby instead of silicates, the high temperature condensates will be carbon-rich compounds such as graphite and carbides, resulting in an entirely different class of planets. These planets are termed carbon planets where carbon is the most abundant component. A carbon planet will have an iron-nickel core just like our Earth. However, the layers of material surrounding the iron core will be very different as the mantle of a carbon planet will be comprised of silicon carbide and titanium carbide. Above the planet’s mantle, a layer of graphite will extend up to the surface of the planet, making up the crust of the planet. The deeper parts of this graphite crust will be subjected to high pressures and it will result in the formation of a global shell of crystalline diamond covering the entire planet.

The atmosphere of a carbon planet will be primarily composed of carbon monoxide or methane and the surface may be covered by precipitations of tar-like substances and other carbon-rich compounds. Such an atmosphere will be reducing instead of oxidizing. A carbon planet that orbits at a very close distance from its host star can loose its atmosphere due to atmospheric escape from the extreme heating, thereby directly exposing its solid surface to the vacuum of space. Such a carbon planet will remain exceptionally stable against the extreme heat as it will be protected by layers of heat resistant shells of graphite, silicon carbide or even diamond. In comparison, a silicate planet will have less protection due to the much lower melting and vaporizing temperatures of silicate compounds. The heat resistance of carbon compounds is exemplified in silicon carbide which is a ceramic used for lining the cylinders of automotive engines and in diamond which remains solid up to a temperature of around 4000 degrees Kelvin.

For a terrestrial planet like our Earth, the atmosphere is characterized by the presence of oxygen-rich gases such as carbon dioxide, oxygen and ozone. However, the atmosphere of a carbon planet will have an absence of these oxygen-rich gases and instead, the atmosphere will be dominated by carbon monoxide or by methane for a cold carbon planet. Cold and low mass carbon planets are conducive for the survival of long chains of photochemically synthesized carbon compounds. On such a planet, the temperatures can even be low enough for methane and ethane to condense and rain out of the atmosphere to form lakes and seas of hydrocarbons, similar to those found on Titan. Carbon planets are probably more common in regions closer towards the galactic centre because the stars there tend to contain a larger proportion of carbon as compared to stars like our Sun which is located further away from the galactic centre.

Worlds Like Titan

2011-04-21T00:16:00.001+08:00

A reddish colour dominated everything, although swathes of darker, older material streaked the landscape. Towards the horizon, beyond the slushy plain below, there were rolling hills with peaks stained red and yellow, with slashes of ochre on their flanks. But they were mountains of ice, not rock. An ethane lake had eroded the base of the hills, and there were visible scars in the hills' profiles.

- Stephen Baxter, Titan

In human terms, Titan is a cold and frigid world with an average surface temperature of minus 180 degrees Centigrade and a surface atmospheric pressure that is 1.45 times the atmospheric pressure at sea-level on Earth. These conditions allow for the existence of liquid methane on Titan’s surface in the form of lakes and seas. A large number of these lakes and seas can be found in Titan’s north polar region and the largest of them is named Kraken Mare - a large sea of liquid methane and ethane that is estimated to be similar in size to the Caspian Sea on Earth. Titan is also characterised by a thick atmosphere which extends hundreds of kilometres above its surface and a global atmospheric haze layer that is transparent to infrared wavelengths but opaque to ultraviolet and visible wavelengths. In this article, I will be considering how Titan will be like if it were to orbit a red dwarf star instead of the Sun and also if it were a rogue planet wandering in the dark depths of interstellar space.

The global atmospheric haze layer of Titan blocks incoming ultraviolet and visible light but allows infrared radiation from the surface to freely escape into space, thereby creating an anti-greenhouse effect. In comparison, a greenhouse effect allows visible light in but blocks outgoing infrared radiation. The clouds in the atmosphere of Titan rain liquid methane and ethane, completing a ‘methanological cycle’ that is akin to the hydrological cycle on Earth. Benner et al. (2004) were the first to suggest that liquid methane on Titan could potentially be the basis for life there, playing the same role as water does for life on Earth. Methane-based life on Titan could consume organic molecules similar to Earthly life, but they would probably inhale hydrogen instead of oxygen and exhale methane instead of carbon dioxide. The discovery of any methane-based life on Titan will have incredibly interesting implications. In this article, it will be assumed that methane-based life on cryogenic Titan-like worlds is a possibility. Hence, the term liquid methane habitable zone (LMHZ) will correspond to Titan-like worlds while the term liquid water habitable zone (LWHZ) will correspond to habitable Earth-like worlds.

Suddenly I was aware of something new. The air in front of me had lost its crystal clearness… I was aware of a faint taste of oil upon my lips, and there was a greasy scum upon the woodwork of the machine. There was no life there. It was inchoate and diffuse; extending for many square acres and then fringing off into void. No, it was not life. But might it not be the remains of life? Above all, might it not be the food of life, a monstrous life, even as the humble grease of the ocean is the food for the mighty whale?

- Arthur Conan Doyle, The Horror of the Heights

Red dwarf stars have much lower masses than our Sun and they comprise the vast majority of stars. Being much more numerous that Sun-like stars, red dwarf stars are particularly interesting in the search for potentially habitable worlds; both in the LMHZ for Titan-like worlds and in the LWHZ for Earth-like worlds. The much lower luminosities of red dwarf stars mean that a planet orbiting a red dwarf star will have to be located much closer in just to receive the same amount of radiation as if it were located around the Sun. For a habitable Earth-like planet around a red dwarf star, the LWHZ will be situated very close to the star, causing the planet to be in a tidally locked state whereby one hemisphere of the planet perpetually faces its host star. However, the LMHZ for a Titan-like planet around a red dwarf star is located much further out from the star and this gives the planet a much better chance of not being in a tidally locked state, thereby creating a less stringent condition for life to exist.

The light from a red dwarf star contains a higher proportion of infrared radiation as compared to the light from the Sun. If Titan were orbiting around a red dwarf star instead of the Sun, a greater proportion of the light from the red dwarf star will reach the surface of Titan as the atmospheric haze of Titan is transparent to infrared wavelengths. If Titan is placed at an appropriate distance from the red dwarf star such that it receives the same amount of radiation as it currently receives from the Sun, the increased infrared fraction of the incoming radiation that makes it to Titan’s surface will warm the surface by an additional 10 degrees Centigrade or so. This warming effect is based on the assumption that a Titan-like world orbiting around a red dwarf star has a haze layer that is as thick as Titan’s. However, because red dwarf stars produce a lower proportion of ultraviolet light as compared to the Sun and because red dwarf stars can also produce a greater deal of high energy radiation that is associated with flares as compared to the Sun, the haze production rate for a Titan-like world in orbit around a red dwarf star can range from being much lower to much higher than that for Titan.

A habitable Titan-like world orbiting within the LMHZ of an M4-type red dwarf star will now be investigated. The M4-type red dwarf star is assumed to have a surface temperature of 3130 degrees Kelvin and a luminosity that is 2500 times less than the Sun’s. For a Titan-like world with a haze layer thickness that is reduced by a factor of 100 in comparison to Titan’s haze layer, it will have to orbit its parent M4-type red dwarf star at a distance of 0.23 AU in order to maintain a surface temperature of minus 180 degrees Centigrade. However, if the haze layer thickness of the Titan-like world is increased by a factor of 100 in comparison to Titan’s haze layer, the planet will need to orbit its parent M4-type red dwarf star at a much closer distance of 0.084 AU in order to maintain the same surface temperature. The temperature of minus 180 degrees Centigrade is the current surface temperature of Titan and it allows for the existence of liquid methane. Therefore, within a range factor of 10000 for the haze layer’s thickness, the liquid methane habitable zone (LMHZ) for a Titan-like world around an M4-type red dwarf star varies from 0.084 AU to 0.23 AU.

Instead of orbiting around the planet Saturn in the solar system, now imagine Titan as a lone planet drifting in interstellar space, with no parent star to provide any form of light and warmth. This is the case of Titan as a rouge planet and how it might still support a surface temperature of minus 180 degrees Centigrade as it drifts in the much colder depths of interstellar space. In order to maintain such a surface temperature, Titan with its current haze layer thickness will require an average geothermal heat flux of 1.4 watts for each square meter of its surface. Nevertheless, this value is around 20 times more than the average geothermal heat flux for the Earth and although this value might be consistent with a planet that is somewhat larger than the Earth, it is not realistic for a world the size of Titan. However, if Titan’s atmosphere is 20 times thicker than its current thickness, a more plausible average geothermal heat flux of 0.1 watts for each square meter of its surface will be sufficient to maintain a surface temperature of minus 180 degrees Centigrade.

Thus, for billions of years, Titan waited… An object looking a little like a comet streaked across the sky of Titan, battering atmospheric gases to a plasma twice as hot as the surface of the Sun itself. Cooling, it fell towards the surface slush. A parachute blossomed above it.

- Stephen Baxter, Titan

If any methane-based life is discovered on Titan, it should be widespread on Titan’s surface because liquid methane is also widespread on the surface. Direct evidence from the Huygens Probe has shown that the surface of Titan at the probe’s landing site is soaked with methane and radar imagery from Cassini has revealed numerous lakes on both the northern and southern polar regions of Titan. Life on a cryogenic world which runs on a methanological cycle will be extremely interesting. This is because the discovery of methane-based life on Titan or on any other Titan-like worlds will greatly improve our understanding of the range of worlds and chemical models that might support liquid-based life.

The supporting paper for a large part of this article is Ashley E. Gilliam and Christopher P. McKay “Titan under a Red Dwarf Star and as a Rogue Planet: Requirements for Liquid Methane” (2011), Planetary and Space Science, doi:10.1016/j.pss.2011.03.012. See also Steven A Benner et al. “Is there a common chemical model for life in the universe?” (2004), Chemical Biology, doi:10.1016/j.cbpa.2004.10.003.

Dark Matter and Alien Planets

2011-04-15T07:50:00.000+08:00

In the dark and immense vastness of interstellar space, there can be lone planets that do not orbit around any parent star. Such planets do not receive warmth from stars and any surface inhabitant will experience perpetual night. It appears very unlikely that these dark and seemingly frigid worlds may support life and sustain alien ecologies. However, a combination of mechanisms such as radiogenic heating, tidal heating or having a thick hydrogen atmosphere that is very effective at trapping heat, can sufficiently raise the surface temperature of such a planet to a point where liquid water can exist on the planet’s surface. In this article, I will consider another possible source of heating which can contribute to raising the surface temperature of a ‘sunless’ planet and that source of heating comes from the annihilation of dark matter particles.

All of the dark matter in the known universe contains a total amount of energy that is on the order of 10 thousand times greater than all of the energy that could be released through the fusion of all the hydrogen in the universe into helium. Unlike normal matter, dark matter has a scattered nature and does not interact at sufficient rates to meaningfully contribute to heating a planet. An exception is when dark matter particles are gravitational captured by a planet, whereby interactions with the matter making up the bulk of the planet can cause the dark matter particles to lose momentum and become gravitationally bound to the planet. This causes dark matter to accumulate in the planet’s interior and the annihilation of dark matter particles produces high energy secondary particles which are then absorbed and deposited as heat into the surrounding bulk of the planet, thereby providing a source of internal heat.

For the Earth, the capture and annihilation of dark matter particles in the planet’s interior does not produce any significant amounts of energy and even in the most optimistic scenarios, the energy contribution from the annihilation of dark matter particles is billions of times less than the energy the Earth receives from the Sun. However, the density of dark matter is expected to be hundreds to thousands of times greater in the central regions of the Milky Way galaxy and in the dense cores of dwarf spheroidal galaxies than it is in our solar system. This means that the energy contribution from the annihilation of dark matter particles for planets located in these regions can be very different.

Furthermore, dark matter residing in this unique regions have extremely low relative velocities and this greatly increases the capture rate of dark matter particles by a planet that is located in such a region. This is due to the fact that the low relative velocities of the dark matter particles make them more efficient in being gravitationally focused toward the planet or becoming gravitationally bound to the planet following collisions in which the particles lose just a small amount of momentum. This enables dark matter particles to accumulate in much greater quantities in planets located in these regions, such that the annihilation of dark matter particles can become the dominant source of energy to the extent of providing sufficient warmth for liquid water to exist on the surfaces of these planets even in the absence of warmth from a parent star.

The energy released from the annihilation of dark matter particles can enable rouge planets that do not orbit around any parent star to become potentially habitable and sustain an alien ecology. Around the center of the Milky Way galaxy, Earth mass planets with very low atmospheric emissivity can efficiently trap the energy released from the annihilation of dark matter particles to maintain surface temperatures that are possible for liquid water to exist. For atmospheres with higher and more Earth-like emissivities, super-Earths with a few times the mass of the Earth will then be required to trap sufficient annihilation energy to maintain surface temperatures that are capable of sustaining liquid water. This is due to the fact that although high emissivity atmospheres are less efficient in trapping heat as compared to low emissivity atmospheres, super-Earths can accumulate more dark matter than Earth-mass planets due to their more massive bulk.

The timescale over which a rouge planet can maintain sufficient warmth to have liquid water on its surface solely by the energy released from the annihilation of dark matter particles is on the order of trillions of years. This surpasses even the exceedingly long lifespans of low mass red dwarf stars. Due to the rarity of very high density dark matter environments, planets that are heated by the annihilation of dark matter particles are expected to be very rare. Nevertheless, such planets can provide the energy required to sustain an alien ecology over trillions of years, even in the absence of warmth from any parent star! Given their exceedingly long lifetimes, these rare alien worlds may prove to be the ultimate cradles of life in the universe.

Black Hole Propelled Starship

2011-04-07T22:29:00.000+08:00

A black hole is essentially an object that is so dense and compact that within a sufficiently close distance from it, its immense gravitational pull does not let even light to escape. This critical distance is the event horizon of the black hole and anything which crosses the event horizon, including light, can never escape. If the entire Earth is crushed to form a black hole, its event horizon will have a diameter of only 18 millimeters! In this article, I am going to describe the possibility of using micro black holes as a means of propulsion for interstellar space travel and also compare it with other forms of propulsion. So far, all black holes known range from monstrous supermassive black holes in the cores of galaxies to stellar mass black holes, spanning in mass from billions of times the mass of the Sun to a few times the mass of the Sun respectively. In this article, the black holes described are micro black holes that are on the order of only a hundred thousand metric tons or so.

In the 1970s, the physicist Stephen Hawking theorized that black holes can emit radiation due to quantum effects and this phenomenon became known as Hawking radiation. In the absence of any mass accretion, an isolated black hole will gradually lose mass via the emission of Hawking radiation until the entire black hole eventually disappears. The power emitted by a black hole in the form of Hawking radiation increases as the mass of the black hole decreases. Therefore, as a black hole shrinks in mass, it will emit Hawking radiation at an ever increasing rate until it eventually disappears in an incredible burst of energy. A black hole with a mass of a billion metric tons will take almost 3 trillion years to complete decay via the emission of Hawking radiation even though it is slightly smaller than the size of the nucleus of an oxygen atom.

The Alpha Centauri star system is located 4.37 light years away and it is among the nearest stars. Traveling at a velocity of say 100 kilometers per second, which is already much faster than the fastest speed attained by any spacecraft to date, it will take over 13000 years to reach Alpha Centauri. Therefore, to get to the stars within a reasonable amount of time, a spacecraft will have to be accelerated up to a significant fraction of the speed of light and an entirely new means of propulsion will be required for such a feat.

As a black hole decays through the emission of Hawking radiation, almost all of the mass of the black hole is directly converted into energy and the only other known process with such a good mass to energy conversion efficiency is the annihilation of matter with antimatter. A 100 percent efficient conversion of mass to energy produces about 90 thousand trillion joules of energy for every kilogram of mass. Today’s best chemical propulsion methods can only get up to a few million joules per kilogram of fuel. Even nuclear fission and nuclear fusion pale in comparison as less than one percent of the mass of the fissile or fusion material is converted into energy. Hence, the almost perfect mass to energy conversion efficiency from the emission of Hawking radiation by decaying micro black holes can make them a viable means of propulsion for interstellar space travel.

A micro black hole with a mass of 100 thousand metric tons or so is able to produce thousands of times more power in the form of Hawking radiation than the average total power consumption by the entire human world in 2008. Such a micro black hole can be use to accelerate a spaceship to the incredibly huge velocities required for interstellar space travel by directing the high energy radiation from the decaying black hole to generate thrust. Because a black hole of this mass has a lifespan of only a few months, matter is continuously required to feed the black hole to sustain it. In fact, any form of matter including the extremely tenuous gases making up the interstellar medium between the stars can be use to feed and sustain the black hole.

To put the numbers into perspective, a micro black hole with a mass of 404 thousand metric tons will have a net power output of 370 petawatts from its emission of Hawking radiation and this is about 25000 times more power than the average total power consumption by the entire human world in 2008! If the total power output of this black hole is sustained for one year to accelerate a 1 million metric ton spaceship which also includes the mass of the black hole itself, the spaceship will acquire a final velocity of almost half the speed of light or 150 thousand kilometers per second. This will get the spaceship to Alpha Centauri in about a decade or so. However, such a scenario assumes that 100 percent of the energy emitted by the black hole is used for the acceleration of the spaceship. The black hole must also be constantly fed with mass such as interstellar gas collected along the way to sustain it as it journeys to the stars.

Forming an initial black hole will first require crushing a large amount of mass into an extremely tiny volume of space. The technology required to accomplish such a feat is probably far beyond today’s capabilities. However, once an initial black hole is created, additional mass can be fed into the black hole to allow it to grow to the required mass for it to be used as a means to accelerate a spaceship for interstellar space travel. Compared to the annihilation of matter with antimatter, the use of micro black holes as a means of propulsion is probably much more energy efficient because the production of antimatter requires vastly more energy as an input that what can be obtained by the annihilation process. In addition, once a micro black hole is created, it can be made to provide power via the emission of Hawking radiation for an indefinite period of time as long as the black hole is fed with mass to sustain it.

Drifting Crust

2011-04-01T07:05:00.000+08:00

Titan is the largest moon of Saturn, the only moon that is known to have a dense atmosphere and the only known object in the Solar System other than Earth with stable bodies of surface liquid. With a diameter of 5150 kilometres, Titan is the second-largest moon in the Solar System as it is slightly smaller than Jupiter’s moon Ganymede. However, when placed together with Ganymede, Titan will actually appear larger because Titan’s dense and opaque atmosphere extends many kilometres above its surface and increases its apparent diameter. NASA’s Cassini spacecraft is currently in orbit around Saturn and it frequently makes flybys of Titan.

On Titan, the average surface temperature is roughly minus 180 degrees Centigrade and the surface atmospheric pressure is 1.45 times the atmospheric pressure at sea-level on Earth. For every square meter of Titan’s surface area, the overlying atmosphere 7.3 times more massive in comparison to the Earth’s. The surface gravity of Titan is one-seventh the surface gravity of the Earth such that when combined with the dense atmosphere, Titan’s gravity is sufficiently low to allow humans to consider flying through the atmosphere on their own strength by flapping artificial wings strapped to their arms!

Beneath an icy crust that has a thickness of perhaps a hundred kilometres or so, Titan is believe to have a global subsurface ocean of liquid water. The presence of a subsurface ocean dynamically decouples the crust of Titan from its much more massive interior bulk, thereby lowering the effective moment of inertia of the moon’s crust. This allows the global circulation of air within Titan’s thick atmosphere to drag and torque the entire crust around such that the crust does not rotate at exactly the same rate as the rest of Titan.

Like the other large moons of Saturn, Titan’s rotation is synchronous, which implies that Titan rotates once with each orbit around Saturn. However, because Titan’s crust is decoupled from its interior by the subsurface ocean, it allows the crust to be freely dragged around by the movement of air in Titan’s thick atmosphere. Surface features imaged by Cassini during one flyby are observed to be offset by as much as a few tens of kilometres when imaged in subsequent flybys. The entire surface of Titan shifts by one-third of a degree each year as the winds in Titan’s thick atmosphere freely torques the entire crust. Therefore, surface features on Titan will be noticeably offset in images of the same locations that were taken on different dates.

Having an atmosphere which pushes around the entire surface of a moon is not something that is new. In fact, the same thing happens on the Earth where the length-of-day changes by about one millisecond over the duration of a year because of winds speeding up and slowing down in the atmosphere. However, that is a tiny amount when compared to Titan because the Earth is much more rigid and more massive than Titan, and the Earth’s atmosphere is less dense than Titan’s atmosphere. On Titan, it seems that the entire world has to be considered, from its thick atmosphere to its icy crust to its interior ocean, just to explain the length of its day and the locations of its surface features. This makes Titan a world that is probably no less complex and dynamic as the Earth.

White Sun

2011-03-25T07:27:00.000+08:00

The search for habitable Earth-size planets has primarily been focused on stars similar to our Sun. In recent years, the search has also gone on to focus on low mass red dwarf stars as these stars are by far the most common and an Earth-size planet around such a star will be much easier to detect due to the lower mass and luminosity of a red dwarf star. In this article, I will be exploring the possibility of detecting Earth-size planets located in the habitable zone of cool white dwarf stars. White dwarf stars are the final evolutionary state of all stars that are not massive enough to explode as supernovae and this includes stars such as our Sun. Typically, a white dwarf star has a mass that is comparable to our Sun and all its mass is contained within a tiny volume that is comparable to the size of the Earth. Hence, a white dwarf star is a very dense object as each cubic centimetre of its material can weight over a metric ton.

White dwarf stars are as common as Sun-like stars and as they slowly cool, they can provide energy to planets in orbit around them for billions of years. A paper entitled “Transit Surveys for Earths in the Habitable Zones of White Dwarfs” describes the prospect of detecting habitable Earth-size planets around white dwarf stars by searching for transits of such planets in front of white dwarf stars. Compared to a typical Sun-like star, the habitable zone around a white dwarf star will be located much closer in due to the much lower luminosity of a white dwarf star. The most common surface temperature for white dwarf stars is around 5000 degrees Kelvin and white dwarf stars with surface temperatures of over 10000 degrees Kelvin are rare because white dwarf stars spend little time at high temperatures as they cool very rapidly at such high temperatures. Furthermore, the high ultraviolet flux from a hot white dwarf star that has a surface temperature of over 10000 degrees Kelvin will affect the retention of an atmosphere around an Earth-size planet. Therefore, only cool white dwarf stars will surface temperatures that are considerably less than 10000 degrees Kelvin are considered for the detection of habitable Earth-size planets.

A white dwarf star does not have an internal source of energy like a typical star and this means that it will gradually radiate away its energy and cool down over a period of billions to trillions of years. Hence, the term “continuously habitable zone” is defined as the range of orbital distances from a white dwarf star where an Earth-size planet can stay habitable for a specified minimum duration. For an Earth-size planet to remain habitable for at least 3 billion years, the continuously habitable zone will extend from a distance of 0.005 AU to 0.02 AU for white dwarf stars with masses ranging from 0.4 to 0.9 times the mass of our Sun, whereby 1.0 AU is basically the mean distance of the Earth from our Sun.

The orbital period of any planet in the continuously habitable zone of white dwarf stars will range from around 4 to 32 hours and the planets are expected to be tidally-locked whereby the star-facing hemisphere of the planet will experience permanent day, while the other hemisphere will experience permanent night. The night side of such a planet can be warmed by the global circulation of heat from the day side of the planet which can prevent the formation of a cold-trap on the night side. Since the orbital period and spin period of a tidally-locked planet are both the same, an Earth-size planet in the continuously habitable zone of a white dwarf star will experience Coriolis and thermal forces that are similar to those on the Earth.

Earth-size planets in or near the continuously habitable zone of white dwarf stars can be detected via the transit method where the individual photometric output of a large number of white dwarf stars can be continuously monitored to look for any dimming that can be associated with the transit of an Earth-size planet in front of a white dwarf star. Due to the small size of a white dwarf star, the transit of an Earth-size planet will block out a significant fraction of the white dwarf star’s total photometric output or even completely block out the entire star if the star is sufficiently small. The small size of a white dwarf star also favours the detection of transiting objects that are smaller than the size of the Earth. The transit durations of Earth-size planets in the continuously habitable zone of white dwarf stars are estimated to last for a couple of minutes or so, thereby requiring high cadence observations to record the proper light curves that are indicative of such transit events.

Measurements of the distance and spectrum of a white dwarf star will allow its mass, luminosity, atmospheric composition and radius to be determined. Therefore, with the size of the white dwarf star known, the measured transit depth of a transiting planet enables the size of the transiting planet to be directly determined. On the contrary, the mass of the transiting planet cannot be determined from Doppler measurements as the spectra of cool white dwarf stars are generally featureless. However, if the white dwarf star has multiple transiting planets, gravitational interactions among the planets can cause measurable transit timing variations which can be use to estimate the mass for each of the planets.

At Mercury

2011-03-18T09:33:00.000+08:00

NASA’s MESSENGER spacecraft was launched into space onboard a Delta II 7925 rocket on 3 August 2004 at 06:15:56 UTC from Space Launch Complex 17B at the Cape Canaveral Air Force Station in Florida. After travelling through space for 6 years, 7 months and 16 days and covering an impressive distance of 7.9 billion kilometres, MESSENGER finally entered orbit around the planet Mercury on 18 March 2011 at 01:00 a.m. UTC after a 15 minutes Mercury orbit insertion (MOI) engine burn. MESSENGER is the second mission to Mercury after a final flyby performed by Mariner 10 in 1975 and it is the first spacecraft to enter orbit around the planet. The primary mission of MESSENGER will be to study the chemical composition, geology and magnetic field of Mercury.

Getting to Mercury from the Earth requires a large velocity change because the closeness of Mercury to the Sun places the planet deep within the Sun’s gravitational potential well. Furthermore, Mercury’s extremely tenuous atmosphere makes it impossible for an aerobraking manoeuvre to be employed to sufficiently slow an incoming spacecraft for capture into orbit around Mercury. To solve this issue, MESSENGER extensively used gravity assist manoeuvres by making flybys of the inner planets to gradually decelerate the spacecraft such that the amount of propellant required to slow the spacecraft into orbit around Mercury is greatly reduced. However, this comes at the cost of prolonging the trip to Mercury by a few years. The trajectory that MESSENGER took through the inner solar system to get to Mercury included one flyby of Earth, two flybys of Venus and three flybys of Mercury itself.

On 18 March 2011 at 12:45 a.m. UTC, the orbital insertion manoeuvre brought MESSENGER into a highly elliptical orbit around Mercury whose lowest point is 200 kilometres above the planet’s surface while the highest point is over 15000 kilometres above the planet’s surface. The three previous flybys of Mercury by MESSENGER have already generated an astonishing amount of interesting science that has changed our understanding of the enigmatic innermost planet of the solar system. However, these flybys are merely a sneak preview of the discoveries that are expected to come as MESSENGER is now the first spacecraft ever to orbit Mercury for long-term observations. Visit http://messenger.jhuapl.edu/index.php to learn more about MESSENGER and its mission around Mercury.

Galactic Blast

2011-03-17T18:25:00.000+08:00

The Fermi Gamma-ray Space Telescope (FGST) is a space observatory which observes the universe in gamma-rays from its vantage point in low Earth orbit. One interesting discovery by Fermi are two enormous gamma-ray-emitting bubbles that extend about 30 thousand light years above and below the centre of the Milky Way galaxy. The existence of the two gamma-ray-emitting bubbles was first hinted by previous detections of a localized excess of radio signals. In this article, the two gamma-ray-emitting bubbles will be referred to as the Fermi Bubble. I recently read a paper entitled “Origin of the Fermi Bubble” and this paper suggests that the periodic capture of stars by the supermassive black hole at the centre of the Milky Way galaxy can inject the required amounts of high energy plasma into the galactic halo to form the Fermi Bubble.

A supermassive black hole with a mass of approximately 4 million Suns sits in the heart of the Milky Way galaxy. Stars which happen to come too close to the supermassive black hole can be destroyed by tidal disruption. When a star gets tidally disrupted by the supermassive black hole, about half of its mass becomes tightly bound to the black hole while the other half gets violently ejected. The amount of energy carried by the ejected mass can significantly exceed the amount of energy released by a normal supernova explosion. Approximations have shown that the supermassive black hole at the galactic centre destroys a star by tidal disruption at a rate of roughly one star every ten thousand years or so. This means that tens of stars are expected to get tidally disrupted every one million years.

The ejecta from each tidally disrupted star expand as a spherically symmetric wind of high energy plasma and ‘snowploughs’ its way out of the galactic centre to form a pair of bipolar outflows which contribute to the existence of the Fermi Bubble. The high energy outflows from each tidal disruption event expand hydrodynamically out of the galactic centre and into the galactic halo, forming shock fronts which accelerate electrons to near the speed of light. Interaction of the high energy electrons with background photons via synchrotron radiation and inverse Compton scattering produces the observed radio and gamma-ray emissions respectively. Since the mean interval between each tidal disruption event is smaller than the timescale for energy loss, the gamma-ray emissions produced from each individual shock front can be approximated to be uniformly distributed over the entire Fermi Bubble.

Finally, the existence of the Fermi Bubble cannot be explained by a previous episode of starburst activity in the galactic centre because there is no evidence of an excessive amount of supernova explosions in the past 10 million years or so in the galactic centre. Furthermore, supernova remnants can be traced by the radioactive aluminium-26 they produce and the sparse concentration of aluminium-26 in the galactic centre does not support a previous episode of starburst activity.

Cool Dwarf

2011-03-09T12:54:00.002+08:00

Brown dwarfs are objects that are too low in mass to sustain hydrogen fusion in their cores and they occupy the mass range between gas giant planets and the lowest mass stars. The upper limit for the mass of a brown dwarf is around 80 times the mass of Jupiter while the lower limit for the mass of a brown dwarf is undefined as it overlaps with the masses of gas giant planets. Methane-bearing spectral class T brown dwarfs are the coolest known class of brown dwarfs. Although a large number of brown dwarfs are know, there remains a large gap between the temperature of the coolest known brown dwarfs and the gas giant planets in our solar system. The coolest known brown dwarfs have temperatures of around 500 degrees Kelvin while the gas giant planets in our solar system have temperatures of around 150 degrees Kelvin. Theoretical studies have shown that brown dwarfs in this temperature range exhibit spectroscopic characteristics that are distinct from the spectral class T brown dwarfs, such as ammonia absorption lines and scattering from clouds of water ice. Any brown dwarfs in this temperature range can be categorized into a new and cooler spectral class known as spectral class Y.

A newly published paper entitled “Discovery of a Candidate for the Coolest Known Brown Dwarf” describes the discovery of what might be the coolest known brown dwarf and a likely prototype for the spectral class Y. With an estimated temperature of 300 degrees Kelvin, WD 0806-661 B is a candidate for the coolest known brown dwarf and also cool enough for its atmosphere to contain clouds of water ice. WD 0806-661 B is in a wide orbit around a white dwarf star and if a similar age to its host star is assumed, WD 0806-661 B will be around 1.5 billion years old. Furthermore, based on evolutionary models of cooling brown dwarfs, WD 0806-661 B is estimated to have a mass of around 7 times the mass of Jupiter and this falls well within the range of masses for the more massive extrasolar planets. There are two mechanisms in which an object like WD 0806-661 B could have formed. Firstly, WD 0806-661 B could have formed from the coalescence of a fragmented cloud of gas at its current large distance from its host star. Secondly, WD 0806-661 B could be a gas giant planet that had been dynamically scattered into a much more distant orbit around its host star. If subsequent observations confirm WD 0806-661 B to be the coolest known brown dwarf, it will become a valuable target for studying atmospheres in an entirely new temperature regime that will consequently aid searches for the coldest brown dwarfs with facilities such as the Wide-field Infrared Survey Explorer (WISE) and the James Webb Space Telescope (JWST).

Interstellar Drifters

2011-03-04T06:44:00.000+08:00

The formation of planets around a young star occurs within a disk of gas and dust encircling the young star called a protoplanetary disk. This results in a close alignment between the rotation axis of the star and the orbital motion of the planets after they have formed. However, measurements of the relative spin-orbit alignment of transiting extrasolar planets via the Rossiter-McLaughlin effect have shown that a number of these planets have orbits that are significantly misaligned with the rotation axes of their host stars. Gravitational interactions with other planets or the Kozai mechanism are two proposed means in which planets in initially aligned orbits can get perturb into misaligned orbits. A recently published paper entitled “Misaligned and Alien Planets from Explosive Death of Stars” proposes an alternative mechanism to explain the planets that are observed to be in misaligned orbits around their host stars.

In this paper, planets whose orbits are misaligned with the rotation axes of their host stars are suggested to have formed from high-speed blobs of gas that are produced in supernova remnants and planetary nebulae. Supernova remnants are formed following the death of massive stars in supernova explosions while planetary nebulae are formed from the death throes of Sun-like stars as they shed off their outer layers into space. These blobs of gas have been observed in great numbers around supernova remnants and planetary nebulae. High resolution images of young supernova remnants and planetary nebulae have shown that each of them is surrounded by thousands of blobs of gas that have cometary-like appearance possibly shaped by overtaking winds. As these blobs of gas travel through interstellar space, they sweep up ambient matter along the way, causing them to increase in mass and decelerate. Over time, these blobs of gas gradually cool by emitting radiation. Once the blobs of gas become sufficiently massive and cool, self-gravity takes over and cause the blobs of gas to collapse gravitationally to form gas giant planets.

Regardless of whether these blobs of gas eventually contract gravitationally to form gas giant planets, they can explain the considerable number of planets that are found to be in misaligned orbits around their host stars through a number of different ways. For a blob of gas that has already collapsed gravitationally to form a gas giant planet, it can be captured into a misaligned orbit around a star or perturb the original planets that have previously formed around a star into misaligned orbits. On the other hand, an uncollapsed blob of gas can be captured into a misaligned orbit around a star to form a misaligned disk of gas and dust from which the in situ formation of planets with misaligned orbits can occur. Furthermore, uncollapsed free-floating blobs of gas can also be captured by stars and strongly perturb their planetary systems.

Around young supernova remnants and planetary nebulae, a typical blob of gas has a similar mass as the Earth and a ‘head’ which spans tens of billions of kilometers across. Shaped into cometary-like morphologies by faster overtaking winds, a typical blob of gas travels with an average projected velocity on the order of a few hundred kilometers per second outwards from its parent supernova remnant or planetary nebula. Blobs of gas in young supernova remnants can be classified into two populations distinguished by their velocities where the low velocity population formed in the ejecta of the progenitor star before its supernova explosion while the high velocity population formed following the supernova explosion.

As the blobs of gas travel outwards, they cool by radiation and grow in mass by accreting gas and dust in the interstellar medium. The accretion process causes the blobs of gas to decelerate, making the blobs of gas or the eventual gas giant planet slow enough to be gravitationally captured during sufficiently close encounters with stars. Finally, very high velocity blobs of gas from supernova explosions that travel through very low density regions of space will escape into the low-density intergalactic space where they will expand before they ever reach the required mass and compactness necessary for them to collapse gravitationally into gas giant planets. These very high velocity blobs of gas will enrich the intergalactic medium with heavy elements produced from supernova explosions.

Vulcanoids

2011-02-24T17:54:00.000+08:00

Asteroids are the most abundant minor objects in the Solar System and most of them are distributed within the main asteroid belt between Mars and Jupiter, and in the two groups of Trojan asteroids located 60 degrees ahead of and behind Jupiter in its orbit around the Sun. Asteroids in these regions of the Solar System can reside stably over billions of years. Apart from these regions, are there other regions of the Solar System where long-lived stable belts of asteroids can possibly exist? In this article, I will describe the possibility of a long-lived stable belt of asteroids existing close to the Sun, in a region of space that is interior to Mercury’s orbit around the Sun.

Mercury is the closest planet to the Sun and it orbits the Sun at an average distance of 57.9 million kilometres or 0.387 AU, where 1.0 AU is basically the mean distance of the Earth from the Sun. The asteroids that belong to this hypothesized belt of asteroids that exists interior to Mercury’s orbit are referred to as the Vulcanoids. The Vulcanoids are a population of intra-Mercurial asteroids that exist in a region between 0.09 AU and 0.20 AU from the Sun. The inner edge of the Vulcanoid belt is approximately 0.09 AU from the Sun as any asteroid orbiting closer to the Sun than this will have an unstable orbit due to perturbation by the intense radiation of the Sun. On the other hand, the outer edge of the Vulcanoid belt is approximately 0.20 AU from the Sun as any asteroid located outside this boundary will be perturbed by Mercury’s gravity.

As the asteroids in the hypothesized Vulcanoid belt orbit the Sun at high velocities in a relatively small volume that is entirely interior to Mercury’s orbit, evolution through the mutual collisions of asteroids will be much more frequent than in the main asteroid belt between Mars and Jupiter. Models of the collisional processes have shown that for the Vulcanoid belt, only a few hundred asteroids larger than one kilometre in size could have survived until the current epoch. Because collisional evolution of asteroids proceeds fastest at smaller distances from the Sun, it has been proposed that a favourable location to search for existing Vulcanoids is near the outer edge of the Vulcanoid belt.

It is estimated that the region between 0.16 AU and 0.18 AU is most likely to contain surviving kilometre-sized Vulcanoids since objects near 0.20 AU from the Sun can get perturbed by the gravity of Venus. The closeness of the Vulcanoids to the Sun makes them extremely challenging to observe from the Earth. However, this closeness is expected to cause the Vulcanoids to be hot enough to give off a significant amount of infrared emission, making infrared detection methods the best choice for detecting the elusive Vulcanoids. To date, no Vulcanoids has yet been discovered and the Vulcanoid belt remains merely as a hypothesis.

Steppenwolf Planet

2011-02-20T00:17:00.000+08:00

During the formation of planetary systems around stars, gravitational interactions with gas giant planets can cause some planets or planetesimals to enter hyperbolic orbits and get ejected into interstellar space. These objects will wander the dark and vast expenses of interstellar space as rouge planets. Several months ago, I wrote an article about the possibilities that a wandering planet in interstellar space can have a dense and high pressure atmosphere of hydrogen gas which can create a greenhouse effect that is strong enough such that liquid water can be maintained on the planet’s surface by the planet’s geothermal heat flux alone, making the planet potentially habitable. In this article, I explore the possibilities in which a planet wandering in interstellar space can be potentially habitable from an alternative mechanism that was recently published in a paper entitled “The Steppenwolf: A Proposal for a Habitable Planet in Interstellar Space”. This paper explores whether an Earth-like planet wandering in interstellar space can be potentially habitable by sustaining a subglacial ocean of liquid water.

In this case, an Earthlike planet means that the planet has a total mass and a water mass fraction that is within one order of magnitude of the Earth’s, and the planet also has approximately the same composition as the Earth. A rouge Earth-like planet wandering in interstellar space and harboring a subglacial ocean of liquid water is referred to in the paper as a Steppenwolf planet. Such a planet will have a layer of ice on top of a subglacial ocean of liquid water where the radiogenic geothermal heat flux from the interior of the planet prevents the liquid water from freezing solid. Furthermore, the overlying layer of ice acts an insulating layer which prevents the radiogenic geothermal heat flux from escaping too quickly into space, thereby trapping sufficient heat energy to sustain the subglacial ocean of liquid water.

The radiogenic geothermal heat flux is produced from the decay of radioisotopes in the interior of the planet and it is estimated to be sufficient to keep the subglacial ocean of liquid water from freezing for the duration of a few billion years. For instance, the Earth has a current average geothermal heat flux of 0.087 watts per square meter of the Earth’s surface and since this geothermal heat flux decays with time, the Earth is estimated to have a geothermal heat flux of twice the current value at around 3 billion years ago. After a few billion years, the decline in the amount of radioisotopes makes the rate of radiogenic heating insufficient to provide enough warmth to keep the subglacial ocean of liquid water from freezing. Hence, a Steppenwolf planet will have a habitable lifetime that is comparable to planets that are found in the traditional habitable zones of Sun-like stars.

The steady-state thickness of the layer of ice on a Steppenwolf planet depends on the amount of radiogenic geothermal heat flux being radiated by the planet. A greater heat flux will allow for a thinner steady-state layer of ice to exist above the subglacial ocean of liquid water while a lower heat flux will result in a thicker layer of ice. If the heat flux is too low, the resulting steady-state thickness of the layer of ice will be greater than the depth of the ocean and no subglacial ocean of liquid water will be possible in this case. In addition, volcanoes on continents or islands that rise above the layer of ice on a Steppenwolf planet can emit significant quantities of carbon dioxide, leading to the eventual formation of a thick cryo-atmospheric layer of carbon dioxide. Such a layer of carbon dioxide can raise the temperature at the top surface of the layer of ice and enable a significantly reduced thickness for the steady-state layer of ice overlying the subglacial ocean of liquid water.

Without a cryo-atmospheric layer of carbon dioxide, a Steppenwolf planet with the same radioisotope composition, age and water mass fraction as the Earth will have to be at least 3.5 times more massive than Earth in order to sustain a subglacial ocean of liquid water. In contrast, a Steppenwolf planet with ten times the water mass fraction of the Earth and with a thick cryo-atmospheric layer of carbon dioxide will require a mass of only 0.3 times the mass of the Earth for it to sustain a subglacial ocean of liquid water. The transport of heat up the layer of ice on a Steppenwolf planet can be assumed to be conductive in nature since any transport of heat by convecting ice is expected to occur only in the lower and warmer regions of the ice layer where it will be capped by an overlying lid of stagnant conducting ice.

Steppenwolf planets will be very challenging to detect as they wander through the dark and immense voids of interstellar space and produce no light of their own. However, if a Steppenwolf planet were to loiter close enough to our Sun, it could make its presence know by detecting the sunlight being reflected off its surface. For instance, a planned wide-field survey telescope known as the Large Synoptic Survey Telescope (LSST) will be able to detect a Steppenwolf planet out to a maximum distance of around 1000 astronomical units where one astronomical unit is the average distance of the Earth from our Sun. It should be known that only surveys that observe large regions of the sky continuously will be likely to discover any Steppenwolf planets because such planets can be anywhere in the sky. Finally, the discovery of any potentially habitable Steppenwolf planets will be rather exciting because it can mean that potentially habitable worlds are truly ubiquitous in the universe.

Hot Rock

2011-02-17T21:29:00.000+08:00

Kepler-10b is the name of the first confirmed rocky planet that was discovered by NASA’s Kepler space telescope. The star around which Kepler-10b orbits is an old Sun-like star designated Kepler-10. This star has an estimated age of 12 billion years and it is located at a distance of 560 light years away. Kepler-10b orbits its host star at a distance of only 3.45 stellar radii, taking just 20 hours to complete one orbit! This means that Kepler-10b transits in front of its host star once every 20 hours. During each transit event which lasts for a duration of 1.81 hours, Kepler-10b induces a 152 parts-per-million dimming of its host star. Being so near to its host star, Kepler-10b is certainly tidally locked where the same hemisphere of the planet is perpetually locked to face its parent star. Therefore, one hemisphere of Kepler-10b is in perpetual daylight while the other hemisphere experiences eternal night. The equilibrium temperature on the dayside hemisphere of Kepler-10b is estimated to be over 1833 degrees Kelvin, which makes it hot enough to melt iron.

The diameter of Kepler-10b is measured to be 1.4 times the diameter of the Earth and the mass of Kepler-10b is estimated from radial velocity measurements to be 4.6 times the mass of the Earth. This gives Kepler-10b an estimated mean volumetric density of 8.8 metric tons per cubic meter, making it on average 1.6 times denser that the Earth. Within two standard deviations of its derived mass and diameter, Kepler-10b is unequivocally a high density rocky planet with a large fraction of its mass being in the form of iron. The official paper announcing the discovery of Kepler-10b is entitled “Kepler's First Rocky Planet: Kepler-10b” and it can be obtained from http://arxiv.org/abs/1102.0605.

As Kepler-10b orbits its host star, the observed amount of reflected starlight from the planet will vary because the planet will present different proportions of its illuminated hemisphere during different phases in its orbit. The amount of reflected starlight from the planet will be the lowest when the planet is directly in front of the star because a distant observer will be looking entirely at just the night side of the planet and none of the day side of the planet will be visible. On the contrary, the amount of reflected starlight from the planet will be the highest when the planet is almost directly behind the star because almost all of the illuminated day side of the planet will be visible. However, the amount of reflected starlight from the planet will be zero when the planet is directly behind the star as the planet will be blocked by the star.

NASA’s Kepler space telescope observed a 7.6 parts-per-million phase curve amplitude for the total photometric output centred on the host star of Kepler-10b each time the planet makes one orbit around its host star. Furthermore, as Kepler-10b passes directly behind its host star, a 5.8 parts-per-million dip in the total photometric output is observed. This gives Kepler-10b an estimated effective geometric albedo of 0.61 which makes the planet unusually reflective because the only objects in our Solar System with such a high albedo are the planet Venus with its reflective layer of photochemically induced hazes and Saturn’s moon Enceladeus with its global coat of fresh ice. One explanation for the high albedo of Kepler-10b is a reflective layer of silicate clouds that cover the entire day side of the planet. Because Kepler-10b is such a hot world, the silicate clouds are basically clouds that are comprised of tiny suspended droplets of molten rock.

Puffy Planet

2011-02-11T00:14:00.000+08:00

A remarkable property of gas giant planets and brown dwarfs is that even though their individual masses can range from less than half the mass of Jupiter up to over 80 times the mass of Jupiter, they all have roughly the same diameters as Jupiter as their sizes do not increase with mass. This is because the volumetric size of a gas giant planet is governed by Coulomb pressure while the volumetric size of a brown dwarf is governed by electron degeneracy pressure and both of these forces neatly compensate for gravitational compression, giving approximately the same diameter as Jupiter for objects ranging in mass from gas giant planets to brown dwarfs. A significant number of extrasolar planets are known with diameters that are much larger than the diameter of Jupiter, making these planets larger than what is theoretically expected for them. Attempts have been made to explain the anomalously large diameters of these extrasolar planets by a number of proposed heating mechanisms that can potentially deliver enough thermal energy into the interiors of these planets to inflate them to their observed diameters.

The largest extrasolar planet currently known is a transiting planet called WASP-17b. This planet has 49 percent the mass of Jupiter and its orbit around its host star happens to be orientated in such a way that the planet is observed to periodically pass in front its host star. WASP-17b is located at a distance of 7.7 million kilometres from the centre of its host star and it takes 3.74 Earth-days to make one orbit around its host star. The host star of the WASP-17b is a spectral type F6V star with 1.3 times the mass of our Sun, an estimated surface temperature of 6650 degrees Kelvin and a luminosity that is over 4 times greater than our Sun’s luminosity. Looking from WASP-17b, its host star will appear almost two thousand times brighter than our Sun as seen from the Earth.

Measuring the fraction of starlight blocked by WASP-17b as it passes in front of its host star gives the planet an estimated size that is twice the diameter of Jupiter. Such a diameter also means that WASP-17b is up to 0.2 Jupiter diameters larger than the next largest planet and up to 0.7 Jupiter diameters larger than the theoretical diameter predicted using the standard cooling theory of irradiated gas giant planets. With twice the diameter of Jupiter and just under half the mass of Jupiter, WASP-17b has a density of just 6 percent the mean density of Jupiter or 8 percent the density of water, making it the least dense planet known. For such an inflated and low density planet, the surface gravity of WASP-17b is less than one-third the surface gravity of the Earth even though WASP-17b has slightly less than half the mass of Jupiter which already translates to 155 times the mass of the Earth.

The orbit of WASP-17b around its host star is slight non-circular with a very small non-zero orbital eccentricity. This means that WASP-17b is a little nearer to its host star than at other times. It has been suggested that a planet can be inflated to twice the diameter of Jupiter or more during a transient phase of heating caused by tidal circularization from a highly eccentric orbit to an almost circular one. In this scenario, the planet can persist in an inflated state for over a billion years after its orbit has circularized considerably. However, such a planet is expected to cool and contract significantly prior to complete orbital circularization where its orbit is still noticeably non-circular. Therefore, under the scenario of transient heating, the near-zero orbital eccentricity of WASP-17b and its highly inflated size means that a transient phase of tidal heating from orbital circularization alone is insufficient to have inflated the planet to its current observed size.

An enhanced atmospheric opacity for WASP-17b will enable its internal heat to be radiated off at a much lower rate and this can slow the contraction of the planet from a previous transient phase of tidal heating. However, even this is still insufficient to account for the current inflated diameter of WASP-17b. Ongoing tidal heating can still occur for WASP-17b if its orbit is being kept non-circular by interaction with another planet in the system. However, this is unlikely to be the primary source of heating to account for the inflated diameter of WASP-17b because its orbital eccentricity is too small for sufficient tidal heating to occur. Finally, the kinetic energy from the strong winds generated in the atmosphere by the large day-night temperature contrasts of the tidally-locked WASP-17b can be transported into the deep interior of planet and be deposited as thermal energy. However, a means to convert the kinetic energy into thermal energy will still be necessary and turbulent dissipation is one of the proposed mechanisms. To conclude, no current theory is able to account for the remarkable inflated diameter of WASP-17b.

New Worlds

2011-02-05T20:12:00.000+08:00

NASA’s Kepler is a space telescope which hunts for planets around other stars by using a method called transit photometry. A transiting extrasolar planet is one which periodically blocks a small fraction of the light from its parent star as it passes in front. Transits only occur when a planet’s orbit around its parent star happens to be orientated almost edge-on with respect to our line of sight. Transit photometry works from observing the amount by which a star dims as a planet passes in front of it to determine the size of the planet. For the same star, a larger planet will occult a greater fraction of the star’s light as compared to a smaller planet. An Earth-size planet transiting a Sun-like star is expected to cause a mere 84 parts-per-million decrease in the star’s brightness and the incredible precision of Kepler’s photometer makes its suited for detecting such planets. Furthermore, by measuring the time between two successive transits, the orbital period and the distance of the planet from its host star can be determined.

The ‘Holy Grail’ for NASA’s Kepler mission is the eventual discovery of Earth-size planets orbiting in the habitable zones of Sun-like stars. However, this will require at least around 3 years of data collection by the space telescope, including a large amount of follow-up observations before these types of planets can emerge from the data. In the mean time, Kepler is already revolutionizing our understanding of extrasolar planets when on 1 February 2011, the Kepler mission team released data for 156453 stars that were observed by the Kepler space telescope from 2 May 2009 to 16 September 2009. During this period of observations, 1235 planetary candidates were detected and these planetary candidates are associated with 997 host stars. These planetary candidates are classified into five classes with 68 Earth-size candidates (less than 1.25 Earth-radii), 288 super-Earth size candidates (between 1.25 to 2 Earth-radii), 662 Neptune-size candidates (between 2 to 6 Earth-radii), 165 Jupiter-size candidates (between 6 to 15 Earth-radii) and 19 super-Jupiter size candidates (between 15 to 22 Earth-radii). For comparison, the radius of Neptune is about 4 Earth-radii and the radius of Jupiter is about 11 Earth-radii.

The planetary candidates in this first 4 months of data released by the Kepler mission team are strictly candidates only as each one of these planetary candidates will require rigorous follow-up observations in order for them to be confirmed as true planets. However, well over 90 percent of the 1235 planetary candidates are expected to be eventually confirmed as true planets due to the extremely meticulous vetting process through which these planetary candidates were extracted from the raw data in order to weed out events that are masquerading as transiting planets. The current set of data is restricted to planetary candidates with orbital periods of less than 125 Earth-days because planets with longer orbital periods will require more time by Kepler for multiple transits to be recorded.

It is interesting to note that out of the 68 Earth-size planetary candidates; around 2 dozen of them are actually somewhat smaller in size than the Earth. Furthermore, 54 of the 1235 planetary candidates are located within the habitable zones of their host stars and they have sizes ranging from Earth-size to larger than that of Jupiter. The habitable zone is basically defined as a region around a star where a rocky planet with an Earth-like atmosphere can have a surface temperature that lies between the freezing point and the boiling point of water. Of the 54 planetary candidates that are located within the habitable zones of their host stars, 5 of them are less than twice the size of the Earth while 2 of them are significantly larger than the size of Jupiter. Additionally, of the 5 approximately Earth-size planetary candidates, one of them is actually smaller than the Earth. Finally, it is also conceivable to expect the presence of potentially habitable Earth-size moons around some of the larger planets.

Of the 997 host stars that contain the 1235 planetary candidates, 170 of the host stars have two or more transiting planetary candidates. Of these 170 host stars, there are 115 stars with 2 transiting planetary candidates, 45 stars with 3 transiting planetary candidates, 8 stars with 4 transiting planetary candidates, one star with 5 transiting planetary candidates and finally, a single star with a staggering 6 transiting planetary candidates! In a recent press conference held on 2 February 2011, the 6 transiting planetary candidates orbiting that single star are no longer just planetary candidates as all 6 of them have been announced to be true planets orbiting the same star which has been named Kepler-11. The 6 planets are named Kepler-11b, 11c, 11d, 11e, 11f and 11g in order of increasing distance from Kepler-11. Since the size of a planet’s orbit is generally much larger than the physical size of its host star, being able to observe 6 planets transiting the same star means that the planetary system of Kepler-11 has to be remarkably flat where the orbital planes of all 6 planets have to be almost perfectly coplanar.

The planetary system of 6 transiting planets around the star Kepler-11 is an extraordinary and unprecedented discovery. The planets Kepler-11b, 11c, 11d, 11e, 11f and 11g have orbital periods of 10.30 days, 13.03 days, 22.69 days, 32.00 days, 46.69 days and 118.38 days respectively, sizes of 1.97 Earth-radii, 3.15 Earth-radii, 3.43 Earth-radii, 4.52 Earth-radii, 2.61 Earth-radii and 3.66 Earth-radii respectively and orbital distances of 13.6 million kilometres, 15.9 million kilometres, 23.8 million kilometres, 29.0 million kilometres, 37.4 million kilometres and 69.1 million kilometres respectively from their parent star Kepler-11. With so many transiting planets for a single star, observing more than one planet transiting the star at the same time is a rather frequent occurrence and on one occasion, the Kepler space telescope was observing three planets transiting the star Kepler-11 at the same time!

All the 6 planets around Kepler-11 have orbits that are almost perfectly circular and one of the most striking features is how close the orbits of the 5 inner planets are to one another. In fact, the inner 5 planets are all closer to their parent star than the planet Mercury is from our Sun and each of the 5 planets are not particularly small either as they have diameters ranging from two to over four times the diameter of the Earth. Dynamically, the inner 5 planets of the star Kepler-11 have one of the most densely packed configurations for any system of planetary orbits ever discovered. The 6th planet orbits significantly further from the star Kepler-11 than the inner 5 planets, but its orbit is still smaller than Venus’ orbit around our Sun!

The transits of a single planet around its host star are strictly periodic. However, for stars with more than one transiting planets, gravitational interactions among planets will cause the orbits of the individual planets to speed up and slow down by small amounts, leading to deviations from perfectly periodic transit timings. Such transit timing variations are strongest for stars with multiple transiting planets whose orbits are particularly close to one another, like in the case for the closely packed inner 5 transiting planets around the star Kepler-11. Hence, analysis of the transit timing variations have allowed the masses of the inner 5 planets around the star Kepler-11 to be estimated and this is the second time transit timing variation measurements have ever been employed to measure the masses of extrasolar planets.

Typically, the mass of an extrasolar planet is determined via radial velocity measurements by observing the Doppler shifts in the star’s spectral lines as the star wobbles back and forth periodically due to the gravitational tug of the orbiting planet. A large number of planets detected by Kepler will be very low mass planets and determining the masses of these planets by radial velocity measurements will not always be possible due to the incredibly small radial velocity amplitudes expected for such planets. Therefore, stars with multiple transiting planets offer a unique opportunity where any low mass transiting planets in such planetary systems can have their masses estimated from observations of transit timing variations which enables much smaller planetary masses to be measured as compared to radial velocity measurements. Knowledge about both the size and the mass of a planet allows the internal composition of the planet to be constrained.

The official paper detailing the release of the 1235 planetary candidates by the Kepler mission team is entitled “Characteristics of planetary candidates observed by Kepler, II - Analysis of the first four months of data” and it can be obtained from http://arxiv.org/abs/1102.0541. Additionally, the official paper on the 170 host stars with multiple transiting planets is entitled “Architecture and Dynamics of Kepler's Candidate Multiple Transiting Planet Systems” and it can be found at http://arxiv.org/abs/1102.0543v1. Finally, the paper confirming the planetary system of 6 transiting planets around the star Kepler-11 is entitled “A Closely-Packed System of Low-Mass, Low-Density Planets Transiting Kepler-11” and it can be found at http://arxiv.org/abs/1102.0291.

Galaxy Defined

2011-02-04T00:39:00.000+08:00

What constitutes a galaxy? Over the past several years, the discovery of a number of very small galaxies has motivated some astronomers to reconsider the definition of a galaxy since there is currently no widely accepted standard definition for a galaxy. The tiniest galaxies have masses and sizes that are known to overlap with the largest globular clusters. Basically, a globular cluster is a gravitationally bound spherical collection of stars which can contain anywhere from a few thousand to several million stars. In comparison, a large galaxy such as the Milky Way galaxy contains hundreds of billions of stars, measures approximately 100000 light years in diameter and has a total mass that is around one trillion times the mass of our Sun. There are about 150 or so globular clusters that are currently known to orbit the Milky Way galaxy.

In recent years, the discovery of objects called Ultra Compact Dwarfs has shown that these objects have properties which blur the distinction between the smallest galaxies and the largest globular clusters. Ultra Compact Dwarfs generally contain on the order of one million to a hundred million times the mass of our Sun and they typically have sizes of around several tens of light years across. In the Local Group, Omega Centauri is the largest globular cluster in the Milky Way galaxy while Mayall II is the largest globular cluster in the neighbouring Andromeda galaxy. Omega Centauri is estimated to contain a total of 5 million times the mass of our Sun and Mayall II is believed to be twice as massive as Omega Centauri. These are among the largest globular clusters known and they clearly show properties which overlap with Ultra Compact Dwarfs. It has also been suggested that Omega Centauri and Mayall II may even be the remaining cores of tidally disrupted dwarf galaxies. Furthermore, the smallest ultra-faint Dwarf Spheroidal Galaxies also have properties which overlap with the largest globular clusters.

A number of requirements are discussed here that can help in distinguishing a small dwarf galaxy from a large globular cluster. To start off, the minimum requirement for a galaxy is that it has to be gravitationally bound and it must contain stars. This means that streams of stars and material that were ejected from the collision of galaxies and masses of starless ‘dark galaxies’ cannot be classified as galaxies. However, these requirements will still include all globular clusters and Ultra Compact Dwarfs as galaxies. Hence, a number of additional requirements and their implications will be discussed to better distinguish objects that have properties which are intermediate between the largest globular clusters and the smallest galaxies.

A gravitationally bound stellar system that is in a dynamically stable state will have stars whose orbits are determined by the mean gravitational field of the system rather than by encounters between individual stars. If a galaxy is defined as a dynamically stable gravitationally bound stellar system with a two-body relaxation timescale that is longer than the age of the universe, then Ultra Compact Dwarfs can be classified as galaxies but not globular clusters. In gravitationally bound stellar systems with deep gravitational potential wells, gases enriched with heavy elements created by nucleosynthesis processes from one or more episodes of star formation will remain gravitationally bound to the system. These enriched gases can be available for subsequent episodes of star formation, leading to populations of stars with different abundances of heavier elements. If a galaxy is required to have multiple populations of stars, then almost all Ultra Compact Dwarfs and the most massive globular clusters can be classified as galaxies.

The Milky Way galaxy is embedded within a massive halo of non-baryonic dark matter. If the presence of a massive dark matter halo is required for a stellar system to be classified as a galaxy, then globular clusters, Ultra Compact Dwarfs and some Dwarf Spheroidal Galaxies cannot be classified as galaxies. A known characteristic of all large galaxies such as the Milky Way galaxy and the Andromeda galaxy is that they all have systems of globular clusters and smaller satellite galaxies in orbit around them. If a system of globular clusters is required, then Ultra Compact Dwarfs and some dwarf galaxies will be unable to meet this requirement to be classified as galaxies. Finally, size can also be a requirement for defining what constitutes a galaxy. Ultra Compact Dwarfs generally have sizes of up to just over 100 light years across, while the smallest ultra-faint Dwarf Spheroidal Galaxies have sizes that are as small as 300 light years across. If a minimum size of 300 light years is a requirement for a galaxy, then all Ultra Compact Dwarfs and globular clusters cannot be classified as galaxies.

Gravitational Recoil

2011-01-29T18:45:00.000+08:00

Supermassive black holes are the most massive type of black holes and they can contain between hundreds of thousands to billions of times the mass of our Sun. Almost all galaxies are known to harbour supermassive black holes within their cores. For example, the Milky Way Galaxy contains a supermassive black hole at its centre called Sagittarius A* and this behemoth packs around 4 million times the mass of our Sun. From time to time, galaxies are known to collide and eventually merge with one another. When two galaxies collide, the two supermassive black holes that are located in each of their galactic cores can eventually come close enough to each other to form a gravitationally bound binary pair.

As two supermassive black holes orbit around each other, the emission of gravitational waves will cause them to loose angular momentum and spiral towards each other, leading to an increasingly tighter orbit. The rate at which angular momentum is lost through the emission of gravitational waves increases dramatically as the two supermassive black holes get closer to each other, leading to a final inspiral that is followed by an eventual coalescence of the two supermassive black holes. During the final inspiral of the two supermassive black holes, the anisotropic emission of gravitational waves is able to impart a large gravitational wave recoil velocity to the final supermassive black hole. The final supermassive black hole can have a gravitational wave recoil velocity that is as large as 4000 kilometres per second.

A large gravitational wave recoil velocity can displace the supermassive black hole arbitrarily far from the core of its host galaxy, or even completely eject the supermassive black hole from its host galaxy if the gravitational wave recoil velocity is larger than the escape velocity of its host galaxy. In reality, the majority of coalescenced supermassive black holes will have gravitational wave recoil velocities that will be significantly less than the escape velocities of typical galaxies. Following a merger, the resultant supermassive black hole will be displaced at some maximum distance from the core of its host galaxy as a result of the gravitational wave recoil.

The supermassive black hole will oscillate a number of times through the core of its host galaxy with decaying amplitudes as it transfers its ‘excess’ kinetic energy gained from the gravitational wave recoil to the surrounding stars. This has the effect of reducing the density distribution of stars in the region of the host galaxy’s core that is near the supermassive black hole. The time required for the amplitude of the oscillatory motion of a ‘kicked’ supermassive black hole to eventually decay down to roughly the core radius of its host galaxy is on the order of a few hundred million years or less. For dwarf galaxies and globular clusters, the gravitational wave recoil velocities are more likely to completely eject most coalesced black holes from these systems due to their much lower escape velocities as compared to typical galaxies.

A supermassive black hole that is displaced from the core of its host galaxy is expected to carry with it an entourage of stars that remains gravitationally bound to the supermassive black hole as a densely packed cluster of stars. Such a cluster of stars is referred to as hypercompact stellar system and it differs from all the other types of star clusters by having an exceptionally high internal velocity distribution due to the deep gravitational potential well of the central supermassive black hole. The total mass of all the stars in a hypercompact stellar system is expected to be on the order of one percent of the mass of the supermassive black hole itself, making the cluster similar in size and luminosity to globular clusters. However, in extreme cases, the size and luminosity of a hypercompact stellar system can approach that of ultra-compact dwarf galaxies.

Although hypercompact stellar systems share similarities with globular clusters, they differ from globular clusters in two fundamental aspects which can help in the identification of these unique clusters. Firstly, hypercompact stellar systems have much higher velocity dispersions than globular clusters as the stars in hypercompact stellar systems have velocities that are on the order of a hundred to a thousand kilometres per second. This is comparable to the velocity imparted to a post-merger supermassive black hole from the gravitational wave recoil. Secondly, the stars in hypercompact stellar systems come from the cores of galaxies and they will exhibit higher metallicities than the stars in globular clusters. This is due to the multiple episodes of stellar evolution that occur in galaxies which enable subsequent generations of stars to be made up of a far higher proportion of elements that are heavier than hydrogen and helium as compared to the stars in globular clusters.

Fiery World

2011-01-20T21:54:00.001+08:00

Well over 500 extrasolar planets have been discovered so far and this number of set to increase ever more rapidly. A large fraction of these extrasolar planets are known to transit their host stars and transiting extrasolar planets allow much more to be known about them than would have been otherwise. An extrasolar planet which passes in front and transits its host star is also likely to pass behind and get occulted by its host star. The occultation of the planet as it goes behind its host star allows the total thermal emission and total reflected light from the planet to be measured, thereby allowing the temperature of the planet to be determined. A recently published paper entitled “Thermal Emission from WASP-33b, the Hottest Known Planet” describes observations of the thermal emission from an extrasolar planet called WASP-33b and this planet is the first known to orbit an A-type star.

WASP-33b orbits its host star at a distance of just 3.82 million kilometres, making it 15.2 times closer to its host star than the planet Mercury is from our Sun. At such a close distance, WASP-33b takes just 1.22 Earth days to complete one orbit around its host star. The host star of WASP-33b is a spectral class A5 star that has 1.44 times the diameter of the Sun, 1.50 times the mass of the Sun, almost 6 times the luminosity of the Sun and a surface temperature of 7430 degrees Kelvin. WASP-33b is expected to be tidally locked whereby one hemisphere is locked to perpetually face its host star while the other hemisphere faces away from its host star. The mass of WASP-33b is deduced to be less than 4.1 times the mass of Jupiter while the diameter of WASP-33b is estimated to be almost 1.5 times the diameter of Jupiter.

Assuming an albedo of zero and uniform heat redistribution to the night-side, the equilibrium temperature of WASP-33b is estimated to be around 2700 degrees Kelvin. In comparison, the planet WASP-12b which was previously the hottest known extrasolar planet has an estimated equilibrium temperature of 2500 degrees Kelvin. Furthermore, the hottest temperature measured for WASP-33b stands at a blistering 3466 degrees Kelvin, making it the hottest temperature ever recorded for an extrasolar planet. At this temperature, metals such as iron, gold and aluminium will not be able to condense into a liquid from their gaseous forms. This huge temperature suggests that the heat transport from the day-side of the planet to its night-side is inefficient and this enables a very high temperature to persist on the planet’s day-side.

The sub-stellar point on the tidally locked WASP-33b is basically a spot on the planet where its host star is always directly overhead. On this blazingly hot spot, the host star of WASP-33b will appear around ten thousand times brighter than our Sun would appear on a clear day on the Earth. The amount of irradiation received by WASP-33b is so enormous that the total amount of energy incident on an effective area of just one square meter on the sub-stellar point on WASP-33b for a period of one year will equal the amount of energy released in a 90 kiloton nuclear explosion.

Ringworld

2011-01-14T06:50:00.002+08:00

The Ringworld is a hoop-shape megastructure that is constructed around a star and its dimensions are so vast that it makes the physical sizes of entire planets seem insignificant in comparison. From afar, the Ringworld will appear as a circular ribbon-like structure which completely encircles a star. The Ringworld spins in order to generate artificial gravity from centrifugal forces to establish a habitable environment across its entire inner surface. In this article, I will be describing a hypothetical Ringworld which exists around a Sun-like star.

Spanning a diameter of 300 million kilometres, a circumference of 942.48 million kilometres and a width of 2 million kilometres, the Ringworld features a habitable Earth-like environment which spans an immense area of 1880 trillion square kilometres over its entire inner surface. This unimaginably huge area is almost 3.7 million times the total surface area of the Earth! This is like having 3.7 million Earths all mapped flat and joined edge to edge. With an average thickness of 40 kilometres and with an average density that is similar to the Earth’s crust, the Ringworld is estimated to have a total mass of about 33000 times the mass of the Earth or about 10 percent the mass of our Sun.

Rim walls that are over a thousand kilometres high are found along the edges at the inner surface of the Ringworld. These walls keep the atmosphere within the inner surface of the Ringworld by preventing the atmosphere from slipping off the edge into space. The Ringworld is so huge that journeying from one rim wall to the other will mean covering a distance that is equal to circling the Earth 50 times. Additionally, circumnavigating the entire Ringworld will mean covering a vastly greater distance that is equal to circling the Earth 23500 times. Travelling at a speed of one kilometre per second, it will take almost 30 years to completely circumnavigate the Ringworld!

The enormous size of the Ringworld allows equal scale maps of entire worlds to be reconstructed on its surface. If all the continents of the Earth were reconstructed to scale in one of the Ringworld’s many great oceans, the entire archipelago of continents will span only one percent the width of the Ringworld. To generate an Earth-like gravity on the inner surface of the Ringworld, the entire megastructure will have to spin at a rate of once every 777000 seconds. At this spin rate, the rim of the structure will be travelling at a speed of 1210 kilometres per second.

Interior to the Ringworld and located at a closer distance to the central sun, an enormous ring of equally spaced rectangular shades block the sun at regular intervals to provide a day-night cycle along the habitable inner surface of the Ringworld. During the day on the inner surface of the Ringworld, the sun always appears directly overhead and night arrives following an eclipse of the sun by one of the rectangular shades.

Between the Ringworld and its sun, a series of climate control shades are positioned into another ring which has the same spin rate as the Ringworld itself such that each climate control shade remains fixed over the same region of the Ringworld’s inner surface. These climate control shades block out various fractions of the Ringworld’s sun and create cold spots on the Ringworld’s inner surface which allow temperature gradients to be generated. These temperature gradients drive large scale atmospheric and oceanic circulations. Sufficient cooling by large enough climate control shades also lead to the creation of ice caps.

Orbital

2011-01-07T06:52:00.001+08:00

An Orbital is a rotating hoop-shaped megastructure which consists of an enormous band of material arranged in a ring with a diameter that is typically measured in millions of kilometres. The spin rate and diameter of the Orbital is made to simulate the length of day and surface gravity of a given planet along the entire inner surface of the megastructure. As the Orbital spins, the lithosphere, hydrosphere and atmosphere are held against the inner surface by centrifugal forces to maintain the desired type of ‘planetary’ environment. In this article, I will be describing an archetypical Orbital that is made to duplicate the same conditions as the Earth.

Credit: Al Brady

To create Earth-like conditions, the Orbital orbits the Sun at around the same distance as the Earth. The inner surface of the Orbital has the same sea level atmospheric pressure as the Earth, experiences a 24 hour day-night cycle and provides the same amount of gravitational acceleration as on the Earth’s surface. To do that, the Orbital will need to have a diameter of 3.71 million kilometres. Because the Orbital spins once every 24 hours, the velocity at its rim is 486000 kilometres per hour or 135 kilometres per second. An atmosphere is held against the inner surface of the Orbital by spin induced centrifugal forces. Walls that are over a hundred kilometres high line both rims at the inner surface of the Orbital. These immense walls along the rims maintain the atmosphere by preventing it from slipping off the edge into space.

Constructing the Orbital will be a challenge because there is still no known material strong enough to withstand the tremendous tensional stresses found within the structure of the Orbital. As always, this yet to be discovered material shall be termed unobtanium and it will be used in the construction of the stress carrying structure of the Orbital. The lithosphere, hydrosphere and atmosphere will all be constructed upon the inner surface of this unobtanium-based stress carrying structure. To construct the rim walls along the two edges of the inner surface of the Orbital, extremely low density and very high strength materials such as self-supporting diamondoid foam can be employed. Furthermore, self-supporting diamondoid foam can also be used to support the underlying contours of the topography.

The entire circumference of the Orbital along its inner surface measures 11.66 million kilometres and it takes light almost 40 seconds to travel that distance. Assuming that the Orbital has a width of 15000 kilometres, the total habitable area of the inner surface of the Orbital will be a staggering 175 billion square kilometres. An area like this is equivalent to 343 times the surface area of the whole Earth or 17800 times the surface area of the United States of America. Assuming an average thickness of 100 kilometres and an average material density that is less than water due to the possible presence of voids within the stress carrying structure, the entire Orbital will be about as massive as the Earth but with 343 times more habitable surface area per unit mass.

An observer standing on the inner surface of the Orbital will see a sky that is similar to that seen from the Earth’s surface as the atmosphere overhead is entirely open to space. However, unlike on the surface of the Earth, the observer will be able to see the approach of dawn and dusk along the Orbital. Furthermore, the observer on the inner surface of the Orbital will be constantly aware of an impressive sight where the world at two ends of the horizon will appear to curve upwards and eventually meet overhead at a great distance of 3.71 million kilometres away.

Nights on the inner surface of the Orbital will be spectacular as the ringlight reflected from the illuminated portion of the Orbital will appear thousands of times brighter than the full moon on Earth. Due to the large amount of reflected light, astronomers on the inner surface of the Orbital will have a great difficulty in trying to observe the nigh sky. However, the dark and pristine vacuum of space is never too far away as the astronomers can easily carryout their observations from the outer surface of the Orbital. At night, an amateur astronomer with an average backyard telescope will be able to distinguish large geological features and any great megalopolis located far away, on the other side of the Orbital.

To create the day and night cycles, the Orbital is tilted at an angle of 23.5 degrees with respect to its motion around the Sun. If the Orbital has no axial tilt with respect to its motion around the Sun, it will eclipse the Sun all of the time from the perspective of an observer on the inner surface of the Orbital. The tilt of the Orbital creates 2 warm seasons and 2 cool seasons each year. The middle of each warm season is marked by a midsummer eclipse of the Sun and the middle of each cool season is marked by the Sun’s lowest position in the sky. Unlike on the Earth, the length of daylight on an Orbital will not vary, resulting in no long summer days or long winter nights. If the orbit of the Orbital around the Sun has some eccentricity, it can cause one warm season to be warmer than the other and one cool season to be cooler than the other.

Regions located near the towering walls along the rims of the inner surface of the Orbital will experience a more pronounced seasonal variation. If the Orbital goes around the Sun in a circular orbit with negligible eccentricity, the region in the immediate vicinity of one rim wall will be in shadow for half a year while the region near the other rim wall will receive extra light reflected from the rim wall itself. This means that during first half of the year, the region located near one rim wall will be cooler than the rest of the Orbital and during the second half of the year, the region located near the same rim wall will be warmer than the rest of the Orbital. During the same period, the opposite is true for the region located near the other rim wall.

Overall, seasonal variations on the Orbital will be very slight in comparison to those that occur on the Earth. The Coriolis Effect will not be significant on the Orbital because the only form of Coriolis Effect is the rising and falling air within the troposphere where most of the weather occurs. With a thickness of around 10 kilometres or so, the depth of the troposphere is insignificant as compared to the 3.75 million kilometres diameter of the entire Orbital. Without major temperature gradients and without the Coriolis Effect, the weather on the habitable inner surface of the Orbital will be gentler and more localized than the weather on the Earth.

Without a moon, the tidal effects on the Orbital will be weaker than those on the Earth since the only form of tides on the Orbital will be those generated by the Sun. With no contrasting cold polar oceans and warm equatorial oceans like those found on the Earth, natural circulation between the surface waters and the deep ocean waters cannot be established. Without such a circulation system, the deep ocean waters will become anoxic. In order to maintain rich oxygen-bearing waters throughout the entire depth of the oceans, artificial heating can be applied to the deep ocean waters at specific locations on the ocean floor to keep the circulation running. Additionally, the underwater topology can be carefully sculptured to enable the mixing of surface water with deep ocean water from tidal effects alone.

Towering walls that are similar to those found on the rims or exceedingly high mountain ranges called ‘bulkhead ranges’ can be used to contain and completely isolated alien environments and ecosystems that are very different from the standard Earth-like environment of the Orbital. Entire pristine prehistoric worlds that are populated by once extinct creatures can also be completely enclosed within these immense barriers. Panoramic views of or from these ‘bulkhead ranges’ will be especially breathtaking as these mountains tower over a hundred kilometres above the surroundings. In comparison, Earth’s Mount Everest is only 8848 meters in height. On the immense scale of the Orbital, mountains that far surpass the height of Earth’s Mount Everest or Mars’ Olympus Mons will appear as mere tiny bumps on the incredibly vast landscape.

‘Bulkhead ranges’ and other mountains of comparable heights rise from the dense and warm lower reaches of the atmosphere and terminate at summits reaching high above the atmosphere into the silent vacuum of space. These mountains are rather interesting as they rise through the full extent of the troposphere, stratosphere and mesosphere. In fact, these mountains are so high that they rise well above the ozone layer and even above the high-flying noctilucent clouds. The summit environments of these mountains are basically bare rock exposed to the vacuum of space. In order to reduce the mass of material required for such mountains, the interior bulk of these mountains can be entirely made of advance self-supporting diamondoid foam or other forms of exotic materials that have very low densities and very high strengths.

Journeying to a distant part of the Orbital will be a challenge due to the sheer size of the Orbital. Even for someone cruising at a speed of 10 kilometres per second onboard a high speed vacuum tube maglev train, it will still take almost 2 weeks to circumnavigate the entire Orbital. Hence, travelling to far-off places on the Orbital will require technologies similar to those employed for large scale commercial interplanetary space travel. Furthermore, interplanetary space voyages disembarking from the Orbital will be much simpler as a spacecraft released from the outer surface of the Orbital will already be travelling at a speed of 135 kilometres per second.

Olympus Mons

2010-12-31T06:48:00.000+08:00

Olympus Mons is a large shield volcano mountain that is located on the planet Mars and it has a morphology similar to the large volcanoes that make up the Hawaiian Islands. Rising to a height of 22 kilometres above the surrounding plains or 21 kilometres above the standard topographic datum of Mars, Olympus Mons is the tallest mountain known in the Solar System. This makes Olympus Mons stand at just under three times the height of Mount Everest. The base of Olympus Mons measures over 600 kilometres across and the outer edge of the mountain is rimmed by an immense cliff which rises up to 8 kilometres above the surrounding terrain.

Due to the sheer size of Olympus Mons and from the fact that the average slope of the volcano’s flank is only 5 degrees, the entire vertical profile of Olympus Mons will not be visible to an observer who is standing at a great distance away on the surrounding plains as the curvature of the planet Mars would obscure the mountain’s summit. Similarly, an observer standing on the summit of Olympus Mons will be unable to view the surrounding plains as the slopes of the volcano would extend well beyond the horizon. However, the immense cliffs which surround almost the entire base of Olympus Mons will definitely make an impressive sight.

Olympus Mons is located on the northwestern edge of the Tharsis Bulge which also has some of the largest volcano mountains known in the Solar System. To the southeast of Olympus Mons are the mountains Arsia Mons, Pavonis Mons and Ascraeus Mons. Like Olympus Mons, these mountains are also immense shield volcanoes that rise to impressive heights, greatly dwarfing even the prominence of Mount Everest. The Tharsis Bulge, on which these colossal mountains are located, covers millions of square kilometres in area and the height of Everest’s summit is merely comparable to the surface elevation of the massive plateau above the standard topographic datum of Mars.

The extraordinary size of Olympus Mons is due to the fact that unlike the Earth, Mars does not have plate tectonics and this enables the crust of Mars to remain stationary over a hotspot. By doing so, magma coming out of the hotspot continuously builds the volcano in the same location and allows Olympus Mons to become so large. A unique observational aspect of Olympus Mons is that it is sufficiently high enough to penetrate above the frequent Martian dust storms that can occasionally be large enough to engulf the entire planet. This was the first observational hint of the incredible height of Olympus Mons, long before the first spacecraft arrived in orbit around Mars.

The atmospheric pressure on the top of Olympus Mons is about 70 Pascal and this is about 11 to 12 percent of the atmospheric pressure at the standard topographic datum of Mars which has a value of 610 Pascal. In comparison, the atmospheric pressure on the top of Mount Everest is about 31400 Pascal while the atmospheric pressure at sea level on the Earth is 101325 Pascal. To put this into an Earthly perspective, the atmospheric pressure on the top of Olympus Mons is like being at an altitude of 50.5 kilometres above sea level while the atmospheric pressure at the standard topographic datum of Mars is like being at an altitude of 34.5 kilometres above sea level.

Orographic clouds that are made up of particles of water ice have long been known to be associated with Olympus Mons and with the other great volcano mountains on Mars. These clouds form when air masses are forced from a lower elevation to a higher elevation as they move up the slopes of these great mountains. The air masses cool as they rise and the moisture content carried within them condenses into particles of water ice, forming orographic clouds.

Magnetar Flare

2010-12-27T23:32:00.000+08:00

A neutron star is a type of compact star that is formed from the gravitational collapse of the core of a massive star during a supernova explosion. A typical neutron star has a diameter of around 20 kilometres and a mass that generally exceeds the mass of our Sun. In comparison, our Sun has a diameter of 1.392 million kilometres. This incredibly compact configuration for a neutron star means that just a single cubic centimetre of its material packs a mass of around a billion metric tons! The extreme compactness of a typical neutron star also gives it a surface gravity that is over 100 billion times the surface gravity on the Earth and an escape velocity of around one third the speed of light!

A magnetar is an exceedingly rare type of neutron star which possesses an extremely powerfully magnetic field. In fact, the magnetic fields of magnetars are the strongest known in the universe as these magnetic fields have intensities on the order of between a billion to a trillion teslas. For comparison, the strength of the Earth’s magnetic field is about 30 microteslas while the strongest permanent magnets can generate magnetic fields of up 5 teslas. Magnetars are so rare that less than 15 of them are known. These exotic stars give rise to occasional burst of X-rays and gamma-rays, thus manifesting themselves as either Soft Gamma-ray Repeaters (SGRs) or Anomalous X-ray Pulsars (AXPs). SGRs are generally more energetic than AXPs and the bursting/flaring events from magnetars can be roughly classified into 3 types – short bursts, intermediate flares and giant flares. Giant flares are far more energetic than the short bursts and intermediate flares, and only 3 giant flares have been recorded in the decades of monitoring high energy astrophysical events since the 1970s.

SGR 1806-20 is a magnetar that is located around 50 thousand light years away, on the other side of the of the Milky Way galaxy. At this distance, it takes light 50 thousand years to travel from SGR 1806-20 to the Earth. The stellar neighbourhood of SGR 1806-20 contains some highly unusual stars, including one of the most massive and luminous star known in the Milky Way galaxy. What makes SGR 1806-20 unique is that this magnetar has the strongest magnetic field ever discovered for any object in the universe and this magnetar is also the progenitor for one of the 3 giant flares recorded so far. The strength of the magnetic field of SGR 1806-20 is estimated to be on the order of a whopping one trillion teslas!

On Monday 27 December 2004, an extremely energetic giant flare was detected from SGR 1806-20. This giant flare was so energetic that it saturated all but the least sensitive particle detectors regardless of where the detectors were pointed and this event became the brightest blast of gamma-rays ever detected from an astrophysical source. The giant flare from SGR 1806-20 is estimated to have released more than 2000 trillion trillion trillion joules of energy in the form of X-rays and gamma-rays. Almost all of the energy released from the giant flare was concentrated in an initial hard spike that lasted for around 0.2 seconds. This initial hard spike was then followed by a gradually decaying pulsating tail which shows about 50 cycles of high-amplitude pulsations over the duration of around 600 seconds. The high-amplitude pulsations show a period of 7.5 seconds and this period matches the rotational period of SGR 1806-20.

To place the amount of energy generated by the giant flare from SGR 1806-20 into perspective, the amount of power produced during the initial hard spike which lasted for around 0.2 seconds is on the order of a thousand times the combined luminosity of all the hundred of billions of stars in the Milky Way galaxy! In fact, the amount of energy produced during the 0.2 seconds of the initial hard spike is greater than the total amount of energy generated by our Sun over a period of 100 thousand years! Already, the amount of energy produced by our Sun in a single second is almost a million times the total worldwide energy consumption in 2009! The giant flare from SGR 1806-20 was so bright that even its echo off our Moon was detectable. Interestingly, if all the energy were converted into visible light, the giant flare would have been brighter than the full Moon during the 0.2 seconds duration of the initial hard spike! If the giant flare from SGR 1806-20 had occurred at a distance of 10 light years from the Earth, it will be similar to standing at a distance of 7.5 kilometres from a 15 kiloton nuclear explosion.

The giant flare detected on 27 December 2004 from SGR 1806-20 is hundreds of times more energetic than the two other known giant flare events. The release of such an immense amount of energy within such a short period of time managed to eject a significant amount of matter from the magnetar. The highly energetic ejecta formed an outflow which interacted with the external interstellar medium and produced a radio afterglow this is at least 500 times more luminous than the only other radio afterglow detected from a giant flare. Finally, it may be possible for ultra-high energy cosmic rays from the giant flare to be detected years after the event form the direction of SGR 1806-20, provided that the deflection of the ultra-high energy cosmic rays by galactic magnetic fields is not too large.

Dark Galaxy

2010-12-24T23:49:00.000+08:00

VIRGOHI 21 is the name given to an intriguing object that is located approximately 50 million light years away in the Virgo Cluster. The Virgo Cluster is a cluster consisting of between one to two thousand member galaxies. VIRGOHI 21 was discovered through radio telescope observations of the 21 centimeter wavelength radio emissions from its neutral hydrogen content. The total mass of hydrogen in VIRGOHI 21 is estimated to be around 100 million times the mass of our Sun. Observations of the motion of hydrogen gas within VIRGOHI 21 shows that the hydrogen gas is moving far too rapidly to be explained by the gravity from just the mass of the detected hydrogen alone. In fact, the total mass of VIRGOHI 21 is inferred to be as large as 100 billion times the mass of our Sun!

Deep observations by the Hubble Space Telescope revealed no optical counterpart to VIRGOHI 21 and this makes VIRGOHI 21 an excellent candidate for a dark galaxy since it has a mass of a galaxy but is entirely devoid of stars. Almost all of the mass which makes up VIRGOHI 21 is expected to be in the form of dark matter and less than a fraction of a percent of its mass is ordinary matter. Dark matter is basically matter whose existence can only be inferred from its gravitational effects due to the fact that dark matter does not scatter nor emit electromagnetic radiation. Interestingly, a paper entitled “Tidal Debris from High-Velocity Collisions as Fake Dark Galaxies: A Numerical Model of VIRGOHI 21” suggests that VIRGOHI 21 may not be a genuine dark galaxy and instead, it could be the result of a high-speed collision between two large galaxies.

Located half a million light years from VIRGOHI 21 is a large spiral galaxy called NGC 4254 and a filamentary structure of hydrogen gas connects VIRGOHI 21 with NGC 4254. This trail of hydrogen gas is almost devoid of stars and its velocity distribution is coherent with the outer disk of the spiral galaxy NGC 4254 to which it is morphologically connected. Furthermore, a tidal origin for this trail of hydrogen gas is unlikely since a counter trail is nonexistent in the opposite direction from the spiral galaxy NGC 4254. Instead, such a feature is consistent with a high speed collision between the spiral galaxy NGC 4254 and another galaxy since an event like this will cause little disturbance to the stars in the main disk of the spiral galaxy NGC 4254, resulting in the lack of stars in the trail of hydrogen gas that connects VIRGOHI 21 with NGC 4254. A high speed collision with another galaxy will also create a counter trail of hydrogen gas that is much fainter and shorter than the main trail. This counter trail will quickly fall back into the disk of the parent spiral galaxy and in a few hundred million years after the collision, a galaxy with just one trail of hydrogen gas will be observed.

The interloper galaxy which collided with the spiral galaxy NGC 4254 is probably a few million light years away by now since the collision is expected to occur at a velocity on the order of a thousand kilometers per second and it is estimated that a couple of billion years would have already elapsed since the collision. VIRGOHI 21 is located along the trail of hydrogen gas and the velocity distribution within VIRGOHI 21 differs remarkably from the rest of the trail. This can occur when denser parts of the trail contract and become self-gravitating. Eventually, a region like this can become an independent object with the mass of a dwarf galaxy, resulting in an object like VIRGOHI 21.

Observing the composition of the filamentary structure which connects VIRGOHI 21 to the spiral galaxy NGC 4254 can provide further evidence to proof if VIRGOHI 21 is a genuine dark galaxy or if it originated from a high speed collision between two galaxies. This is due to the assumption that genuine dark galaxies will be made up of pristine metal-poor gases as there will be no stars to fuse the hydrogen and helium into heavier elements. On the other hand, if VIRGOHI 21 formed out of matter spewed out from the spiral galaxy NGC 4254 after a high speed collision with another galaxy, VIRGOHI 21 will be observed to be enriched with elements heavier than hydrogen and helium from the many episodes of stellar fusion prevalent in the main stellar disk of NGC 4254. In conclusion, a high speed collision could provide an explanation for the origin of putative dark galaxies such as VIRGOHI 21.

Consuming Worlds

2010-12-16T23:24:00.000+08:00

Red giant stars have diameters of around tens to hundreds of times larger than that of the Sun and they occur when stars like the Sun eventually exhaust the supply of hydrogen in their cores and switched to fusing hydrogen in a shell external to the core. The increased temperatures and reaction rates causes the star to expand into a red giant and as the star expands, it spins down due to the conservation of angular momentum. Therefore, red giant stars are expected to rotate much more slowly about their spin axis as compared to stars like the Sun.

An unusual class of red giant stars known as red giant rapid rotators are basically red giant stars that are known to spin much faster than what is predicted for them. Ordinary red giant stars have equatorial velocities of around 2 kilometres per second while red giant rapid rotators have equatorial velocities of around 10 kilometres per second or more. It has been suggested in a recently published paper entitled “The Fate of Exoplanets and the Red Giant Rapid Rotator Connection” that as a red giant star expands; it can consume and accrete planets that happen to be orbiting in close vicinity. Planets accreted in this way can dump sufficient angular momentum into the red giant star and cause the star to spin up to become a red giant rapid rotator.

This mechanism of accreting planets only works for planets whose orbital periods are shorter than the rotational period of their host stars. In other words, the time it takes for the planet to orbit once around its star has to be shorter than the times it takes for the star to complete one rotation about its spin axis. In such a configuration, the tidal bulge raised on the star by the orbiting planet will always be trailing the planet and this allows angular momentum to be transferred from the orbiting planet to the rotation of the star. This causes the planet to lose orbital angular momentum, fall closer towards its host star and eventually getting accreted by the star.

The amount of angular momentum that is dumped into a red giant star by an accreted planet can be many times greater than the angular momentum of the star itself. In our solar system, the Sun holds less than 2 percent of the total angular momentum while the planet Jupiter holds 60 percent of the total angular momentum. However, the orbit of Jupiter is too distant for it to get consumed by the Sun when the Sun expands into a red giant star billions of years from now.

Most of the 500 or so extrasolar planets known to date are Jupiter-like planets which orbit very close to their parent stars, many of which have orbital periods in the range of a few days. These planets are termed hot-Jupiters and they form a large proportion of the currently known planets due to observational biases as these planets are the easiest to detect. Such a hot-Jupiter can dump a huge amount of angular momentum into its host star via accretion when the star expands into a red giant, turning the red giant into a rapid rotator. For example, a Jupiter-mass planet in a Mercury-like orbit around a star that is identical to our Sun will have about 10 times more angular momentum than the star itself.

The lithium abundance of a red giant rapid rotator can also provide further evidence to correlate it with accreted planets. Red giant stars are known to be depleted in lithium due to convective mixing and the accretion of a Jupiter-mass planet can significantly raise the lithium abundance of the red giant star. However, a better understanding of stellar evolution is still required to ensure that any observed lithium abundance or any other observed abundance anomalies are indeed anomalous for a given red giant rapid rotator such that it can be attributed to an accreted planet.

Historic Flight

2010-12-10T21:58:00.001+08:00

SpaceX successfully launched its Dragon spacecraft into low-Earth orbit atop a Falcon 9 rocket on Wednesday, 8 December 2010 at 10:43 AM EST (4:43 PM UTC) from Launch Complex 40 at Cape Canaveral Air Force Station in Florida. The Falcon 9 rocket inserted the Dragon spacecraft into an orbit with a low point of 288 kilometers, a high point of 301 kilometers and an orbital inclination of 34.53 degrees. This orbit is remarkably close to the targeted orbit which called for an almost circular orbit 300 kilometers above the Earth’s surface with an orbital inclination of 34.5 degrees. Traveling at a velocity of nearly 28000 kilometers per hour, the Dragon spacecraft made almost two orbits around the Earth before reentering the Earth’s atmosphere and eventually landing on the surface of the Pacific Ocean at 3 hours and 19 minutes after liftoff.

This launch event marks the first time in history a commercial company has successfully recovered a spacecraft reentering from low-Earth orbit. Such a feat has been performed by only six nations or government agencies: the United States, Russia, China, Japan, India and the European Space Agency. Wednesday’s launch of the Dragon spacecraft marks a historic first for the future of space travel. No one was on onboard the Dragon spacecraft on its maiden flight even though the spacecraft has enough room for 7 astronauts. The entire mission from launch to splashdown in the Pacific Ocean was flawless and if there had been people in the Dragon spacecraft, they would have enjoyed the whole ride.

Timeline of Events:

T+00:00:00 – Liftoff

T+0:02:58 - 1st Stage Shut Down (Main Engine Cut Off)

T+0:03:02 - 1st Stage Separates

T+0:03:09 - 2nd Stage Engine Start

T+0:09:00 - 2nd Stage Engine Cutoff

T+0:09:35 - Dragon Spacecraft Separates from Falcon 9

T+0:13 - On-Orbit Operations

T+2:32 - Deorbit Burn Begins

T+2:38 - Deorbit Burn Ends

T+2:58 - Reentry Phase Begins (Entry Interface)

T+3:09 - Drogue Chute Deploys

T+3:10 - Main Chute Deploys

T+3:19 - Water Landing

As the Dragon spacecraft reenters the Earth’s atmosphere at a velocity of over 7 kilometers per second, the spacecraft experiences temperatures of around 2000 degrees Centigrade. To keep the interior of the spacecraft at room temperatures, against the ferocious heating during reentry, SpaceX worked with NASA to create a phenolic impregnated carbon ablator (PICA) heat shield called PICA-X. This heat shield is probably the most advanced heat shield ever to fly as it can be reused hundreds of times with little degradation, somewhat like an “on steroids” version of a Formula One racing car’s carbon brake pads.

Dragon is a reusable spacecraft that was developed by SpaceX under NASA’s Commercial Orbital Transportation Services (COTS) program and it was initially conceptualized by SpaceX in 2005. The Dragon spacecraft is made up of a pressurized capsule and an unpressurized trunk for the transportation of pressurized cargo, unpressurized cargo and/or crew members to low-Earth orbit. Basically, the Dragon spacecraft has 10 cubic meters of pressurized volume, 14 cubic meters of unpressurized volume and it can support up to 7 passengers in crew configuration. The crew and cargo versions of the Dragon spacecraft are designed to be nearly identical to facilitate a rapid transition between cargo and crew.

SpaceX’s Falcon 9 rocket was used to launch the Dragon spacecraft into space on this historic voyage. The Falcon 9 rocket is basically a two stage launch vehicle that is powered by liquid oxygen and rocket grade kerosene (RP-1). The first stage rocket booster is powered by nine Merlin 1C rocket engines which generate a combined thrust of 5 million Newton at liftoff, while the second stage rocket booster is powered by a single Merlin Vacuum rocket engine which generates a thrust of 411 thousand Newton in a vacuum. The Falcon 9 rocket can launch over 10 metric tons into low-Earth orbit and a yet to be launched heavy variant called the Falcon 9 Heavy can launch almost 30 tons into low-Earth orbit.

SpaceX is developing a family of launch vehicles and spacecraft that will increase reliability and performance of space transportation, while ultimately reducing costs by a factor of ten. Next year, the Falcon 9 rocket and the Dragon spacecraft will start delivering cargo, including live plants and animals to and from the International Space Station for NASA. Both the Falcon 9 rocket and the Dragon spacecraft were developed to one day carry astronauts.

SpaceX has also revealed plans for future rocket designs, namely the Falcon X, Falcon X Heavy and Falcon XX. All these launch vehicle designs are in the heavy-lift to super heavy-lift range. The Falcon X can deliver up to 38 metric tons to low-Earth orbit while the Falcon X Heavy can deliver up to 125 metric tons to low-Earth orbit. Finally, the Falcon XX is a behemoth which can deliver up to 140 metric tons to low-Earth orbit. If developed, SpaceX’s Falcon X Heavy and Falcon XX will be among the largest and most powerful rockets ever built, with the long retired legendary Saturn V rocket being the closest rival. For comparison, NASA’s Space Shuttle can deliver 24 metric tons to low-Earth orbit while the Saturn V rocket can deliver a massive 120 metric tons to low-Earth orbit.

Dark Matter

2010-12-04T20:19:00.000+08:00

The existence of dark matter in the universe can only be inferred from its gravitational effects on ordinary matter and electromagnetic radiation as dark matter can neither emit nor scatter electromagnetic radiation. Dark matter constitutes 80 percent of the matter in the universe while ordinary matter makes up the remaining 20 percent. Ordinary matter is basically everything which makes up the Earth, the planets, the stars and the vast quantities of gas and dust across interstellar and intergalactic space.

Although ordinary matter can account for a tiny proportion of dark matter, the vast majority of dark matter is made up of something else entirely. While the properties of dark matter can be somewhat constrained, the particle constituents of dark matter continue to elude detection. Dark matter is not made up of atoms and it does not interact with ordinary matter via electromagnetic forces. Hence, the study of dark matter has so far been largely based on the observable gravitational effects that dark matter imposes on ordinary matter and on electromagnetic radiation.

There are a number of independent sources of evidence for the existence of dark matter. Stars are known to orbit around the centre of galaxies and their orbital speeds do not decrease with increasing distance from the galactic centre. This is rather unexpected because the galaxy must have much more mass than can be attributed to ordinary matter alone; otherwise the orbital speeds of stars should decrease with increasing distance from the galactic centre. Thus, dark matter can make up the mass that can’t be attributed to ordinary matter alone.

Gravitational lensing is another independent piece of evidence for the existence of dark matter and this phenomenon occurs when light from a background object gets deflected by the gravitational field of a foreground object. This can distort the image of the background object and also change its observed brightness. A more massive foreground object will create a more pronounced gravitational lensing effect. Observations of gravitational lensing by foreground galaxies have shown that the amount of mass required by the galaxy to generate the observed lensing far exceeds the combined mass of all its stars.

Not long ago, I came across a paper entitled “Planet-Bound Dark Matter and the Internal Heat of Uranus, Neptune, and Hot-Jupiter Exoplanets” by Stephen L. Adler from the Institute for Advanced Study at Princeton. This paper explores the possibility that the accretion of planet-bound dark matter by gas giant planets could significantly contribute to their internal heat.

The Milky Way galaxy is situated at the centre of a vast halo of dark matter. The dark matter in the vicinity of the solar system is believed to be distributed in a way such that a cubic volume of space measuring 10000 by 10000 by 10000 kilometres contains around 500 grams of dark matter. Although this may seem sparse, scaling up the volume of space to one cubic light year will give a mass of dark matter this is around 80 times the mass of the Earth. One light year is the distance light travels in a period of one year. The dark matter in the vicinity of the solar system orbits around the galactic centre of mass along with the solar system.

It is not known if there is dark matter that is gravitationally bound to the Sun or to the planets in the solar system. Gravitational conglomerations such as stars and planets can accrete the ambient galactic dark matter over time such that Sun-bound and planet-bound dark matter can have densities that are many orders of magnitude greater than the ambient density of dark matter.

Planet-bound dark matter can contribute to internal heating of the planet by depositing energy inside the planet as the dark matter particles lose orbital energy by interacting with the particles of ordinary matter that make up the planet. Planet-bound dark matter can also contribute to internal heating if the particles that comprise them are self-annihilating. When self-annihilating dark matter particles meet, they annihilate each other and convert their mass into energy which can be deposited within the planet as internal heating.

Interestingly, the contribution to internal heating of a planet by the accretion of dark matter can explain the anomalously low rate of internal heat production for Uranus as compared to Neptune. Uranus has an almost identical internal structure and composition as Neptune and it should be producing the same amount of internal heating as Neptune. However, one key difference between Uranus and Neptune is that Uranus is tiled 98 degrees with respect to the plane of the solar system, whereas Neptune is only tiled 28 degrees. For comparison, the Earth has an axial tilt of 23.4 degrees.

The large axial tilt of Uranus is believed to be caused by a massive impact event, whereby an object around the mass of the Earth slammed into Uranus. This impact event would have pushed Uranus out of its accreted planet-bound cloud of dark matter and leave it with a much lower rate of internal heat production than Neptune. Before the impact event, Uranus would have a similar rate of internal heat production as Neptune.

Faraway Sedna

2010-11-21T23:53:00.001+08:00

It is amazing to see how much our view of the solar system has changed over the past few years. Once upon a time, the solar system was known to be just a system of several planets in neat orbits around the Sun, together with a population of asteroids and comets. Back then, not much was known to exist beyond Pluto and the solar system seemed to be a simple place to be in.

Today however, the solar system is far from being a simple place as new objects are frequently being discovered as far out as Pluto and beyond. In fact, Pluto is far from being at the edge of the solar system as a huge number of newly discovered worlds are known to exist far beyond Pluto. Many of these newfound worlds rival Pluto in size and in 2005, a newly discovered object named Eris is found to be more massive and probably larger in size than Pluto.

This population of objects that orbit the Sun beyond Neptune are knows as Trans-Neptunian objects (TNOs) and they include objects such as Pluto and Eris. Several TNOs are known to be over 1000 kilometers in diameter and many more of such large TNOs are yet to be discovered. Since the discovery of Pluto in the 1930s, one of the most intriguing discoveries of a TNO was of an object named Sedna in 2003.

What makes the discovery of Sedna so interesting is its extremely elongated and far-flung orbit that is unlike any other TNOs. Sedna’s orbit brings it as close as 76 AU from the Sun out to as far as 960 AU from the Sun and it takes Sedna around 12 thousand years to complete one orbit around the Sun. An AU is a unit of measurement and one AU is basically the mean distance of the Earth from the Sun. When Sedna was discovered in 2003, it was located at a distance of 90 AU from the Sun and approaching perihelion. At its furthest distance of 960 AU, the Sun will appear as a point of light with less than half the brightness of the full moon.

Sedna is so distant that it never comes close enough to Neptune for it to be gravitationally scattered by Neptune into its current highly elongated orbit. In fact, the Earth comes closer to Neptune than Sedna ever does! Since its discovery in 2003, the answer as to how Sedna got kicked into its crazily elongated orbit is still not yet known, making it probably the only known object in the solar system whose orbit cannot be explained. Is something lurking in the outer parts of the solar system that could account for Sedna’s orbit?

Sedna could not have formed in it current orbit since the large relative velocities between planetesimals would have been disruptive rather than constructive. Hence, Sedna’s initial orbit must have been circular otherwise its formation by the accretion of planetesimals would not have been possible. A number of possibilities have been thrown in that might explain Sedna’s intriguing orbit.

The first possibility is that there is a large Earth-sized planet orbiting the Sun beyond Neptune that could have gravitationally scattered Sedna into its current orbit. This hypothesis might be a long shot because any Earth-sized planet located within 100 AU would have been easily detected, especially from its gravitational interactions with other TNOs. However, such a planet might once exist but may have been ejected from the solar system after the formation of the Inner Oort Cloud. The ejection of this planet would not substantially modify the orbits of the objects that have been scattered into Sedna-like orbits.

The second possibility that might explain Sedna’s odd orbit is a chance close encounter with a passing star. Such a star would have to come as close as 200 AU to 1000 AU from the Sun in order to excite TNOs into Sedna-like orbits. An encounter like this would have been “extremely close” give that the closest stars are already a few hundred thousand AU distant. In fact, the probability for such a close encounter in the past 4.5 billion years of the solar system’s history is around 1 percent. This is probably not good odds to base a theory on. For the second possibility, it can also be that Sedna once orbited a brown dwarf or a low mass star, and it was stripped from its parent star when it came too close to the Sun.

The third possibility, which is also the most likely one, assumes that the Sun was formed on a dense cluster of stars and perturbations from numerous neighboring stars gradually excited Sedna into its current elongated orbit. The view from inside one of these clusters would have been an incredibly awesome sight. After 4.5 billion years, the stars that once formed this cluster would have been long lost amongst the hundreds of billions of stars in the vast Milky Way galaxy. If this third possibility is true, then Sedna could serve as a “fossil record” of what happened during the Sun’s birth 4.5 billion years ago!

With two-thirds the diameter of Pluto, far-flung Sedna is already an interesting world in its own right. However remote the possibility may be, the thought that Sedna once orbited another star is rather fascinating because that will make Sedna the first known extra-solar dwarf planet in the solar system. What is Sedna trying to tell us? With just a single object, there will be no way of finding out and the next practical step will be to continue to search the skies for more objects like Sedna. All these explain why I personally think that Sedna is the most interesting TNO discovered so far since the discovery of Pluto.

Measuring Worlds

2010-11-13T07:22:00.003+08:00

An extrasolar planet is a planet which orbits a star other than the Sun and from the Paris Observatory’s online Extrasolar Planets Encyclopedia, there are about 500 known extrasolar planets as of November 2010. This number is expected to increase dramatically in the next several months with follow-up observations of the hundreds of candidate transiting extrasolar planet released in the first data set by NASA’s Kepler space observatory - a ‘planet hunting’ space telescope. A transiting extrasolar planet is one which periodically blocks a small fraction of the light from its parent star as its orbit happens to bring it in front of the star.

As the number of known extrasolar planets continues to increase rapidly, it undoubtedly brings up the possibility of detecting moons orbiting around these extrasolar planets. Detecting moons around extrasolar planets will be very challenging since such objects are expected to be smaller and less massive than the Earth. However, NASA’s Kepler space observatory might have the sensitivity necessary to detect the largest of such moons around extrasolar planets. Moons around extrasolar giant planets that are close to the size of the Earth can be particularly interesting because a large number of extrasolar giant planets are know to orbit their parent stars at ‘comfortable’ distances where Earth-like surface conditions are possible on such moons!

Recently, I did some research on transit timing variations (TTV) and transit duration variations (TDV) caused by the presence of a planet’s moon perturbing the periodic transit of the planet in front of its parent star. I used the methods outlined in two papers published by David M. Kipping in 2008 and in 2009 respectively, and wrote a program which allows me to play around with the parameters. I used stars, planets and moons of different masses in various combinations and orbital configurations. Additionally, I also used various TTV and TDV inputs to determine the corresponding mass of the moon and the corresponding planet-moon orbital configuration that is responsible the various signals.

In one of my analysis, I have a star with the mass of our Sun and a planet with the mass of the Earth which orbits the star at a mean distance of 100 million kilometers. This planet has a moon that is one-twelfth its mass and the moon orbits the planet at a mean distance of 130000 kilometers. It is also assumed that the planet takes 40000 seconds to transit in front of its parent star. As the periodic transits of the planet in front of its parent star is measured, the moon will induce an observed TTV of around 20 seconds and a TDV of around 35 seconds.

In light of a recently published paper entitled “How to Weigh a Star Using a Moon”, I wrote a separate program to study the methods outlined in this paper. Basically, if a star has a planet, and if that planet has a moon, and if both of them transit in front of their parent star, then the sizes and masses of the star, planet and moon can be precisely measured. Furthermore, knowing the size and mass of an object allows its bulk composition to be constrained. This particular method employs the TTV and TDV signals, and it requires a star to have both a planet and moon that transit it. Although no star is yet know to have both a planet and moon that transit it, NASA’s Kepler space observatory is expected to discover several of such systems.

This method of measuring the mass of a moon of an extrasolar planet is rather interesting because such a moon is likely to be less massive than the Earth and the mass of such an object will not be measurable with radial velocity measurements. Therefore, a method like this offers a means to accurately pin down the masses and sizes of the star, planet and moon respectively. The masses of moons measured in this way could well be the smallest masses that can be directly measured outside of our Solar System!

Interstellar Traverse

2010-10-21T14:34:00.001+08:00

The desire to reach for the sky runs deep in our human psyche.

- Cesar Pelli

Interstellar space travel refers to unmanned or manned travel to the stars and it is vastly more difficult that interplanetary space travel as the distances involved are many orders of magnitude greater, even for the nearest stars. The distances to the stars are so immense that a light-year is employed as the unit of measurement, where one light-year is the distance a beam of light travels in one year and it has a value of 9.46 trillion kilometers.

Alpha Centauri is one of the closest stars and it is already located at a distance of 4.37 light-years or 41.34 trillion kilometers away from us. To put this impressive distance into perspective, 41.34 trillion kilometers is over a billion times the circumference of the Earth, or over 100 million times the distance of the Moon from the Earth. Even traveling at a velocity of 100 kilometers per second, it will take over 13000 years to traverse that distance! Hence, in order to reach the nearest stars within a reasonable amount of time, a spacecraft will have to be accelerated to much larger velocities and this is where the immense difficulty of interstellar space travel arises.

If the total worldwide energy consumption in 2009 were used to accelerate a 10 ton spacecraft, it will only accelerate the spacecraft to a velocity of only 10 percent the speed of light and that spacecraft will still have to take over 4 decades to reach Alpha Centauri. Furthermore, upon reaching Alpha Centauri, the spacecraft will not be able to spend any meaningful amount of time at its destination since it will simply speed past Alpha Centauri at 10 percent the speed of light unless a similar amount of energy is employed to decelerate the spacecraft.

In this article, I will assume that the immense scientific and technological barriers of interstellar space travel have been crossed and the capability to accelerate to near the speed of light is possible. This possibility is enabled by having a propulsion system that can generate exhaust velocities at close to the speed of light and some hypothetical form of antimatter-based propulsion system can be a possible candidate. It should be noted that the speed of light in a vacuum is exactly 299792458 meters per second since one meter is officially defined as the distance traveled by light in a vacuum in 1/299792458 of a second.

To begin, I shall describe a set of equations that I developed not long ago which extends the classical rocket equations into the relativistic regime. In other words, the relativistic rocket equations that I have derived account for the effects of relativity as the rocket’s velocity approaches a significant fraction of the speed of light and such relativistic effects include time dilation and length contraction. Additionally, I have also written a program which employs the equations to compute the characteristic of various mission scenarios.

In almost all other literature that I have reviewed, a constant acceleration is assumed for the relativistic rocket equations. However, in the equations that I have derived, a constant proper thrust is assumed rather than a constant acceleration because in practice, it is more realistic for a rocket to maintain a constant thrust rather than having a varying thrust to maintain a constant acceleration. It should be noted that the thrust is constant from the perspective of an observer traveling together with the rocket. This observer will also experience a gradual increase in acceleration as the total proper mass of the rocket decreases due to the burning of propellant, while the thrust remains constant throughout.

Using the set of equations and the computer program that I have developed, I will start with an unmanned spacecraft that has a total initial mass of 1 million kilograms (one thousand metric tons). This spacecraft is in orbit around the Earth and it is poised for a one-way journey to the stars. Which destination should the spacecraft visit? Alpha Centauri? Tau Ceti? Sirius? In this mission, I shall choose the red dwarf star Gliese 581 as the interstellar destination for the spacecraft.

… to explore strange new worlds, to seek out new life and new civilizations, to boldly go where no one has gone before.

- Gene Roddenberry

Why Gliese 581? The reason is that Gliese 581 has a total of six known planets in orbit around it and in my previous post, I wrote about one of the planets which is the most Earth-like one discovered so far. This planet is designated Gliese 581 g and it orbits Gliese 581 at a comfortable distance where the temperatures are estimated to be just right to support Earth-like conditions. The star Gliese 581 is located 20.3 light years or 192 trillion kilometers away from us and the spacecraft will need to accelerate to close to the speed of light to get there within a reasonable period of time. Upon reaching Gliese 581, the spacecraft will also have to decelerate itself from its incredibly huge velocity so that it will not merely zip pass Gliese 581.

As stated previously, the spacecraft has a total initial mass of 1 million kilograms and most of which is in the form of fuel. The spacecraft also has a propulsion system which can generate an exhaust velocity that is 80 percent the speed of light. Furthermore, the spacecraft’s propulsion system is able to generate a constant 24 million Newton of thrust and this force is equivalent to approximately 7 times the weight of a Boeing 747 airliner. It is important to note that the mass of the spacecraft and the generated thrust is measured from the perspective of an ‘observer’ traveling with the spacecraft since the effect of relativity will give a different measured reading for an observer at rest.

To generate a constant 24 million Newton of thrust, the spacecraft will have to burn its propellant at a rate of 0.1 kilograms per second and direct the high energy exhaust out at 80 percent the speed of light. From rest, the spacecraft will accelerate at a constant thrust for a total duration of 7.5 million seconds (86.8 days) as measured from onboard the spacecraft. However, due to the effect of relativistic time dilation, 8.6 million seconds (99.6 days) would have already elapsed on Earth during the entire acceleration phase!

Initially, the spacecraft will experience an acceleration of 2.40 g’s which gradually increases to 9.59 g’s at the end of the acceleration phase because the proper mass of the spacecraft decreases while the thrust remains constant. At the end of the acceleration phase, the spacecraft will attain a final velocity of 240949550 meters per second which is slightly over 80 percent the speed of light. During the acceleration phase, the spacecraft would have already traveled over a trillion kilometers, or approximately one-tenth of a light year.

The spacecraft then shuts off its engine and begins its high speed cruise across the vast expanses of interstellar space, towards the direction of Gliese 581. It should be noted that the total mass of the spacecraft is now 250 thousand kilograms. Cruising at an incredible velocity of 240949550 meters per second, the spacecraft still has to take 25 years to get to Gliese 581! Additionally, the effect of relativistic time dilation means that only 15 years would have elapsed for a hypothetical observer onboard the spacecraft.

Upon reaching Gliese 581, the spacecraft will turn on its engine and commence its deceleration phase with the same constant thrust of 24 million Newton. The spacecraft will take 1.875 million seconds (21.7 days) to decelerate from 80 percent the speed of light so that it will be slow enough to enter orbit around Gliese 581. However, due to relativistic time dilation, 2.151 million seconds (24.9 days) would have elapsed back on Earth during the entire deceleration phase. Initially, the spacecraft will experience a deceleration of 9.59 g’s which gradually increases to 38.4 g’s at the end of the deceleration phase because the proper mass of the spacecraft decreases while the thrust remains constant. The spacecraft would have traveled another quarter of a trillion kilometers during the deceleration phase.

Orbiting around Gliese 581, the spacecraft now has a total mass of just 62.5 thousand kilograms as 93.75 percent of its initial mass is basically the propellant required for the journey. It is up to you to imagine the various kinds of payloads that can makeup the 62.5 metric tons of the spacecraft’s final mass. Due to the finite speed of light, the Earth will only receive the first signals from the spacecraft 20.3 years after the spacecraft has reached Gliese 581…

Now when we think that each of these stars is probably the centre of a solar system grander than our own, we cannot seriously take ourselves to be the only minds in it all.

- Percival Lowell

Resembling Earth

2010-10-08T08:31:00.001+08:00

For many planet hunters, though, the ultimate goal is still greater (or actually, smaller) prey: terrestrial planets, like Earth, circling a star like the Sun. Astronomers already know that three such planets orbit at least one pulsar. But planet hunters will not rest until they are in sight of a small blue world, warm and wet, in whose azure skies and upon whose wind-whipped oceans shines a bright yellow star like our own.

- Ken Croswell

Located 20 light-years or 190 trillion kilometers away is a humdrum red dwarf star called Gliese 581. As of October 2010, the star Gliese 581 has a total of six known planets in orbit around it and one of which is the most Earth-like planet discovered so far. This planet is designated Gliese 581 g and it is the fourth planet from its parent star. Gliese 581 g orbits its parents star at a distance of 22 million kilometers, taking 36.6 days to complete one orbit.

The orbit of the planet Gliese 581 g is located well within the habitable zone where the distance from its parent star is just right to support Earth-like surface temperatures! Thus, Gliese 581 g is located neither too close nor too far from its parent star. Since the star Gliese 581 is much less luminous that our Sun, the planet Gliese 581 g is able to support Earth-like surface temperatures even though it is located much closer to its parent star than our Earth is from the Sun.

In addition to orbiting its parent star within the “Goldilocks Zone”, the mass of Gliese 581 g is estimated to be between 3.1 to 4.3 times the mass of the Earth. If Gliese 581 g is a dense rocky planet like the Earth, its diameter will be somewhere between 1.3 to 1.5 times of the Earth’s diameter. The surface gravity of Gliese 581 g is also expected to be between 1.1 to 1.7 times the surface gravity of the Earth, making it not too different from the Earth. In fact, it will not be much of a problem for a human being to walk on the surface of Gliese 581 g.

With an Earth-like greenhouse effect, the average surface temperature of Gliese 581 g is estimated to be between 236 to 261 degrees Kelvin. In comparison, the average surface temperature of the Earth is 288 degrees Kelvin or 15 degrees Centigrade. However, because Gliese 581 g is more massive than the Earth, it is possible that the planet will have a more massive atmosphere which can create a larger greenhouse effect than Earth’s atmosphere. This can increase the average surface temperature of Gliese 581 g closer to the average surface temperature of the Earth.

One key difference between Gliese 581 g and the Earth is that Gliese 581 g is probably tidally locked whereby the same hemisphere of the planet perpetually faces its parent star. This is somewhat like the Earth-Moon system where the same side of the Moon always faces the Earth. In such a scenario, one side of Gliese 581 g will be in eternal daylight while the other side will experience eternal night. On such a world, temperatures can range from blazing hot at the sub stellar point on the day side to freezing cold on the night side. The sub stellar point on the surface of Gliese 581 g is where its parent star is forever directly overhead and it is the spot with maximum insolation. Between the two extremes, Earth-like temperatures can exist where a world that is not too different from ours is both easily conceivable and highly probable.

The mere fact that a potentially habitable planet has been discovered so soon around such a nearby star, suggests that habitable planets are far more common than previously believed. This means that potential of having many billions of Earth-like planets in our Milky Way galaxy alone is extremely probable! The paper detailing the discovery of this first ever potentially habitable planet is entitled “The Lick-Carnegie Exoplanet Survey: A 3.1 M Earth Planet in the Habitable Zone of the Nearby M3V Star Gliese 581” and it can be obtained from http://arxiv.org/abs/1009.5733v1.

We live in a changing universe, and few things are changing faster than our conception of it.

- Timothy Ferris

Of the 500 or so known extrasolar planets, Gliese 581 g is probably the most interesting one discovered so far. Apart from the remarkable discovery of Gliese 581 g, I’ve also read a paper entitled “A frozen super-Earth orbiting a star at the bottom of the Main Sequence” and it basically describes the discovery of a super-Earth which orbits a faint red dwarf star whose mass is close to the lower limit for a star.

This planet is designated MOA-2007-BLG-192Lb and its mass is 3.2 times the mass of the Earth. The planet orbits its parent star at a distance of about 100 million kilometers and this is about two-thirds the distance of the Earth from the Sun. However, because the parent star of MOA-2007-BLG-192Lb is a mere 8.4 percent the mass of the Sun, the planet receives over a thousand times less insolation that the Earth gets from the Sun even though it is located closer to its parent star than the Earth is from the Sun.

MOA-2007-BLG-192Lb was discovered using the gravitational microlensing technique as it provides a unique opportunity for the detection of low mass planets that are currently beyond the reach of most other methods. Gravitational microlensing occurs when a foreground star passes in front of a background star and the gravity of the foreground star acts as a lens and magnifies the apparent brightness of the background star. If the foreground star has a planet orbiting it, the gravity of the planet can induce a perturbation to the microlensing light curve. The duration of the perturbation depends on the mass of the planet, where a more massive planet will induce a perturbation with a longer duration. The discovery of the MOA-2007-BLG-192Lb shows that planet formation can occur down to the very low mass end of the stellar population.

The surface temperature of MOA-2007-BLG-192Lb is estimated to be around 55 degrees Kelvin and this is just below the melting temperature of pure nitrogen. However, internal heat generated from the decay of radioisotopes can raise the temperature on the surface of MOA-2007-BLG-192Lb to beyond the melting temperature of pure nitrogen. This can enable seas and oceans of liquid nitrogen to exist on the planet’s surface as long as the atmospheric pressure on the planet’s surface exceeds 0.1 bars.

The flow of internal heat onto the surface of a terrestrial planet can be strongly heterogeneous, making it highly probably that the surface temperatures on specific locations on MOA-2007-BLG-192Lb can exceed not just the melting point of nitrogen, but also the melting point of methane and even water! Therefore, lakes of liquid hydrocarbons like those on Saturn’s moon Titan can exist on MOA-2007-BLG-192Lb and open bodies of liquid water can exist in volcanically active locales.

Blazing Genesis

2010-09-17T15:48:00.003+08:00

Quarks are elementary particles and they are a fundamental constituent of matter. For instance, a neutron is made up of one up-quark and two down-quarks, and a proton is made up of two up-quarks and one down-quark. Neutrons and protons then make up the nuclei of atoms. Quarks have never been studied individually because when two quarks move apart, the force between them increases until it becomes more energetically favorable at some point for a new quark-antiquark pair to appear out of the vacuum than for the two quarks to continue separating. The phenomenon whereby quarks cannot be individually isolated is called color confinement and the phenomenon whereby quark-antiquark particles can appear out of the vacuum is called hadronization.

A quark star is a type of exotic star that is made up of ultra-dense quark matter and they are even denser than neutron stars. Given sufficient pressure from a neutron star’s immense gravity, individual neutrons can break down into their constituent quarks and a neutron star can turn into an even more compact quark star. A typical quark star has roughly the mass of the Sun packed into a diameter of only around 10 kilometers and just a cubic centimeter of its ultra-dense material can have a mass of a few billion metric tons!

Gamma-ray bursts are the most energetic electromagnetic events known to occur in the universe and they emit titanic bursts of gamma-rays which last anywhere from milliseconds to several minutes. Gamma-ray bursts are believed to be narrow bipolar beams of incredibly intense radiation created during powerful supernova explosions and a typical gamma-ray burst produces as much energy in a few seconds as the Sun does over its entire 10 billion years lifespan!

There are two types of gamma-ray bursts – the long duration gamma-ray bursts and the less common short duration gamma-ray bursts. Long duration gamma-ray bursts last longer than 2 seconds and they are generally linked to the deaths of very massive stars. Additionally, long duration gamma-ray bursts are followed by bright and lingering afterglows. On the other hand, short duration gamma-ray bursts last less than 2 seconds and they produce very little afterglows as compared to long duration gamma-ray bursts. The true nature of short duration gamma-ray bursts still remains an enigma and the leading hypothesis is that these events originate from the coalescence of binary neutron stars.

A gamma-ray burst is generally characterized by an initial powerful blast of gamma-rays followed by an afterglow with a rapidly decaying intensity. In this article, I will only focus on the afterglows of long duration gamma-ray bursts and the observed plateau in the light curves of a number of these gamma-ray burst afterglows. Such a plateauing of the afterglow light curve of a gamma-ray burst can be attributed to the cooling behavior of a newly formed quark star.

Immediately after a gamma-ray burst, the newly formed quark star cools by emitting vast amounts of neutrinos and photons. This initial afterglow phase is characterized by a light curve with a gradually decaying intensity. The light curve of the afterglow then stops decaying and plateaus out with a constant intensity. This observed phenomenon can be explained by the solidification of the quark star as it undergoes a phase transition from liquid to solid. The latent heat released during the phase transition can provide a steady and constant supply of energy to power the afterglow of the gamma-ray burst. This is because the temperature of the central quark star will remain constant as it undergoes its phase transition.

After the phase transition, the light curve of the afterglow abruptly decays due to the extremely low heat capacity of the solid quark star. The entire phase transition of the quark star from liquid to solid occurs over a timescale of roughly 1000 seconds and the amount of energy generated from the phase transition alone is roughly equal to the total amount of energy the Sun gives off over a period of 10 billion years!

Therefore, the amount of energy produced during the phase transition of a quark star is sufficient and steady enough to produce the plateau in the light curve observed in the afterglow of a gamma-ray burst. Gamma-ray bursts are the most powerful explosions in the universe and when they do occur, they blaze with the glory of a billion billion Suns. Nevertheless, as magnificent as they are, their fleeting nature makes them elusive and challenging to study.

Deep Center

2010-09-03T08:23:00.002+08:00

Our Sun is one of the hundreds of billions of stars in the Milky Way Galaxy and it is located approximately 26000 light years from the center of the galaxy. One light year is the distance light travels in a year and its value is 9.46 trillion kilometers. A supermassive black hole named Sagittarius A* sits right in the center of the Milky Way Galaxy and this colossal black hole is estimated to have a mass of a few million Suns!

Located within the close vicinity of Sagittarius A* is a very intriguing group of stars called the “S stars”. These stars are the closest known stars to the center of the Milky Way Galaxy and they orbit around Sagittarius A* at very high orbital velocities. Each of the “S stars” are several times more massive than our Sun and being much more massive than our Sun, these stars undergo a much more rapid rate of nuclear fusion which means that they have a lifespan of just several million years. In comparison, our Sun has a lifespan of over 10 billion years.

The “S stars” are intriguing because the environment around such a supermassive black hole is hostile to the formation of stars and the “S stars” would have to form somewhere much further out before migrating to their current extraordinarily close proximity to Sagittarius A*. However, the timescales involved in such a migration is longer than the short ages of the “S stars” and hence, these “S stars” constitute a “paradox of youth”.

S2 is the designation given to one of the “S stars” and what distinguishes S2 from the other stars is that S2 is by far the closest star in orbit around the supermassive black hole - Sagittarius A*. S2 orbits Sagittarius A* in a highly elliptical orbit and in such an extreme gravitational environment near a supermassive black hole, S2 takes just 15.5 years to complete one orbit around Sagittarius A* even though S2 is located at an average distance of about 140 billion kilometers from Sagittarius A*! In comparison, Pluto orbits the Sun once every 248 years at an average orbital distance of 6 billion kilometers from the Sun.

At its closest approach, S2 comes within just 17 light-hours from Sagittarius A* and this is roughly three times the distance of Pluto from the Sun. The highly elliptical orbit of S2 also brings it out as far as 10 light-days from Sagittarius A*. During closest approach, S2 zips around Sagittarius A* at a incredible velocity of over 5000 kilometers per second (about 2 percent the speed of light)! The remarkable orbit of S2 around Sagittarius A* makes it uniquely valuable for testing various general relativistic and even extra-dimensional effects!

Solar Probe

2010-08-20T06:20:00.001+08:00

The Sun is our nearest star and it has a diameter of 1.392 million kilometers and a surface temperature of 5778 degrees Kelvin. Well over a million Earths can fit within the volume of the Sun. The Earth orbits the Sun at a mean distance of 149.6 million kilometers with a mean orbital velocity of almost 30 kilometers per second. To match the amount of energy given off by the Sun in a single second, the total worldwide energy consumption in 2009 will have to be sustained for a period of almost a million years!

Deep beneath the surface of the Sun, enormous forces were gathering. At any moment, the energies of a million hydrogen bombs might burst forth in the awesome explosion…. Climbing at millions of miles per hour, an invisible fireball many times the size of Earth would leap from the Sun and head out across space.

- Arthur C. Clarke

NASA’s Solar Probe Plus is a proposed mission which will approach the Sun much closer than any other spacecraft and it is scheduled for launch sometime in the middle of the year 2015 using an Atlas V rocket. This will be the first ever spacecraft that will venture into the searing outer atmosphere of the Sun. At launch, Solar Probe Plus will have a total mass of 610 kilograms.

The 4 primary scientific objectives of Solar Probe Plus are:

1. Determine the structure and dynamics of the magnetic fields at the sources of both fast and slow solar wind.

2. Trace the flow of energy that heats the corona and accelerates the solar wind.

3. Determine what mechanisms accelerate and transport energetic particles.

4. Explore dusty plasma phenomena near the Sun and its influence on the solar wind and energetic particle formation.

Solar Probe Plus will approach as close as 8.5 solar radii from the surface of the Sun where one solar radius is basically the radius of the Sun. This translates to a distance of just 5.9 million kilometers from the Sun’s surface or 6.6 million kilometers from the center of the Sun. In comparison, the Earth orbits the Sun over 22 times further away, at a mean distance of 149.6 million kilometers. For that reason, when Solar Probe Plus is at its minimum distance from the Sun, the intensity of solar radiation that it will experience is over 500 times greater than at the Earth’s distance from the Sun. To put this into further perspective, at a distance of 8.5 solar radii from the surface of the Sun, the intensity of sunlight is a blazing 700 kilowatts per square meter, whereas at Earth’s mean distance from the Sun, the intensity of sunlight is just 1.37 kilowatts per square meter.

The planned nominal mission duration of Solar Probe Plus is 6 years and 321 days. During this period, the spacecraft will complete 24 elliptical orbits around the Sun where the minimum distance of the spacecraft from the center of the Sun will gradually decrease from 35 solar radii on the first orbit to 9.5 solar radii from the last 3 orbits onward. Solar Probe Plus does that by making 7 flybys of the planet Venus. These flybys enable Solar Probe Plus to make use of Venus’ gravity to decelerate and loose orbital energy in order for the spacecraft to gradually shrink its orbit around the Sun. It is highly likely that Solar Probe Plus will have its mission extended beyond the nominal 24 orbits around the Sun.

From the last 3 orbits onward, Solar Probe Plus will remain in the same final elliptical orbit around the Sun which will take it as close as 9.5 solar radii (6.6 million kilometers) from the center of the Sun out to as far as 109 million kilometers from the Sun. In this final orbit, Solar Probe Plus will go around the Sun once every 88 days and this means that the spacecraft will make closest approach to the Sun with an average frequency of 4 times per year.

At the minimum distance from the Sun in its final orbital configuration, Solar Probe Plus will be traveling at a speed of almost 200 kilometers per second with respect to the Sun! Throughout the nominal mission of 24 orbits around the Sun, the total amount of time Solar Probe Plus will spend within various distances from the Sun are - 2149 hours within 30 solar radii, 961 hours within 20 solar radii, 434 hours within 15 solar radii and 30 hours within 10 solar radii.

The extremely high temperatures and the harsh environment in the near vicinity of the Sun make cooling the spacecraft a vital and intricate challenge. The thermal protection system of the Solar Probe Plus consists primarily of a carbon-carbon composite heat shield with a diameter of 2.7 meters and a thickness of 17 centimeters. When the spacecraft is at its minimum distance of 8.5 solar radii from the surface of the Sun, the heat shield will be exposed to a total incident solar radiation flux of approximately 4 million watts which will cause the Sun-facing side of the heat shield to reach a temperature of around 1700 degrees Kelvin.

To keep the spacecraft within its safe operating temperature, the heat shield will significantly reduce the heat flux transmitted to the spacecraft to just 50 watts or 1/80000th of the original incident solar radiation flux. Additionally, the rest of the spacecraft will be concealed within the shadow created behind the heat shield when in close proximity to the Sun.

Solar Probe Plus will derive its power from two separate sets of solar arrays. The primary solar arrays will be employed when the spacecraft is at greater distances from the Sun and at close proximity to the Sun, the primary solar arrays will be folded within the shadow created behind the heat shield. Instead, a pair of much smaller secondary solar arrays will be employed when the spacecraft is near the Sun. This pair of secondary solar arrays will be liquid-cooled and when the spacecraft gets even closer to the Sun, the pair of secondary solar arrays will be partially retracted into the shadow created behind the heat shield. The spacecraft also utilizes a lithium-ion battery as its secondary power source.

Solar Probe Plus will rely on a lot of the heat-resistant technologies that have been developed for NASA’s MESSENGER spacecraft which is currently on a mission to study the planet Mercury. NASA’s Solar Probe Plus will be a historic and challenging mission which will journey to one of the least explored regions in the Solar System. It will be the first spacecraft to fly into the corona of the Sun. The homepage of NASA’s Solar Probe Plus mission can be found at http://solarprobe.jhuapl.edu/.

Around Dwarfs

2010-08-13T06:45:00.002+08:00

An extrasolar planet is basically a planet which orbits a star other than our Sun and a transiting extrasolar planet is one which periodically blocks a small fraction of the light from its parent star as its orbit brings it in front of the star’s luminous disk. Current missions such as NASA’s Kepler space telescope are sensitive enough to detect transiting extrasolar planets that are as small as the Earth around Sun-like stars.

Transits only occur when a planet’s orbit around its parent star happens to be orientated nearly edge-on with respect to our line of sight. Stars have randomly orientated planetary orbits and the probability that a planet is observed to transit its parent star is inversely proportional to the distance of the planet from its star. Therefore, a planet orbiting at a smaller distance from its star will have a higher probability of being observed to transit its star as compared to a planet orbiting at a larger distance.

For stars like the Sun, the fraction of light that a transiting planet blocks is small because the size of the star is much larger than the size of the planet. For example, if the Earth were to be observe transiting the Sun, it will cause the brightness of the Sun to dip by approximately 0.01 percent while if Jupiter were to be observe transiting the Sun, it will cause the brightness of the Sun to dip by approximately 1 percent.

White dwarfs are basically the end result of the evolution of main sequence stars such as the Sun and stars with less than 8 times the mass of the Sun eventually end up as white dwarfs. The Sun is 333 thousand times more massive than the Earth and a typical white dwarf can contain as much mass as the Sun gravitationally compactified into a dense sphere that is approximately the size of the Earth. In comparison, the Sun has a diameter that is 109 times larger than the Earth’s and a volume that can fit 1.3 million Earths.

As a result, when it comes to the relative fraction of starlight that can be blocked by a transiting planet, white dwarfs do offer a huge advantage over main sequence stars like the Sun. This is because the size of a white dwarf is much smaller than the size of a main sequence star and this allows a much greater fraction of a white dwarf’s light to be blocked by a transiting planet to generate a proportionally stronger transit signal. In fact, a Jupiter-size transiting planet can completely block out the light from a white dwarf since such a planet will be larger in size than the white dwarf.

The transit of an Earth-size planet across the luminous disk of a white dwarf will block out a significant fraction of the white dwarf’s light. In certain cases, it is even possible for the Earth-size planet to completely block out the white dwarf. Even the transit of an object as small as the Earth’s Moon in front of a white dwarf will occult a few percent of the white dwarf’s light. In comparison, the transit of an object as small as the Moon in front of a star like our Sun will only block out a minuscule 6 parts-per-million of the star’s light and such a weak signal will probably be hardly distinguishable from background noise. Therefore, white dwarfs offer an enormous advantage over main sequence stars as a means to detect small planetary objects.

Nonetheless, the detection of small transiting planetary objects around white dwarfs will require a higher observational cadence as compared to the detection of planets around main sequence stars like our Sun. This is because the small size of a white dwarf means that any transiting planet takes on the order of only a few minutes to transit across the entire luminous disk of the white dwarf. In comparison, the transit of a planet across the luminous disk of a main sequence star like our Sun takes on the order of a few hours which permits a much lower observational cadence.

White dwarfs are the final result of the evolution of main sequence stars like our Sun and there are ways in which planets can exist around white dwarfs. Before a star becomes a white dwarf, it will undergo a red giant phase where it will swell to many times its original size and engulf or vaporize planets that might be orbiting it in close vicinity. However, planets that orbit further out and planets that are sufficiently massive can survive the star’s evolution to a white dwarf and continue to orbit the white dwarf.

As a star evolves to a white dwarf, it can lose a significant fraction of its mass and this can destabilize any system of planets that is in orbit around the star. In such a scenario, planets can gravitationally interact with each other and can either be scattered into a tighter orbit around the white dwarf or get boosted into a more distant orbit around the white dwarf. It is also possible for planets to get completely ejected from the system. Planets that are scattered into a tighter orbit around the white dwarf will improve their probability of being observed to transit the white dwarf because the transit probability of a planet is inversely proportional to the distance of the planet from its star.

Interestingly, there is also an alternate mechanism that can allow planets to exist around white dwarfs. When a closely spaced pair of white dwarfs eventually merges due to the loss of orbital energy via the emission of gravitational radiation, a second generation of planets can form out from the disk of debris from the tidal disruption of the lower mass white dwarf. Therefore, the existence of an entire second generation system of planets, moons and asteroids in close orbit around a white dwarf is quite plausible.

The detection of transiting planetary objects around white dwarfs will help constrain the effectiveness of the mechanisms in which planets can exist around white dwarfs. To detect such transits, a large number of white dwarfs have to be continuously sampled at a high observational cadence. Currently, NASA’s Kepler space telescope is most suited for the detection of transiting planetary objects around white dwarfs. This is because Kepler has an extremely high observational cadence since its CCDs are read out every six seconds and Kepler is theoretically sensitive enough to detect objects as small as asteroids as they transit in front of white dwarfs!

Rogue Worlds

2010-08-06T10:25:00.001+08:00

The formation of planets in protoplanetary disks around stars is a messy process and a significant number of protoplanets with masses similar to planets such as the Earth or Mars may get ejected from their solar systems by gravitational interactions with massive gas giant planets. It is even possible that more protoplanets are ejected than retained in protoplanetary disks around stars and a considerable number of such ejected worlds might be wandering in the immense and dark expanses of interstellar space. Interstellar space is basically the vast and enormous spaces between stars.

An ejected planet with the mass of the Earth can retain an atmosphere of hydrogen in the frigid temperatures of interstellar space since the atmosphere of hydrogen will be cold enough to be bounded by the gravity of the planet. In comparison, at the distance where the Earth is from the Sun, a planet has to be over 10 times more massive than the Earth for its own gravity to be sufficiently strong enough to keep an atmosphere of hydrogen from escaping into space. In this article, I shall denote such ejected worlds as interstellar planets. From a distance, an interstellar planet will appear as a dark silhouette against a field of distant background stars.

The effective temperature of an interstellar planet is expected to be just several degrees Kelvin above absolute zero and any form of water at this temperature will be frozen solid. However, if an interstellar planet has a sufficiently thick atmosphere of hydrogen where the pressure at the bottom of such an atmosphere ranges from a hundred to a few thousand bars, the pressure-induced infrared opacity of molecular hydrogen will greatly insulate the planet from dissipating its internal radiogenic heat into space. At high pressures, molecular hydrogen is very effective at trapping heat and this significantly reduces the amount of heat lost into space.

With such an overlying atmosphere of hydrogen, the surface temperature of an interstellar planet can potentially exceed the meting point of water! In an environment like this, it will be possible for oceans of liquid water to exist on the planet’s surface and the ocean can be kept warm by the persistent flux of heat generated by the decay of radioactive isotopes in the planet’s interior.

On the surface of an interstellar planet, the sky will appear totally back and it is highly unlikely that stars will be visible from the planet’s surface due to the thick hydrogen atmosphere. However, areas of the planet’s surface can still be illuminated by occasional flashes of lightning. In the lower and warmer reaches of the atmosphere, it is possible for clouds of water droplets to form and drive a hydrological cycle. The atmospheric temperature decreases higher up in the atmosphere, possibly enabling other gases such as ammonia and nitrogen that have lower condensation temperatures to condense into clouds at these higher altitudes. Therefore, an interstellar planet can have several cloud layers of different condensates in its atmosphere.

To consider the Earth as the only populated world in infinite space is as absurd as to assert that in an entire field sown with millet, only one grain will grow.

- Metrodorus of Chios, 4th century BC

An interstellar planet can provide a stable environment for life for billions of year until the surface temperature declines below the meting point of water as the heat generating radioisotopes in the interior of the planet slowly gets depleted. Interstellar planets will be extremely difficult to detect as the amount of radiation they emit is exceedingly miniscule compared to the large amount of solar radiation the Earth reflects back into space as the Earth basks in the warm vicinity of the Sun. Finally, if interstellar planets do exist in significant numbers in the vast and uncharted expanses between the stars, these dark worlds might serve as common sites for life-supporting environments in the universe.

Triton's Sky

2010-07-30T14:34:00.003+08:00

Triton is by far the largest moon in orbit around the planet Neptune and with a diameter of 2700 kilometers; Triton is the seventh largest moon in the Solar System. Interestingly, Triton is the only large moon in the Solar System with a retrograde orbit, whereby it orbits Neptune in the opposite direction to Neptune’s rotation. Triton has a relatively high rock/ice ratio of approximately 70/30 and this composition is remarkably similar to large Kuiper Belt objects such as Pluto and Eris. Because of Triton’s retrograde orbit and its similar composition to objects such as Pluto and Eris, Triton is believed to be a Kuiper Belt object that was captured into orbit around Neptune.

Triton orbits Neptune at a distance of 354800 kilometers and it takes 5 days and 21 hours for Triton to orbit once around Neptune. Like the other large moons in the Solar System, Triton is tidally locked to Neptune where it keeps the same hemisphere oriented towards Neptune at all times. Triton was discovered by British astronomer William Lassell in 1846, only several days after Neptune itself was discovered. The first and only closed-up images of this elusive and far-flung moon of Neptune came on August 1989, after a close fly-by of Triton by NASA’s Voyager 2 spacecraft.

Apart from Saturn’s moon Titan, Neptune’s moon Triton is the only other moon in the Solar System with an appreciable atmosphere. Similar to both the Earth and Titan, nitrogen is the main constituent of Triton’s atmosphere. The atmospheric pressure on the surface of Triton is over 50000 times less than the atmospheric pressure at sea-level on the Earth. This is actually equivalent to the atmospheric pressure up in the Earth’s mesosphere at well over 50 kilometers above the Earth’s surface. Triton’s extremely frigid and cold environment allows the nitrogen in its atmosphere to be deposited onto the surface as frozen nitrogen. On the surface of Triton, the Sun will appear almost a thousand times dimmer than from the Earth’s surface.

The atmosphere of Triton contains thin clouds of nitrogen ice particles that are located a few kilometers above the surface. Above the clouds of nitrogen ice particles is a haze layer which extends up to 30 kilometers above Triton’s surface. This haze layer is made up of hydrocarbons and nitriles created when ultraviolet light from the Sun breaks down methane in Triton’s atmosphere. The surface of Titan also contains numerous erupting geysers of nitrogen gas. By itself, nitrogen gas is invisible and it is the entrained dust within the nitrogen gas which allows the geysers to be seen as plumes rising from the surface up to a height of 8 kilometers above Triton’s surface. The entrained dust within a plume can be deposited to over a hundred kilometers downwind of the plume and these plumes are responsible for creating the long and dark streaks on the surface of Triton.

Although Triton’s atmosphere is rather tenuous compared to the Earth’s atmosphere, it is still dense enough to ablate micrometeoroids when they pass through. The combination of a micrometeoroid’s orbital velocity around the Sun, the orbital velocity of Triton around Neptune, the orbital velocity of Neptune around the Sun and the gravitational accelerations of both Triton and Neptune, can allow the micrometeoroid to achieve a fast enough impact velocity that will enable the micrometeoroid to be sufficiently heated to visibility by friction with the air molecules in Triton’s atmosphere.

As Triton orbits Neptune, most micrometeoroids will impact the leading hemisphere of Triton and this is where most micrometeoroids can be observed. When a micrometeoroid penetrates an atmosphere, it heats up due to friction with the air molecules in the atmosphere. This causes material to sputter off the surface of the micrometeoroid particle in a process called ablation. Sufficiently strong heating and ablation can enable a micrometeoroid to become a visible meteor.

Due to the thin atmosphere of Triton and the much lower speeds at which micrometeoroids will impact the atmosphere of Triton as compared to the Earth, micrometeoroids can penetrate all the way down to the surface of Triton. This is unlike the Earth where micrometeoroids vaporize entirely high in the atmosphere. On Triton, visible meteor trails can extend all the way down to the surface!

Additionally, large variation in the brightness of meteors is expected to occur along different phases of Triton’s orbit around Neptune. The brightness of a meteor trail is expected to be the greatest when Triton’s orbital velocity around Neptune adds up most positively with Neptune’s orbital velocity around the Sun. Finally, icy micrometeoroids are expected to produce brighter meteor trails than stony micrometeoroids because icy micrometeoroids have a greater rate of ablation and the brightness of a meteor trail is directly related to the rate of ablation of the micrometeoroid.

In addition to producing visible meteor tails, the ablation of incoming micrometeoroids can deposit metallic atoms and molecules that were once part of the micrometeoroid into the atmosphere of Triton. These metallic atoms and molecules can condense into dust particles in the atmosphere of Triton and these dust particles could serve as nucleation centers for the condensation and formation of cloud and haze particles observed in Triton’s atmosphere. Most of these metallic atoms and molecules will eventually get deposited together with the condensates onto the surface of Triton.

A few days ago, I created a program to compute the various velocity distributions of meteoroids on trajectories that will impact Triton. I computed the velocity distributions as functions of variables such as the impact angles of meteoroids, the orbital phase positions of Triton in its orbit around Neptune and the heliocentric velocities of meteoroids. This model can also be extended to compute the meteoroid flux for other planet-satellite systems such as the Earth-Moon system, the Jupiter-Europa system and the Saturn-Titan system. To conclude, the Neptune-Triton system is unique, because unlike other planet-satellite systems, it features a remarkably large variation in the meteoroid impact velocity onto Triton as a function of Triton’s orbital phase position around Neptune.

Stellar Behemoth

2010-07-24T00:05:00.001+08:00

The Tarantula Nebula is an extremely luminous nebula that is located approximately 165000 light years away in the Large Magellanic Cloud and it is the largest and most active star formation region known in the Local Group of galaxies. The Large Magellanic Cloud is basically a nearby irregular galaxy which is roughly one-tenth as massive as the Milky Way Galaxy.

At the heart of the Tarantula Nebula lies an exceptionally dense cluster of stars called R136 which generates most of the light that illuminates the Tarantula Nebula. 4 exceedingly massive and luminous stars sit in the core of the R136 star cluster and they are designated R136a1, R136a2, R136a3 and R136c respectively. Each star is well over 100 times more massive than the Sun and each star is millions of times more luminous than the Sun.

The most massive and luminous of the 4 central stars in the R136 star cluster is the star called R136a1. This behemoth is currently the most massive and luminous star discovered so far. R136a1 has a current mass that is 265 times the mass of the Sun and an initial mass that is estimated to be 320 times the mass of the Sun! Since its birth, R136a1 has shed over 50 times the mass of the Sun in extremely powerful stellar winds.

R136a1 also shines with an “off-the-charts” luminosity that is approximately 10 million times greater than the Sun’s luminosity! To put this extreme luminosity into perspective, R136a1 emits as much energy in 3 seconds as the Sun emits in an entire Earth year! With an age of just over a million years, R136a1 is already a middle-aged star. In comparison, our Sun is already 5000 million years old and the Sun is only halfway through its lifespan.

Although the R136 star cluster contains approximately 100000 stars, its 4 brightest stars (R136a1, R136a2, R136a3 and R136c) account for approximately half of the wind and radiation power of the entire cluster of stars! Because very massive stars are so exceedingly rare, it is unlikely that there is any other star in the Tarantula Nebula or possibly even in the entire Local Group of galaxies that will be comparable to the brightest components of the R136 star cluster. An ultra-massive star like R136a1 is considered to be a very extreme case for a star and R136a1 might very well be literally one in a trillion!

If you want to read more about R136a1 and associated discoveries, you can access the paper entitle “The R136 Star Cluster Hosts Several Stars Whose Individual Masses Greatly Exceed the Accepted 150 M_Sun Stellar Mass Limit” from http://arxiv.org/abs/1007.3284v1.

Mercurian Ice

2010-07-23T23:04:00.001+08:00

Mercury is the closest planet from the Sun and it orbits the Sun once every 88 Earth days. Due to its eccentric orbit, Mercury has a minimum distance of 46.0 million kilometers from the Sun and a maximum distance of 69.8 million kilometers from the Sun. This causes the intensity of sunlight on Mercury to vary by over a factor of two as Mercury orbits the Sun. Mercury is also locked in a spin-orbit resonance where the ratio of orbital period to spin period is precisely 3:2. This means that Mercury completes three rotations about its axis for every two orbits around the Sun.

With a diameter of 4880 kilometers, Mercury is the smallest of the terrestrial planets and despite its size; it is the only other terrestrial planet besides the Earth that has a global magnetic field. Mercury also has a large and dense iron core which makes up well over half the planet’s mass. This makes Mercury the second densest planet in the Solar System with Earth being the densest due to gravitational compression effects. Had it not been for Earth’s gravitational compression effects, Mercury would have been the densest planet in the Solar System.

Mercury has a large surface temperature range which goes from a maximum of 700 degrees Kelvin at the subsolar point when Mercury is closest to the Sun to a minimum of below 100 degrees Kelvin at the bottoms of craters located around the poles. The subsolar point on a planet such as Mercury is where the Sun is directly overhead and hence, the Sun’s rays strike the surface perpendicularly at the subsolar point.

Permanently shadowed craters are known to exist around the north and south poles of Mercury. Since Mercury has an axial tilt that is almost zero, the rims of many of these craters are able to shield the Sun and keep the floors of the craters in permanent darkness. The temperatures within these permanently shadowed craters can go below 100 degrees Kelvin as the Sun never rises above the crater rims to warm the frigid interiors of these craters. Within these permanently shadowed craters, water can exist in the form of ice and remain stable over billions of years. In fact, radar observations have revealed the presence of ice deposits within permanently shadowed craters in Mercury’s polar regions.

These permanently shadowed regions within craters around the poles of Mercury receive only scattered sunlight and thermal emissions from the surrounding topography. The temperatures within these permanently shadowed regions are therefore sensitive to the orientations of the surface and surrounding topography. Finally, burial under a thin regolith layer can enable the ice deposits to remain stable at higher temperatures and can extend the presence of ice deposits to lower latitudes.

Currently, NASA’s MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Ranging) spacecraft is on its way to Mercury and MESSENGER is expected to enter orbit around Mercury on 18 March 2011. Since its launch on 3 August 2004, MESSENGER has made one Earth flyby, two Venus flybys and three Mercury flybys. These flybys are a form of gravity assist maneuvers which greatly reduce the amount of fuel required to fly MESSENGER on the right trajectory that will allow it to eventually enter orbit around Mercury. The three flybys of Mercury by MESSENGER have already generated an astonishing amount of interesting science that is poised to greatly change and increase our understanding of the elusive closest planet from the Sun. If you want to find out more, visit the mission homepage at http://messenger.jhuapl.edu/.

Frozen Oasis

2010-07-18T23:49:00.004+08:00

Liquid water cannot exist on the surface of the Moon as it will rapidly evaporate on its airless surface or get broken down into hydrogen and oxygen by sunlight. However, water can be present in the form of ice within permanently shadowed craters at the Moon’s poles. In such places, the Sun never gets high enough over the horizon to cast its rays over the rims of these craters and illuminate the floors of the craters.

Without ever being warmed by the Sun, these permanently shadowed regions can maintain incredibly low temperatures which make them ideal for water in its frozen form to exist over billion-year timescales. In fact, an instrument onboard NASA’s Lunar Reconnaissance Orbiter recorded temperatures as low as 25 degrees Kelvin or -248 degrees Centigrade in areas within these permanently shadowed regions, making them amongst the coldest known places in the Solar System!

[29] From whose womb comes the ice? Who gives birth to the frost from the heavens [30] when the waters become hard as stone, when the surface of the deep is frozen?

- Job 38:29-30 (New international Version)

The permanently shadowed craters at the Moon’s poles can serve as cold traps where water brought to the Moon by impacting comets can accumulate in these places. This week, I researched on the retention of water from the impacts of comets onto the surface of the Moon. Comets are small icy objects which orbit the Sun and they range in sizes from a few hundred meters to tens of kilometers across. Comets are known to contain a large amount of volatiles, especially water in the form of ice. When a comet impacts the Moon, a fraction of the water from the comet can eventually end up in these permanently shadowed craters and accumulate there in the form of ice.

In my research, I derived a method which estimates the fraction of the comet’s mass that remains gravitationally bound to the Moon after the impact. I carried out the computations and analysis for various impact velocities and various impact incident angles. From my results, a significant fraction of the comet’s mass remains gravitationally bound to the Moon after the impact as long as the impact velocity of the comet is less than 30 to 40 kilometers per second. In my analysis, the comet’s mass is assumed to be entirely made up of water.

Of the fraction of the comet’s mass in water which remains gravitationally bound to the Moon after the impact, a portion can survive long enough in its migration across the surface of the Moon to eventually accumulate in the permanently shadowed craters at the Moon’s poles. The presence of water on the Moon is an important factor in determining lunar habitability since a large and easily accessible source of water on the Moon will render needless the prohibitively expensive feat of transporting water from the Earth. Water can be separated into hydrogen and oxygen to provide breathable oxygen and to serve as a form of rocket fuel.

I have also extended my method of analysis in estimating the retention of water from the impacts of comets onto the surface of the Moon to other worlds such as the planet Mercury. With a stronger gravitational field, Mercury is able to retain a larger fraction of a comet’s watery mass and like the Moon; Mercury also has permanently shadowed craters at its poles where frozen water can accumulate.

Visiting Europa

2010-07-10T13:59:00.001+08:00

Jupiter has over twice the mass of all the other planets in our Solar System combined and it is the archetype of large gas giant planets, especially so for the countless giant planets now known to orbit other stars. Orbiting Jupiter are 4 large moons named Io, Europa, Ganymede and Callisto. Additionally, Jupiter also has a few dozen small irregular satellites and a ring system in orbit around it. The 4 large moons of Jupiter, also known as the Galilean satellites, are particularly fascinating. Io is by far the most volcanically active world in the Solar System while Europa has a huge global ocean of water hidden beneath just a thin layer of ice. Ganymede and Callisto are large moons that are believed to also harbor internal oceans. Ganymede is the largest moon in the Solar System and it is even larger than the planet Mercury.

Today, I came across a paper entitled - “Return to Europa: Overview of the Jupiter Europa Orbiter Mission (2010), K. Clark, J. Boldt, R. Greeley, K. Hand, I. Jun, R. Lock, R. Pappalardo, T. Van Houten, T. Yan” and this paper describes a mission to explore Jupiter’s ocean moon Europa. The Europa Jupiter System Mission (EJSM) is a proposed mission by the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA) to explore Jupiter and its moons. The Jupiter Europa Orbiter (JEO) will make up the NASA-led portion of the EJSM and it is a satellite that will be placed into orbit around Europa.

Europa is a particularly interesting and intriguing moon of Jupiter because its subsurface ocean is in direct contact with the rocky interior of Europa, enabling the water to be infused with minerals and energy that is necessary for life through features such as hydrothermal vents. Thus, conditions at the bottom of Europa’s ocean could be very similar to the Earth’s ocean floor. Europa’s subsurface ocean is estimated to contain far more water than all the oceans on the Earth combined and the aquatic environment of Europa’s ocean is likely to be within the constraints of known life on Earth.

The Jupiter Europa Orbiter (JEO) is expected to be launched onboard an Atlas V 551 launch vehicle in the first quarter of 2020 and it will use a Venus-Earth-Earth gravity assist interplanetary trajectory to get to Jupiter. It will take approximately 6 years for JEO to get to Jupiter where it is expected to arrive at the end of 2025 or the beginning of 2026. Upon reaching Jupiter, JEO will perform a series of gravity assist with the moons Io, Europa, Ganymede and Callisto over a 30 month period to reduce its orbital energy with respect to Europa. This mission phase provides a unique opportunity to explore the Jovian system as it also includes several flybys of each of the 4 large moons of Jupiter.

In the middle of 2028, the Europa Orbit Insertion (EOI) will occur whereby a main engine burn will decelerate JEO into a low circular orbit around Europa. JEO will then begin its nominal Europa science campaign which is expected to last for 9 months and a mission extension beyond 9 months is very likely because the compounding effect of applying worst-case assumptions at every level in the design of the spacecraft tends to severely underestimate the mission lifetime.

JEO will be powered by Multi-Mission Radioisotope Thermoelectric Generators (MMRTGs) where the radiogenic heat from the decay of Plutonium-238 will be used to power the onboard systems. The huge amount of radiation that JEO will be subjected to throughout its mission poses a unique technical challenge. The 4 main sources of radiation are solar radiation, galactic cosmic rays, high energy particles trapped within the Jovian magnetosphere and neutrons and gamma ray photons from the onboard MMRTG nuclear power source. The original paper briefly addresses the radiation risks and various mitigation methods for the JEO mission.

The JEO mission science objectives, as defined by the international EJSM Science Definition Team are:

1. Europa’s Ocean: Characterize the extent of the ocean and its relation to the deeper interior.

2. Europa’s Ice Shell: Characterize the ice shell and any subsurface water, including their heterogeneity, and the nature of surface-ice-ocean exchange.

3. Europa’s Chemistry: Determine global surface compositions and chemistry, especially as related to habitability.

4. Europa’s Geology: Understand the formation of surface features, including sites of recent or current activity, and identify and characterize candidate sites for future in situ exploration.

5. Jupiter System: Understand Europa in the context of the Jupiter system.

If you want to read more about the Europa Jupiter System Mission (EJSM), you can download the final report from http://opfm.jpl.nasa.gov/library/.

Alien Planets

2010-07-05T12:14:00.002+08:00

Kepler is NASA’s first mission that is capable of finding Earth-sized worlds orbiting other stars and Kepler is a space telescope that is named after German astronomer Johannes Kepler. Kepler was launched into space on 7 March 2009 onboard a Delta II rocket. On 12 May 2009, Kepler completed its commissioning phase and started searching for planets around other stars.

Kepler utilizes the transit photometry method to detect extrasolar planets by precisely monitoring the brightness of about 150000 selected stars in its field-of-view. A transit occurs when a planet passes in front of its parent star and causes a slight decrease in the star’s apparent brightness from the blocking of a small fraction of the star’s light by the planet’s opaque disk. Therefore, transits only occur when a planet’s orbit around its parent star happens to be orientated nearly edge-on with respect to an observer’s line of sight. A larger planet will block out a greater fraction of the star’s light compared to a smaller planet and Kepler is sensitive enough to observe the miniscule drop in brightness when an Earth-sized planet transits a Sun-like star.

An Earth-sized planet transiting a Sun-like star will cause an 84 parts-per-million decrease in the star’s apparent brightness while a Jupiter-sized planet transiting a Sun-like star will cause a one percent decrease in the star’s apparent brightness. For the same star, the dip in its apparent brightness from the transit of a Jupiter-sized planet is over 100 times greater than the signal from a transiting Earth-sized planet. Kepler has to observe at least three transits to be sure that the dimming of a star is caused by a planet. Therefore, the discovery of Earth-sized planets in Earth-like orbits around Sun-like stars is expected to take three years or longer. However, Kepler has already turned out a myriad of potential planetary candidates during its first several days of observations.

On 15 June 2010, the Kepler mission team released data on all but 400 of the 706 targets from data collected during the first 43 days of Kepler’s nominal observation phase. This set of data contains viable extrasolar planet candidates with sizes ranging from as small as that of the Earth to larger than that of Jupiter! The paper detailing this is entitled “Characteristics of Kepler Planetary Candidates Based on the First Data Set - The Majority are found to be Neptune-Size and Smaller (2010)” and the appendix of this paper shows a list of 306 viable extrasolar planet candidates.

The publicly released list of 306 targets with viable extrasolar planet candidates shows that most candidate planets are significantly smaller than Jupiter. In fact, most of the candidate planets range in size from being slightly larger than the Earth to Neptune-sized worlds. Interestingly, 5 of the 306 targets are stars with multiple transiting candidate extrasolar planets and a paper entitled “Five Kepler Target Stars that Show Multiple Transiting Exoplanet Candidates (2010)” further describes this.

Data for 400 of the 706 targets with viable planetary candidates were not publicly released because they are bright enough for high-quality Doppler measurements or contain candidate planets with less than 1.5 times the diameter of the Earth, or both. Data for these 400 targets will only be released in February 2011.

Considering that there are approximately 470 known extrasolar planets as of July 2010, follow-up confirmations of the targets in this first set of data from Kepler is expected to dramatically increase the known planet count! Besides the discovery of Earth-sized planets, Kepler is also able to detect interesting objects such as a Saturn-style ring system around a planet or an Earth-sized moon of a gas giant planet. I bet that this initial set of data with 706 targets containing viable extrasolar planet candidates is just a sneak preview of things to come!

Trillion Years

2010-06-28T22:23:00.004+08:00

Red dwarf stars are by far the most common stars in the galaxy and they have masses ranging from 0.08 to 0.4 times the mass of the Sun. 0.08 times the mass of the Sun is just about the lowest possible mass a star can have and still be able to sustain hydrogen fusion within its core. The least massive red dwarf stars shine at only 0.01 percent the luminosity of the Sun while the most massive ones do not exceed 10 percent the luminosity of the Sun.

Assuming that the lifespan of a star is the total duration in which it is able to sustain nuclear fusion reactions, the lowest mass red dwarf stars can have lifespans that exceed 10 trillion years. In comparison, the current age of the universe is a mere 13.7 billion years and the estimated lifespan of the Sun is just 12 billion years. At the current age of 13.7 billion years, all the red dwarf stars in the universe have only just begun their seemingly eternal existence.

One reason why red dwarf stars have such incredibly long lifespans when compared to more massive stars is because red dwarf stars have fully convective interiors and this means than almost all of the hydrogen within such stars is available for sustaining nuclear fusion within the cores of these stars. A more massive star such as the Sun has a mostly radiative interior and this means that only the hydrogen within the core of the Sun is available for nuclear fusion due to the absence of any convective mixing between the matter in the core with the matter in the overlying layers. The other reason for the longevity of red dwarf stars is that such stars burn their hydrogen via nuclear fusion at a much smaller rate than more massive stars.

In this article, we shall follow the evolution of a red dwarf star that has 0.1 times the mass of the Sun. It takes an estimated 2 billion years for this red dwarf star to contract from an initial cool cloud of hydrogen and helium to the point where is able to sustain hydrogen fusion within its core. At the onset of stable hydrogen fusion within its core, the newly formed red dwarf star will have a surface temperature of 2200 Kelvin and shine at 0.04 percent the luminosity of the Sun. Since the red dwarf star has a fully convective interior, almost all of its hydrogen is available to sustain the fusion reactions within its core. With 0.1 times the mass of the Sun, this red dwarf star is estimated to have a nuclear burning lifespan of over 6 trillion years. In fact, the current age of the universe is not even a quarter of a percent of the multi-trillion year lifespan of this red dwarf star.

At age zero, the mass of the red dwarf star is three quarters hydrogen and one quarter helium. Over the subsequent trillions of years, the red dwarf star will fuse hydrogen into helium within its core, gradually converting more of its fraction by mass into helium. The steady rise in the helium mass fraction of the red dwarf star increases the rate at which energy is being generated by nuclear fusion in the core of the star, causing the surface temperature and the overall luminosity of the star to also increase.

After 3.1 trillion years, the red dwarf star’s fraction of mass that is helium surpasses the fraction of mass that is hydrogen. At this point, the red dwarfs star will have a surface temperature of 2500 Kelvin and shine at 0.1 percent the luminosity of the Sun. As the red dwarf star crosses the age of 5.7 trillion years, over 85 percent of its mass will now be in the form of helium and this is the point where radiative transport of energy replaces convection in the core of the star. At this stage, the red dwarfs star will have a surface temperature of 3500 Kelvin and shine at 0.3 percent the luminosity of the Sun.

The creation of the radiative core within the red dwarfs star signifies the closing stages of its near eternal lifespan as the evolution of the star begins to accelerate. The core of the red dwarf star increases in mass via the buildup of helium as the remaining hydrogen undergoes fusion into helium in a shell surrounding the core which gradually moves outward through the star. During this process, the surface temperature and luminosity of the red dwarf star continues to increase until it eventually reached a maximum surface temperature of 5800 Kelvin and shines with just under one percent the luminosity of the Sun. In fact, the surface temperature of the red dwarf star is now slightly greater than the surface temperature of the Sun even though its overall luminosity is much lower due to its vastly smaller size compared to the Sun. At this stage, the red dwarf star is a far cry compared to what it initially was.

After the red dwarf star attains its maximum surface temperature, it beings to turn around and evolves towards a lower surface temperature and a lower luminosity. At this point, the red dwarf star is still producing energy by burning hydrogen into helium in a shell surrounding a large and inert helium core. The rate at which energy is being generated by the fusion of hydrogen into helium in the shell gradually diminishes and it is eventually extinguished at 540 billion years after the red dwarf star first develops its radiative core. At this point in time, the red dwarf star has a surface temperature of 1700 Kelvin and shines with 0.0005 percent the luminosity of the Sun, 80 times dimmer than its luminosity at birth. Since the onset of the radiative core occurs 5.7 trillion years into the lifespan of the red dwarf star, the total duration of nuclear burning within the star adds up to just over 6 trillion years.

The red dwarf star now ends its life as a low mass helium white dwarf star with a final mass fraction where 99 percent of it is helium with the remaining 1 percent being hydrogen. This final mass fraction shows the extraordinary efficiency in which the red dwarf star generates energy by burning its hydrogen into helium through nuclear fusion. In comparison, the Sun burns only 10 percent of its hydrogen throughout its entire lifespan.

A 6 trillion year lifespan is not the longest a red dwarf star can possibly have. In fact, a red dwarf star with 0.08 times the mass of the Sun has an estimated lifespan of 12 trillion years, making it twice as long as a red dwarf star with 0.1 times the mass of the Sun. In this incredibly distant future universe, the red dwarf star with 0.1 times the mass of the Sun has finally evolved into a white dwarf star. After many trillions of years of further cooling, this white dwarf star will eventually become a black dwarf where its surface temperature gets ever nearer to absolute zero.

Given that red dwarf stars make up the vast majority of stars in a galaxy and that these stars can live for trillions of years, most of the stellar evolution that will occur has yet to occur. In the far future universe, red dwarf stars will play an increasingly important role in contributing to the total luminosity of a galaxy as the rate of star formation decreases and as the more massive stars in the galaxy age and fade away. This is because the gradual increase in the luminosities of red dwarf stars nearly compensates the loss in luminosity as the rate of star formation declines and as the more massive stars fade away. This ultimately causes the total luminosity of the galaxy to remain fairly constant over trillions of years.

Intergalactic Wanderer

2010-06-04T14:13:00.002+08:00

Hypervelocity stars are stars with sufficiently large velocities that they are no longer gravitationally bound to the galaxy. While ordinary stars have velocities on the order of 100 kilometers per second, hypervelocity stars have velocities on the order of 1000 kilometers per second. At such velocities, hypervelocity stars will escape their home galaxy forever and become lone wanderers of intergalactic space. In February 2010, I wrote and posted a short article about hypervelocity stars, including possible mechanisms that can lead to such stars.

For a moment, imagine a Sun-like hypervelocity star with an entire system of planets orbiting it, where one of the planets is an Earth-like world that is not too different from ours. The Sun-like star and its system of planets are traveling in excess of 1000 kilometers per second, on a trajectory that has already taken them far from the home galaxy. How will the night sky from the surface of such an Earth-like world appear as it wanders the dark and immense distances of intergalactic space?

In the vast and starless voids of intergalactic space, the night sky from the surface of this Earth-like planet will be totally devoid of any stars. Assuming that the Earth-like planet and its parent star left their home galaxy a hundred million years ago, an alien observer on the surface of the Earth-like planet will be able to see the entire galaxy as a disk of wispy arms spiraling out from a glowing central bulge. The galaxy will span across a huge swath of the starless night sky and none of the hundreds of billions of stars that make up the galaxy will be individually distinguishable with unaided eyes. Every several years or so, a star in the galaxy will end its life in a supernova explosion and it will appear as a brilliant point of light which will suddenly appear and gradually fade.

It will take a huge leap of imagination and ingenuity for an extraterrestrial civilization living on this planet to realize that each of the hundreds of billions of miniscule points of light that comprise the galaxy are actually stars not too different from their yellow Sun. Imagine the idea of interstellar space travel in such a scenario where instead of a mere 4.37 light years away, the nearest stars are many thousands of light years away! Additionally, this extraterrestrial civilization might even contemplate about the possibilities of other extraterrestrial civilizations living on worlds within the galaxy and how these civilizations might possibly figure out the shape of the galaxy in which they live in!

Discovering Tyche

2010-04-30T12:39:00.003+08:00

Is there an undiscovered massive object orbiting the Sun in the Oort Cloud, elusively hidden in the perpetual frigid darkness? The Oort Cloud occupies an immense region of space surrounding the Sun; from a couple of thousand AU to as far as 50000 AU from the Sun! The term AU is the acronym for Astronomical Unit, where one AU is the mean distance of the Earth from the Sun and it has a value of 149.6 million kilometers.

The Oort Cloud is estimated to contain several trillion objects larger than 1 kilometer in diameter, with each object spaced tens of millions of kilometers away from its closest neighbor! To put the size of the Oort Cloud into perspective, even the distance of Pluto from the Sun is less than 0.1 percent the distance to the edge of the Oort Cloud!

A recent paper entitled “Persistent Evidence of a Jovian Mass Solar Companion in the Oort Cloud” describes the possibilities of a Jupiter-mass object orbiting the Sun at a distance large enough to place it within the Oort Cloud. This paper can be found at http://arxiv.org/abs/1004.4584v1. Tyche is the name recently suggested for this hypothetical object. The name Tyche, which means “luck” in Greek, is also the good sister of Nemesis in Greek mythology.

In this paper, the possible existence of Tyche was inferred from dynamical and statistical analysis of the orbits of comets entering the Solar System from the Oort Cloud. The gravitational perturbations from a distant Jupiter-mass object like Tyche could also explain the peculiar orbits of extended scattered disc objects such as Sedna. These objects orbit the Sun on highly elliptical orbits that take them out to hundreds of AU from the Sun. Sedna for example, has a very elongated orbit which takes it from a minimum of 76 AU from the Sun out to an incredible 976 AU from the Sun and it takes over 12 thousand years to orbit the Sun once.

Being located at such a huge distance from the Sun, the amount of insolation that Tyche gets from the Sun will be negligible. Tyche will be a gas giant world like Jupiter and it is expected to glow feebly at a temperature of about 200 Kelvin from heat emanating from its warm interior. Therefore, Tyche can only be detected in the infrared band since such a cool object is expected to emit almost no visible light. Interestingly, NASA’s recently launched Wide-field Infrared Survey Explorer (WISE) will be able to easily detect the presence of such an object in the Oort Cloud! Visit http://wise.ssl.berkeley.edu/ to find out more about WISE.

Although a positive detection of Tyche might not be much of a surprise, the discovery of such a world will be extremely fascinating! Jupiter is currently by far the most massive known object in orbit around the Sun and the discovery of something more massive than Jupiter will have interesting implication regarding our perspectives of things in orbit around the Sun. What kind of moons will orbit this object and might some of these moons be similar to the ones in orbit around Jupiter? What kind of exploratory robotic spacecraft might possibly be sent there? Additionally, since the formation mechanisms for such an object are probably be very different compared to the formation mechanisms for the planets in our Solar System, should Tyche be classified as a planet or a brown dwarf?

Quaoar-Weywot

2010-04-23T12:32:00.002+08:00

Quaoar is the name of a Kuiper Belt object which orbits the Sun at a mean distance of 43.6 times the distance of the Earth from the Sun. At that orbital distance, Quaoar takes 288 Earth years to go around the Sun once. Traveling at a speed of 10 kilometers per second, it will take roughly 2 decades to travel from the Earth to Quaoar! Additionally, Quaoar also has a moon named Weywot which orbits it with period of close to twelve and a half days. Weywot orbits Quaoar at a mean orbital distance of approximately 14500 kilometers from Quaoar.

I read a number of interesting newly published papers this week and a paper entitled “Quaoar: a Rock in the Kuiper Belt” was the most attention grabbing. This paper describes the unique properties of the Quaoar-Weywot system and some new observations of this fascinating far-flung system. This paper can be read at http://arxiv.org/abs/1003.5911v1.

Quaoar is about 900 kilometers in diameter and in comparison; Pluto has a diameter of 2300 kilometers. The orbit of Weywot around Quaoar reveals that Quaoar has a mass that is approximately 12 percent of Pluto’s. This gives Quaoar an estimated mean density of 4.2 grams per cubic centimeter which makes Quaoar one of the densest known objects in the Kuiper Belt. Additionally, Quaoar’s moon Weywot is estimated to have a diameter of 74 kilometers. A human being with a weight of 70 kilograms on the Earth’s surface will weigh less than 4 kilograms on the surface of Quaoar!

Quaoar unusually high density implies that it contains proportionally less icy materials than other Kuiper Belt objects and its high density is also reminiscent of objects in the main asteroid belt which are located much closer to the Sun than Quaoar. Therefore, a substantial bulk of Quaoar is probably made up of much denser rocky material instead of the less dense icy materials.

One theory which explains Quaoar’s unusually high density states that Quaoar collided with another object which stripped away most of Quaoar’s less dense icy mantle and left behind the denser rocky core. This collision event increased the mean density of Quaoar to the current observed value as a larger proportion of Quaoar’s mass is now comprised of denser rocky material.

Another theory which might explain Quaoar unusually high density states that Quaoar formed much closer to the Sun in the main asteroid belt where objects formed there typically have densities similar to the current density observed for Quaoar. Subsequently, gravitational interaction with the planets scattered Quaoar further from the Sun into the frigid realm of the distant Kuiper Belt objects where Quaoar has been residing ever since.

Finally, regarding the exploration of objects in the Kuiper Belt, NASA’s New Horizons spacecraft is currently on its way to Pluto and it is scheduled to make closest approach to Pluto on 14 July 2015. In fact on 17 October 2010, New Horizons will have traveled half the flight time to reach Pluto since its launch on 19 January 2006. After making its flyby of Pluto and its moons, Charon, Nix and Hydra, New Horizons is also scheduled to flyby one or more Kuiper Belt objects. Visit http://pluto.jhuapl.edu/ to obtain all the latest news about this mission.

Incredibly Massive

2010-04-09T20:52:00.002+08:00

Over the past two weeks, I wrote about some fascinating stuff in which small black holes can be utilized for and I also explored an interesting alternative to true black holes! In this post, I am going to write about the most massive black hole currently known in the universe, even though there are probably a lot more yet-to-be-discovered black holes which could be more massive than the one which I am about to describe.

OJ 287 is a pair of supermassive black holes residing in the heart of a distant galaxy located 3.5 billion light years away, where one light year is the distance light travels in one year. The primary black hole of OJ 287 contains an incredible 18 billion times the mass of the Sun while the secondary black hole contains 150 million times the mass of the Sun! This makes the primary black hole of OJ 287 one of the most massive known black holes in the universe! To put things into perspective, the supermassive black hole in the core of our Milky Way Galaxy is a mere 4 million times the mass of our Sun and the Sun alone is already 333 thousand times more massive than the Earth!

With 18 billion times the mass of the Sun, the event horizon of the monstrous primary black hole of OJ 287 will span an astonishing 110 billion kilometers in diameter. This means that about 80 thousand Suns or 9 million Earths placed end-to-end are required to span the diameter of the black hole’s event horizon! Note that the event horizon of a black hole is a region surrounding it where gravity becomes so strong that it does not let even light to escape.

The much less massive secondary black hole of OJ 287 orbits the primary black hole with a period of 11 to 12 years. Two outbursts are observed from OJ 287 every 11 to 12 years as the secondary black hole plows through the accretion disk of the much more massive primary black hole twice per orbit. The orbit of the secondary black hole around the primary black hole is gradually decaying via the emission of gravitational radiation and the secondary black hole is expected to merge with the primary black hole within 10 thousand years.

Ultra Compact

2010-04-02T23:19:00.002+08:00

In my previous post last week, I wrote about some fascinating theoretical uses of black holes and in my first sentence, I defined a black hole as an object that is so dense and compact that within a certain distance from it, its gravitational pull becomes so strong that it does not let even light to escape. This certain distance is known as the black hole’s event horizon and anything that happens to enter it will never escape. By this definition, a black hole does not have a true physical surface and it is basically a region of space from which nothing, including light, can escape.

In this article, I am going to describe a kind of black hole that is very different from what was defined in the above paragraph and I will be using the phase “progenitor star” to denote an object that is currently in the process of collapsing under its own immense gravity towards forming a black hole. As the progenitor star collapses, it never reaches a true black hole state, but instead becomes a General Relativistic Radiation Pressure Supported Star (GRRPSS). What a mouthful! Although this scenario is purely theoretical, its exciting properties are definitely worth considering!

The name “General Relativistic Radiation Pressure Supported Star” somewhat speaks for itself. So, how does radiation pressure works? Take for example our Sun - a ferociously hot ball of hydrogen and helium with 333 thousand times the mass of the Earth sitting in the middle of our Solar System. The enormous amount of radiation produced via nuclear fusion within the Sun’s core produces an incredible amount of radiation pressure which tries to blow the Sun apart, while gravity tries to crush the Sun inwards. It is this perfect balance between radiation pressure and gravity which gives our Sun the size that we constantly observe it to be.

As the progenitor star collapses, the gravitational field in the vicinity of the star becomes increasingly stronger as the star becomes ever more dense and compact. As the physical diameter of the progenitor star collapses and approaches the diameter of the event horizon for a black hole of its mass, radiation emitted from the surface of the progenitor star will become increasingly redshifted. This means that the radiation emitted from the progenitor star’s surface gets stretch into ever longer wavelengths.

When the progenitor star collapses below one and a half times the diameter of the event horizon for a black hole of its mass, light emitted at or near the tangent to the star’s surface will not be able to escape into space and will eventually fall back to the surface. As the progenitor star collapses until its physical size approaches the diameter of the event horizon for a black hole of its mass, only light that is being emitted vertically upwards from the star’s surface will escape into space instead of falling back somewhere else on the star’s surface. Therefore, as the progenitor star collapses, only light that is emitted at an ever decreasing angle from the vertical will escape into space and the self-gravitational trapping of radiation by the progenitor star becomes more and more effective.

The radiation pressure created from the self-gravitational trapping of radiation by the progenitor star prevents it from collapsing to a true black hole state. Instead, the progenitor star will continue collapsing as an incredibly hot ball of quark gluon plasma which asymptotically tends towards a true black hole state but never reaches it. In fact, such an object will appear totally dark since almost no radiation will be able to escape the super strong gravity of the star and it will appear very much like a true black hole!

It might also be true that as the collapsing General Relativistic Radiation Pressure Supported Star tends to become a true black hole, its lifetime in this phase becomes infinite. Such as object can be called an Eternally Collapsing Object (ECO) and this class of objects represents an alternative to black holes!

The titles of the papers which I have made some reference to in writing this article are “Likely Formation of General Relativistic Radiation Pressure Supported Stars or Eternally Collapsing Objects”, “Radiation Pressure Supported Stars in Einstein Gravity - Eternally Collapsing Objects” and “Sources of Stellar Energy, Einstein-Eddington Timescale of Gravitational Contraction and Eternally Collapsing Objects.” If you are interested to learn more, you can obtain these papers from http://arxiv.org/.

Like Magic

2010-03-26T19:47:00.003+08:00

A black hole is an object that is so dense and compact that within a certain distance from it, its gravitational pull becomes so strong that it does not let even light to escape! This distance is where the event horizon of the black hole is located and anything which crosses the event horizon, including light, can never escape. If the entire Earth is crushed to form a black hole, its event horizon will have a diameter of only 1.8 centimeters! In comparison, the black hole version of the Sun will have an event horizon that is 5910 meters in diameter.

In this article, I will briefly describe the theoretical possibilities of using small black holes as energy generators, antimatter factories, propulsion for interstellar space travel and gravity wells for artificial planets! These ideas are just theoretical possibilities that are probably possible in the far distant future. The small black holes that I’m referring to are those with masses at or below the planetary mass regime. Unless it can be found naturally, forming such a black hole will first require compressing a large amount of mass into an incredibly tiny volume of space. After the creation of an initial black hole, additional matter can be thrown into the black hole to increase its mass.

Hawking radiation is a form of radiation that is predicted to be emitted by black holes due to quantum effects and it is named after the physicist Stephen Hawking who theorized its existence in the 1970s. Since the emission of Hawking radiation allows black holes to lose mass, black holes that lose more mass than they gain will eventually disappear. In the absence of any mass addition, a black hole will eventually vanish by emitting all of its mass in the form of Hawking radiation and the lifespan of a black hole is directly proportional to its mass. The amount of Hawking radiation and the mean energy of the radiation particles being emitted by the black hole are both inversely proportional to the mass of the black hole. Thus, small black holes are expected to emit much more Hawking radiation than their more massive counterparts. I recently came across an online Hawking radiation calculator at http://xaonon.dyndns.org/hawking/.

I shall first touch on the use of small black holes as antimatter factories! Compared to ordinary matter particles, antimatter particles have the same mass but opposite charge. For example, the antimatter counterpart of an electron is a positron and it has a positive charge instead of a negative charge. On its own, antimatter is stable. However, when an antimatter particle meets an ordinary matter particle, they will annihilate with total conversion of matter to energy. The amount of energy produced when one gram of matter annihilates with one gram of antimatter is 3 times the amount of energy produced from the detonation of the Hiroshima atomic bomb!

Micro black holes can be used as antimatter factories since matter and antimatter are expected to be produced in equal quantities as black holes evaporate via the emission of Hawking radiation. Since the mean energy of the radiation particles being emitted increases as the mass of the black hole decreases, the production of more massive particles will require smaller black holes. For example, a black hole with a mass of 65 billion tons is optimal for the production of electrons and positrons. For the much heavier protons and antiprotons, a black hole with a much smaller mass of 35 million tons will be required.

Black holes of planetary mass can be used to create artificial planets by providing the source of mass necessary to generate the required amount of gravity. An artificial planet can be created by constructing a large spherical shell with the black hole in the center. For example, a spherical shell that is 12760 kilometers in diameter can be constructed around an Earth-mass black hole to form an artificial planet with Earth-like gravity on the external surface of the shell. In another example, a spherical shell that is 227000 kilometers in diameter can be constructed around a Jupiter-mass black hole to form an artificial planet that has over 300 times the surface area of the Earth, with Earth-like gravity on its external surface!

Such artificial worlds with Earth-like environments can range from hundreds of kilometers to hundreds of thousands of kilometers in diameter! This concept is especially useful in planetary systems with insufficient silicate and metallic elements to build solid planets, and hydrogen and helium from the gas giant planets or from the local star can be used as a source of mass to form the black hole. In addition, energy can be generated by dropping mass into the black hole located at the center of such an artificial world as the accretion of even a small amount of mass into the black hole is expected to generate a tremendous quantity of energy.

Now, I shall describe the evaporation of a small black hole with an initial mass of a billion metric tons and the amount of energy emitted by the black hole as it evaporates via the emission of Hawking radiation. The event horizon of this billion metric ton black hole is about the same size as the atomic nucleus of a hydrogen atom and it will have a luminosity of 356 million watts due to the emission of Hawking radiation, which is approximately twice the power output of a Nimitz-class aircraft carrier. This black hole will have a lifespan of over 2 and a half trillion years, which is much longer than the current age of the Universe.

As the black hole evaporates by emitting Hawking radiation over 2 and a half trillion years or so, it will eventually reach a mass of 10 million metric tons. At this mass, the size of the black hole’s event horizon is about 100 times smaller than the atomic nucleus of a hydrogen atom and it will have a luminosity of 3.56 trillion watts from the emission of Hawking radiation, which is roughly the average total power consumption of the entire United States in 2008. At this mass, the black hole still has a life span of another 2 and a half million years.

Now, I’ll fast forward until the black hole has just one year remaining. At this time, the black hole will have a mass of 72 thousand metric tons, an event horizon that is 15000 times smaller than the atomic nucleus of a hydrogen atom and it will shine with a luminosity of 68.5 thousand trillion watts, which is approximately 4000 times the average total power consumption of the human world in 2008! A black hole around this order of magnitude of mass can be use as a propulsive device to accelerate a spaceship to relativistic velocities, tens of thousands to hundreds of thousands of kilometers per second! This can be done by directing the high energy radiation particles emitted from the black hole to produce thrust and matter can also be fed into the black hole to sustain it.

As the black hole gets smaller and smaller, it will lose more and more of its mass in the form of Hawking radiation at an increasing rate. When the black hole reaches a remaining lifespan of 10 seconds, it will have a mass of 492 metric tons, a diameter of 1.46E-021 meters and a luminosity of 1.47E+021 watts. At this stage, the black hole is just over a million times smaller than the atomic nucleus of a hydrogen atom and in one second, it emits more energy than the detonation of 23 million Hiroshima atomic bombs!

Finally, when the black hole reaches the final second of its existence, it will have a mass of 22.8 metric tons, a diameter of 6.78E-022 meters and a luminosity of 6.84E+021 watts. At this stage, the black hole is over two million times smaller than the atomic nucleus of a hydrogen atom and in its final second, it will emit more energy than the detonation of 300 million Hiroshima atomic bombs! To further put it into perspective, the amount of energy emitted in the final second of the black hole’s existence is over 40 times the total worldwide energy consumption in 2008! You will certainly want to be very far away during the final moments of the black hole’s existence as it disappears in an incredible burst of energy!

All the values which I have used in this article to describe black holes were calculated using a program which I have developed a few years ago. It is interesting to note that there is a possible natural source for small black holes. A primordial black hole is a hypothetical type of black hole that is theorized to form out from the extreme densities present during the beginning of the Universe and these black holes are expected to be very low in mass. On way to detect such black holes is via their Hawking radiation, but none have been detected so far. A primordial black hole with a mass of 173 million metric tons will have a lifespan that is equal to the current age of the Universe and if such primordial black holes were created in sufficient numbers, their demise might be detectable as they emit an extraordinary burst of Hawking radiation in their final seconds. NASA’s Fermi Gamma-ray Space Telescope which was launched in 2008 might have the sensitivities necessary to detect the demise of primordial black holes if they exist.

Blasted Away

2010-03-13T16:12:00.002+08:00

A super-Earth is an extrasolar planet with a mass between roughly 1 to 10 times the mass of the Earth and our Solar System does not have such planets. A number of super-Earths have already been discovered around other stars. The four distinct types of materials that could make up a super-Earth with different proportions are iron alloys, silicates, volatiles/ices and hydrogen–helium gas. For a given mass, a less dense super-Earth will have a larger diameter while a denser super-Earth will have a smaller diameter. Thus, a pure hydrogen–helium gas planet will have the largest possible diameter while a pure iron planet with have the smallest possible diameter. However, the upper and lower limiting diameters for a super-Earth of a given mass are highly unlikely with regard to the physical processes involved in planet formation.

A recent paper entitled “Minimum Radii of Super-Earths: Constraints from Giant Impacts” examines the smallest possible diameter a super-Earth of a given mass could have. Therefore, volatiles/ices and hydrogen–helium gas are not considered and only rocky planets with an iron core and a silicate mantle are considered here. The only way to significantly increase the density of a planet requires the removal of the silicate mantle while preserving the iron core. The most effective way to do that is by the stripping of the silicate mantle by giant impacts.

An example of mantle stripping via collision in our own Solar System is the planet Mercury. By mass, Mercury is 70 percent iron and 30 percent silicate, while the Earth is one-third iron and two-thirds silicates and other materials. Proportional to its mass, Mercury has a higher iron content than any other planet in the Solar System. It is currently theorized that Mercury was initially over twice its current mass with an iron core and a substantial silicate mantle. A large object, roughly one-third Mercury’s current mass, struck the planet and stripped away much of the planet’s original crust and silicate mantle, leaving behind the iron core together with a thin layer of the original crust and silicate mantle.

The conclusions derived from this paper show that the collision stripping of mantle material is an effective mechanism in producing a super-Earth with a higher mean density by increasing the iron mass fraction. It is easier for the collision stripping of mantle material for a lower mass super-Earth to produce a large iron mass fraction as compared to a higher mass super-Earth.

However, even with the most extreme impact conditions, the collision stripping of mantle material from a super-Earth is still unable to produce anything close to a pure iron planet. The maximum mass of a super-Earth with over 70 percent iron by mass is most probably 5 Earth masses since its formation via the stripping of its silicate mantle by a giant impact requires an initial object of 10 Earth masses. The maximum mass of a super-Earth is expected to be around 10 times the mass of the Earth since a more massive planet will probably undergo runaway growth via accretion of hydrogen-helium gas and become an even more massive gas giant planet.

NASA’s Kepler space observatory will find a few hundred planets in the super-Earth mass regime and a sample of them will probably have masses too large for their observed diameters based on standard planet formation. The formation of such dense “cannonball” super-Earths can then be explained by the collision stripping of mantle material to produce a larger iron mass fraction. The original paper examining all these can be found at http://arxiv.org/abs/1003.0451v1.

Alien Earths

2010-03-06T22:52:00.003+08:00

A huge number of extrasolar planets similar to the Earth are expected to reside through the galaxy, orbiting stars not too different from our Sun. How will such Earth-like worlds differ from our own? A recent paper entitled “Habitable Climates: The Influence of Eccentricity” examines how factors such as obliquity, spin rate, orbital eccentricity, orbital semimajor axis and the fraction of surface covered by ocean might affect the habitability of Earth-like extrasolar planets. In this paper, regions of a planet that are at temperatures between 273 degrees Kelvin and 373 degrees Kelvin are considered habitable while regions outside that temperature range are considered uninhabitable.

Obliquity refers to the tilt of a planet’s axis, spin rate refers to the time required for a planet to complete one rotation about its axis, orbital eccentricity refers to how much a planet’s orbit around its star deviates from a perfect circle and orbital semimajor axis refers to the mean distance of a planet from its star. An orbital eccentricity of zero denotes a perfect circle and an orbital eccentricity of one denotes a parabola. The Earth for example, has an obliquity of 23.4 degrees, a spin rate of 24 hours, an orbital eccentricity of 0.0167 and an orbital semimajor axis of 149.6 million kilometers. In addition, the surface of the Earth is 70 percent ocean and 30 percent land.

Of all the extrasolar planets with measured orbital eccentricities, a large fraction of them have significant orbital eccentricities and this suggests that Earth-like planets in near circular orbits, like ours, probably represent only a small subset of potentially habitable worlds. This paper basically studies the numerous possible types of Earth-like planets and many of the models of Earth-like planets presented are particularly interesting!

Take for example, a desert planet with an obliquity of 90 degrees, an orbital semimajor axis of 1.225 AU and an orbital eccentricity of 0.2. Winter at the southern hemisphere of this planet occurs when the planet is furthest from its star and during this long winter, the southern pole freezes and reaches an incredibly cold temperature of minus 120 degrees Centigrade! For this planet, the southern pole becomes transiently habitable only during northern winter when the planet is closest to its star. The southern pole of this planet experiences the most extreme temperature variations. During southern winter, the planet is furthest from its star and the southern pole experiences perpetual darkness. During southern summer, the planet is closest to its star and the southern pole experiences perpetual daylight.

Imagine an Earth-like planet whose spin axis is tilted 90 degrees, such that the entire northern hemisphere can be in constant daylight while the entire southern hemisphere can be in constant darkness and vice versa, during specific points of the planet’s orbits around its star. Imagine a world where one day has a length of 8 hours or another where one day has a length of 72 hours! Imagine a planet whose highly eccentric orbits around its star brings it from a distance where most of its surface is scorching hot out to a distance where most of the planet’s surface is in a “deep freeze.” This paper can be obtained at http://arxiv1.library.cornell.edu/abs/1002.4875 and it will show you the many types of Earth-like worlds that can exist!

Dissipating World

2010-02-27T17:02:00.002+08:00

A newly published paper entitled “WASP-12b as a Prolate, Inflated and Disrupting Planet from Tidal Dissipation” describes some interesting properties of an extrasolar planet. WASP-12b is an extrasolar gas giant planet that has 1.4 times the mass of Jupiter and it is located so incredibly close to its parent star that it takes only a little over an Earth day to orbit the star! In fact, WASP-12b orbits its parent star at a distance of just 2 stellar radii from the star’s surface and the planet is distorted by the star’s gravity into a prolate shape, similar to that of a rugby ball.

The nonzero orbital eccentricity of WASP-12b makes it very susceptible to the strong tidal effects from its parent star which heats the planet’s interior and causes the planet to expand. WASP-12b has an orbital eccentricity of about 0.05 and this is odd because orbital circularization should have already circularized its orbit into a zero eccentricity orbit. Another planet with a mass of a few Earths is probably responsible for perturbing WASP-12b to maintain its nonzero orbital eccentricity. WASP-12b is extremely “bloated” and it has a diameter that is 80 percent larger than Jupiter’s. The atmospheric temperature of WASP-12b is estimated to be a scorching 2500 degrees Kelvin!

WASP-12b has ballooned so much that its own gravity is unable to retain its mass from the gravitational pull of its parent star. Gas from WASP-12b is flowing towards the parent star through a nozzle that is located at the L1 Lagrangian point, a region that is situated between the planet and the star. WASP-12b is losing mass to its host star at a rate of a few billion metric tons each second. The material that is pulled off from WASP-12b forms an accretion disk around the parent star and gradually spiral inwards into the star.

Another interesting paper which I read yesterday is entitled “Forming Low Mass Stars and Brown Dwarfs in Protoplanetary Disks of Very Massive Stars.” The stars considered in this paper are very massive stars that have over 100 times the mass of the Sun. The basic assumption here is that objects with ever larger masses can form via fragmentation of the protoplanetary disks of increasingly massive stars. Thus, objects beyond the planetary mass regime such as brown dwarfs and low mass stars can form via fragmentation of the protoplanetary disks of very massive stars. Such objects are expected to have orbital separations ranging from about a hundred to thousands of times the Earth-Sun distance from very massive stars. This is because the formation of such objects via fragmentation of the protoplanetary disks of very massive stars does not occur at smaller orbital separations.

This paper also considers the possibility that extremely metal-poor stars in the galaxy might have originated from fragmentation of the protoplanetary disks of the first stars in the Universe which are expected to be extremely massive. Metal-poor means that the proportion of the star’s mass that is not hydrogen and helium is exceedingly small with respect to that for our Sun; on the order of a million times smaller than the Sun’s. The first stars in the Universe are thought to be extremely metal-poor and very massive. Very massive stars explode eventually and the brown dwarfs and low mass stars that formed out of the protoplanetary disks of such a star can become unbound if a sufficient mass from the star is lost in its explosion.

Intergalactic Space

2010-02-24T15:05:00.002+08:00

This week, I researched on the Local Group, covering areas such as the morphological structures of the different galaxies, the satellite galaxies of the Andromeda and Milky Way galaxies, the motion of galaxies and the past and future of the Local Group. Over here, I shall just briefly define what the Local Group is and some of its basic properties. The Local Group contains dozens of galaxies contained within a volume about 10 million light years across and it is dominated by the Andromeda Galaxy and the Milky Way Galaxy which together account for over 80 percent of the visible light of the Local group.

Within the Local Group, many more tiny galaxies are still probably waiting to be discovered. The third most prominent galaxy in the Local Group is the Triangulum Galaxy and it has approximately one-fifth the luminosity of the Milky Way Galaxy. The rest of the galaxies in the Local Group are irregular galaxies, dwarf irregular galaxies, dwarf elliptical galaxies and dwarf spherical galaxies. Many of these smaller galaxies are either in orbit around the Andromeda Galaxy or the Milky Way Galaxy.

The mutual gravitational attraction from matter within the Local Group dominates over the general expansion of the Universe. The galaxies in the Local Group range in size from the Andromeda and Milky Way galaxies which contain hundreds of billions of stars each to tiny galaxies which hold fewer than a million stars. The Andromeda Galaxy and the Milky Way Galaxy are separated by a distance of 2.5 million light years and they are approaching each other at a rate which will most probably result in a collision in the next few billion years. Such a merger will result in the formation of a giant elliptical galaxy. It is very unlikely that stars will collide when both galaxies merge as stars in galaxies are spaced extremely far apart from each other.

Yesterday, I read a few papers about the Boötes Void and the very few galaxies located within this immense void. The Boötes Void is one of the largest known voids in the Universe and it is estimated to be 400 million light years across. For comparison, the Milky Way Galaxy is 100000 light years across and you’ll need 4000 Milky Way Galaxies placed end to end to cover the span of the Boötes Void. Within this incredibly vast emptiness, several galaxies have been detected to extend in a rough tube-shape through the middle of the void. An astronomer named Greg Aldering once said, “If the Milky Way had been in the center of the Boötes void, we wouldn’t have known there were other galaxies until the 1960s.”

Evolved Binaries

2010-02-19T19:19:00.002+08:00

NASA’s Kepler space observatory was launched on 6 March 2009 with the primary purpose of detecting Earth-like planets orbiting other stars. Among Kepler’s first major discoveries are two small objects which are hotter than the stars they orbit and both objects are likely to be very low mass white dwarfs. KOI-81b is estimated to have 40 percent the luminosity of the Sun and a surface temperature of 13000 degrees Kelvin while KOI-74b is estimated to have 3 percent the luminosity of the Sun and a surface temperature of 12000 degrees Kelvin. These objects are definitely not planets as they have surface temperatures an order of magnitude higher than would be consistent with stellar heating alone.

A newly published paper entitled “Transits and Lensing by Compact Objects in the Kepler Field: Disrupted Stars Orbiting Blue Stragglers” describes the prospects of Kepler in discovering a large number of such objects as described above. This paper can be obtained at http://arxiv.org/abs/1002.3009v1. These hot compact objects are most likely the cores of stars that have evolved to their present state through a process of stable mass transfer with their current stellar companions. The Kepler space observatory offers a unique opportunity to study a large sample of such white dwarfs and the role of mass transfer in the evolution of binary star systems.

Cloudy Planets

2010-02-16T15:28:00.003+08:00

Today, I read a newly published paper entitled “Clouds in the Atmospheres of Extrasolar Planets.” This paper studies the impact of clouds on the surface temperatures of Earth-like planets orbiting different types of stars. Clouds play a vital role in determining the climatic conditions of planetary atmospheres by reflecting incident stellar radiation back into space via the albedo effect or by trapping infrared radiation in the atmosphere via the greenhouse effect. For the Earth, low-level clouds cause cooling via the albedo effect and high-level clouds cause heating via the greenhouse effect. For mid-level clouds, the albedo effect and greenhouse effect generally balance each other.

In this paper, studies were done for Earth-like planets orbiting F, G, K and M-type stars. Note that our Sun is a G-type star and for this range of stellar types, F-type stars are the hottest while M-type stars are the coolest. With respect to a clear sky model, a planet with a higher percentage of high-level clouds will have a higher surface temperature while a planet with a higher percentage of low-level clouds will have a lower surface temperature. This characteristic applies to all Earth-like planets orbiting F, G, K and M-type stars. For the Earth, the presence of clouds creates a net cooling effect with respect to the clear sky model and the mean value of the Earth’s surface temperature is 288.4 degrees Kelvin.

Using just low-level clouds for maximum cooling effect, planets can be located up to 15 percent closer to their parent star compared to a clear sky planet to achieve the mean Earth’s surface temperature of 288.4 degrees Kelvin. On the contrary, using just high-level clouds for maximum heating effect, planets can be located up to 35 percent further from their parent star compared to a clear sky planet to achieve the mean Earth’s surface temperature. To read more about this study, you can obtain the paper from http://arxiv.org/abs/1002.2927v1.

Stellar Exotica

2010-02-13T12:10:00.012+08:00

The first stars that form in the early Universe can be powered by dark matter heating instead of nuclear fusion and these stars are termed dark stars. These dark stars still remain to be discovered and future telescopes such as the James Webb Space Telescope (JWST) may have the sensitivities required to detect such stars. The term “dark” does not mean that these stars appear dark, but rather it means that the energy from the annihilation of dark matter particles is the primary mechanism keeping these stars shining in hydrostatic equilibrium. The recently published paper entitled “Supermassive Dark Stars - Detectable in JWST” basically described such stars and their detectability by the JWST.

Weakly Interacting Massive Particles (WIMPs), an excellent candidate for dark matter, may be their own antiparticles and this allows them to annihilate among themselves wherever the density is sufficiently high, providing a heat source that can power such dark stars for millions to billions of years. The WIMP annihilation products thermalize within the star, providing the energy required to keep the star shining in hydrostatic equilibrium. Dark stars are made primarily of hydrogen and helium, with only a fraction of a percent of their mass in the form of dark matter. Due to the high efficiency of energy production via WIMP annihilation, a small proportion of it is sufficient to sustain the star against its own gravity over astronomical timescales.

The much cooler surface temperatures of dark stars is the primary reason which allows darks stars to grow to become so much more massive than ordinary fusion powered stars, as long as heating from the annihilation of dark matter persists. Ordinary fusion powered stars produce ionizing photons that provide a variety of feedback mechanisms that cut off further accretion of baryonic matter, whereas dark stars, with their cooler surface temperatures, allow the continued accretion of baryonic matter all the way up to colossal stellar masses. Dark stars can grow up to millions of times the mass of the Sun and exceed billions of times the luminosity of the Sun. Such massive stars can lead to the formation of supermassive black holes when they run out of dark matter fuel to sustain them, causing them to contract, heat up and undergo a brief phase as fusion powered stars before collapsing into massive black holes.

The titles of some interesting papers that I’ve read recently are “Irregular Satellites of Jupiter - Capture Configurations of Binary Asteroids”, “Capture and Annihilation of Dark Matter in Milky Way Globular Clusters”, “The Observed Infall of Galaxies towards the Virgo Cluster” and “M 54 + Sagittarius = Omega Centauri.” All these papers can be downloaded from http://arxiv.org/.

On Thursday night, I watched the live telecast launch of NASA’s Solar Dynamic Observatory (SDO) online from NASA Television. The SDO was launched on Thursday, 11 February 2010 at 10:23 a.m. EST (03:23 p.m. UTC) onboard an Atlas V rocket. The primary mission of the SDO is scheduled to last 5 years and 3 months, with expendables expected to last for 10 years. For more information about the SDO, visit a previous post of mine at http://ultimate-infusion.blogspot.com/2010/02/nearest-star.html or visit http://sdo.gsfc.nasa.gov/.

Runaway Star

2010-02-10T20:47:00.003+08:00

Hypervelocity stars are a class of extreme velocity stars that travel at such an extreme velocity with respect to the Milky Way’s rest frame that their velocity exceeds the Milky Way’s local escape velocity. Gravitational interactions and supernova explosions in binary systems are two possible mechanisms to produce such “runaway stars.” Today, I read a newly published paper entitled “The Nature of the Hyper-Runaway Candidate HIP 60350” and it is about a hypervelocity star called HIP 60350. Kinematic observations of HIP 60350 have shown that it has a velocity which slightly exceeds the Milky Way’s local escape velocity and that it is probably moving out of the Milky Way Galaxy on an unbound trajectory.

Tracing its origin back to the plane of the Milky Way Galaxy, the estimated travel time of HIP 60350 is about 14 million years. Given an estimated ago of 45 million years for the star, a shorter travel time is consistent with the runaway nature of the star. So far, neither gravitational interaction nor a supernova explosion in a binary system can be strictly confirmed or rejected for HIP60350. The paper mentioned above can be obtained from http://arxiv.org/abs/1002.1848v1. In addition, a neutron star that is created from an asymmetric supernova explosion can also achieve such hypervelocity speeds and be on an unbound runaway trajectory from the Milky Way Galaxy. One such neutron star is RX J0822-4300 and it was measured to move at a record speed of over 1300 kilometers per second!

NASA’s Space Shuttle Endeavour took off from Kennedy Space Center on Monday, 8 February 2010 at 04:14 EST (09:14 UTC) for a mission to the International Space Station. This mission is designated STS-130 and its primary payloads are the Tranquility module and the Cupola. The Cupola is an observation deck which will offer astronauts a panoramic view of space and of the Earth from within the International Space Station. In addition, the Cupola is also the best and largest set of windows to have ever been flown in space. Visit http://www.nasa.gov/ for the latest in news!

Solstice Mission

2010-02-05T18:05:00.001+08:00

NASA’s Cassini mission has been extended to explore Saturn and its moons to 2017! NASA’s fiscal year 2011 budget provides US$60 million per year for the extended mission. Since Cassini arrived at Saturn in 2004, it has been sending back incredible data for nearly 6 years! The Cassini mission was scheduled to end in 2008, but the mission received a 27 month extension to September 2010 and this mission extension to 2017 calls for an additional 155 orbits around Saturn, 54 flybys of Titan and 11 flybys of the icy moon Enceladus. This extended mission is named the Cassini Solstice Mission and you can find out more about it at http://saturn.jpl.nasa.gov/.

This week, I was at the Singapore Airshow 2010 on a 4 day trade visitor pass and I took hundreds of pictures and videos at the event. On Thursday, 4 February 2010, a South Korean pilot steered his T-50 Golden Eagle jet too close to spectators which caused the remaining part of the flying display to be aborted. Anyway, I managed to take the entire video of the incredibly close flyby of the T-50 Golden Eagle! During the Singapore Airshow 2010, I sat in various aircraft and collected numerous brochures, catalogues, journals, posters and other kinds of publications. I have posted some pictures taken at the Singapore Airshow 2010 at http://alien-biosphere.blogspot.com/.

Nearest Star

2010-02-01T18:23:00.003+08:00

Another ordinary star amongst hundreds of billions of other stars in the vast Milky Way Galaxy… Composed of hydrogen, helium and trace amounts of other elements, the Sun is the Earth’s closest star and it is located in the center of the Solar System. The Sun is 1.392 million kilometers in diameter, has 332900 times the mass of the Earth, a volume which can fit 1.3 million Earths, a surface temperature of 5778 degrees Kelvin and a core temperature of 15.7 million degrees Kelvin. The amount of energy the Sun emits in just one second is approximately one million times the total worldwide energy consumption in the year 2008! Each second, 700 million tons of hydrogen is converted into helium in the core of the Sun. The Sun has been doing this for the past 5 billion years and will continue to do so for another 5 billion years!

NASA’s Solar Dynamics Observatory (SDO) is scheduled for launch on Tuesday, 9 February 2010 from Cape Canaveral Air Force Station in Florida. The SDO will be launched into space onboard an Atlas V rocket and it will be placed into an inclined geosynchronous orbit around the Earth, about 36000 kilometers above the Earth’s surface. Any object in such an orbit will have an orbital period that matches the Earth’s rotational period, thus maintaining approximately the same position relative to the Earth’s surface. This orbit allows the SDO to maintain a continuous science data downlink rate of well over 100 Megabits per second (Mbps) to a dedicated ground station in New Mexico.

The instruments carried onboard the SDO are the Extreme Ultraviolet Variability Experiment (EVE) (http://lasp.colorado.edu/eve/), the Helioseismic and Magnetic Imager (HMI) (http://hmi.stanford.edu/) and the Atmospheric Imaging Assembly (AIA) (http://aia.lmsal.com/). The SDO will observe the Sun with unprecedented sensitivity and observational cadence, with the goal of developing the necessary scientific understanding to effectively address the effects of the Sun on the Earth’s environment and how it affects life and technology. The SDO will generate approximately 1.5 Terabytes of data per day. Visit the main site for this mission at http://sdo.gsfc.nasa.gov/.

Sunday, 7 February 2010 is the scheduled launch date for NASA’s Space Shuttle Endeavour for a mission to the International Space Station. This is just two days before the scheduled launch date for NASA’s Solar Dynamics Observatory! Space Shuttle Endeavour will be launched from Kennedy Space Center Launch Complex 39A in Florida. This shuttle mission is designated STS-130 and it will deliver the Tranquility module and the Cupola to the International Space Station. Visit http://www.nasa.gov/ for more information about this mission.

This week, I’ll be at the Singapore Airshow 2010 as a trade visitor on Wednesday, Thursday and Friday, from 9:30am to 5:00pm each day. The Singapore Airshow 2010 will be held at the Changi Exhibition Center (CEC) from 2 to 7 February 2010. The first 4 days are exclusively for trade attendees while the final 2 weekend days are open to the public. Visit http://www.singaporeairshow.com.sg/ for more information about the Singapore Airshow 2010.

High Flyer

2010-01-30T00:15:00.001+08:00

The International Space Station (ISS) measures almost 110 meters across and it is by far the largest artificial satellite in orbit around the Earth. The ISS orbits the Earth at an altitude of well over 300 kilometers above the Earth’s surface, at an average orbital velocity of almost 8 kilometers per second and taking just over 90 minutes to complete an orbit around the Earth. On Tuesday, 26 January 2010 and on Thursday, 28 January 2010, I managed to observe the ISS passing overhead. On both occasions, the ISS passed directly above Singapore and it only takes a few minutes for the ISS to move across the sky.

My observation of the ISS on Tuesday, 26 January 2010 occurred at around 8:30pm. I caught first glimpse of the ISS while looking in the northwesterly direction and I followed the ISS until it disappeared into the Earth’s shadow, seconds after it made closest approach to the zenith. The ISS appeared like a brilliant white star as it crossed the evening sky. In fact, the ISS is many times brighter than even the brightest star in the nigh sky and most of its brightness is due to the reflection of sunlight off its huge solar array wings.

My second observation of the ISS on Thursday, 28 January 2010 took place at around 7:30pm where I observed the ISS rose from the northwest until it set in the southeast. The entire passage of the ISS across the sky lasted a few minutes and it was clearly visible throughout. There were some scattered clouds in the western sky that evening and it was interesting to watch the ISS fading and brightening as it passed behind the clouds. On this occasion, I managed to record the entire passage of the ISS on video!

A few days ago, I wrote about two interesting objects which were discovered by NASA’s Kepler space observatory. The two objects are called KOI-74b and KOI-81b, and the paper entitled “Kepler Observations of Transiting Hot Compact Objects” describes the discovery, but was unable to determine what these two objects are. However, a recent paper entitled “A Paradox Resolved using Observations of Doppler Boosting in Kepler Lightcurves” describes the newly inferred masses for KOI-74b and KOI-81b, and demonstrated that both objects are likely to be very low mass white dwarfs.

Lunar Terrain

2010-01-26T18:42:00.003+08:00

I shall describe a typical period of observations of the Earth’s Moon through my telescope! Just past midnight on Monday, 25 January 2010, I went about observing the surface of the Moon through my telescope, with a lot of observations carried out at very high magnifications. At that time, the day-night terminator is located at a longitude of about 20 degrees west on the Moon and so, the surface of the Moon west of that longitude is dark while the surface of the Moon east of that longitude is illuminated and experiencing day. The Moon keeps nearly the same face pointed towards the Earth at all times and the side of the Moon that faces Earth is called the near side, while the opposite side is called the far side. Small variations in the angle from which the Moon is seen from the Earth allows small areas of the far side to be visible.

The lunar maria (singular: mare) are large basaltic plains on the Moon that were formed when molten basalt from ancient volcanic eruptions flowed into low lying impact basins and solidified. I could easily identify Mare Imbrium (Sea of Rains), Mare Tranquillitatis (Sea of Tranquility), Mare Serenitatis (Sea of Serenity), Mare Fecunditatis (Sea of Fertility), Mare Nectaris (Sea of Nectar), Mare Crisium (Sea of Crises), Mare Frigoris (Sea of Cold), Mare Nubium (Sea of Clouds) and Mare Vaporum (Sea of Vapors). The lunar maria are made up of less reflective material and this makes them appear darker than the surrounding lunar landscape. In fact, the lunar maria are found almost exclusively on the near side of the Moon. The largest mare on the Moon is Oceanus Procellarum (Ocean of Storms) and it was not visible at the time of my observation as it was still hidden in the darkness of lunar night.

My first stop was Mare Imbrium, a vast basaltic plain located in the northern hemisphere of the Moon. Only the eastern half of Mare Imbrium was visible as the western half was still in lunar night. Rimming the eastern edge of Mare Imbrium are some impressive mountain ranges. Through my telescope, I could see the mountain ranges in great detail and the small oblique angles at which the Sun illuminated that region brought out the contrasts in amazing detail! I focused primarily on the Apennine Mountains, located along the southwestern edge of Mare Imbrium, and the Caucasus Mountains, located along the eastern edge. NASA’s Apollo 15 mission landed near the Apennine Mountains on 26 July 1971 and some of these mountains actually formed the backgrounds for many of the pictures taken on the surface of the Moon! On this mission, astronauts David Scott and James Irwin spent three days on the surface of the Moon and this was also the mission when Scott simultaneously dropped a falcon’s feather and a hammer to prove Galileo Galilei’s theory that in a vacuum, objects of different mass fall at the same rate.

Moving west from the great mountains, still within Mare Imbrium, I came to three prominent craters named Archimedes, Aristillus and Autolycus, and they appeared in remarkable “crispy clear” detail through my telescope. Scattered across the vast basaltic plains of Mare Imbrium are numerous other craters. Near the day-night terminator towards the western part of Mare Imbrium, the elevated rims of these craters cast long shadows across the lunar landscape.

A very striking feature on the surface of the Moon is an impact crater named Copernicus and it is located at the easternmost edge of Oceanus Procellarum, south of Mare Imbrium. During my observations, Copernicus was lying along the day-night terminator and the glancing angles at which the Sun’s rays made with respect to the crater made the entire scene an exceptionally breathtaking sight. The floor of the crater was entirely in darkness as the Sun hasn’t risen over the rim of the crater. The western half of the inner wall of the crater’s rim was illuminated by the Sun and it looked surreal as it rose above the dark crater floor. Copernicus is an impact crater that is almost 100 kilometers across and the terraced structure of the inner wall of the crater’s rim was clearly visible through my telescope.

Going down south, past the lunar equator to the southern hemisphere of the Moon, I came to Mare Nubium. Located in the southeastern part of Mare Nubium is a linear fault named Rupes Recta, or more commonly called the “Straight Wall.” The fault is 110 kilometers in length, 2 to 3 kilometers in width and 200 to 300 meters in height. Whenever the Sun illuminates the fault at a glancing oblique angle, Rupes Recta will cast a wide shadow which gives it the appearance of a steep cliff even though grade of the slope is relatively shallow. However, at the time of my observations, the Sun was already rather high in the sky at the location of Rupes Recta and there was hardly any shadow.

Continuing south from the “Straight Wall”, I observed the contrasting shadowed and illuminated rims of craters such as Pitatus, Wurzelbauer, Tycho, Maginus, Longomontanus and numerous other impact craters. Located in the southern highlands of the Moon, south of the crater Tycho, is an interesting crater named Clavius. One of the most remarkable features of Clavius is a curving chain of craters located completely within Clavius itself. The chain of craters arc across the floor of Clavius and form a sequence of ever smaller crater diameters. Even the smallest crater comprising this chain was clearly visible through my telescope. With a diameter of 225 kilometers, Clavius is the third largest crater on the near side of the Moon.

The best regions on the Moon to explore through a telescope are those that are located near to the day-night terminator at the time of observation. This is because the small grazing angle at which the Sun illuminates these regions brings out more contrast. Close enough to the day-night terminator, even small surface undulations can cast incredibly long shadows across the lunar surface. The best times to observe surface morphological features of the Moon are from crescent phase through first quarter phase to waxing gibbous phase and from waning gibbous phase through last quarter phase to crescent phase. During a full moon and the few days before and after the full moon, are the worst times to observe morphological features on the lunar surface.

That orbed maiden
With white fire laden
Whom mortals call the Moon
- Percy Bythe Shelley

Yesterday, I created a program for performance analysis for both thermoelectric generation and thermoelectric refrigeration systems. I played around with numerous coefficients such as Seebeck coefficients, thermal conductance, thermal conductivity, electrical resistance, electrical resistivity, the thermoelectric figure of merit for couple and etc. I even did computations for real world radioisotope thermoelectric generators and the results agreed very well. I also did computations for various materials such as copper, constantan, gold, cadmium, platinum and various semiconductor materials.

Twinkle Twinkle

2010-01-23T01:49:00.013+08:00

Shining like a brilliant red star in the constellation Cancer is the planet Mars. During the wee hours of Friday morning, I set up my telescope and started observing astronomical objects in the region of the sky around the constellations Orion and Gemini. I switched my telescope’s eyepieces for higher magnification and started observing the planet Mars. Surface features such as the northern polar ice cap, Valles Marineris, Acidalia Planitia and various other albedo features were clearly visible through my telescope. The northern polar ice cap of Mars appeared brilliantly white, in stark contrast to the vast Martian desert.

The northern hemisphere of Mars is currently experiencing spring while the southern hemisphere of Mars is currently experiencing autumn. Like the Earth, Mars also has seasons and currently, the Sun never sets on the northern most regions of Mars as Mars crossed Vernal Equinox on October 2009 while Autumnal Equinox falls on November 2010. The seasons on Mars are approximately twice the duration of those on the Earth since Mars takes almost twice as long as the Earth to orbit the Sun. All these observations of Mars were done on Friday, 22 January 2010, at around 0300 hrs Singapore time (1900 hrs UTC, Thursday, 21 January 2010). At that time, Mars was 99.9 million kilometers away from the Earth and viewing Mars will be like viewing a tennis ball from a distance of one kilometer.

Which maketh Arcturus, Orion, and Pleiades, and the chambers of the south.

- Job 9:9 (King James Version)

Canst thou bind the sweet influences of Pleiades, or loose the bands of Orion?

- Job 38:31 (King James Version)

During that night, I also viewed objects such as the Beehive Cluster, the Orion Nebula and the Pleiades. I also went “star-hopping” around the constellations Taurus, Orion, Gemini, Canis Major, Cancer and Auriga. Bright stars like Sirius, Betelgeuse, Rigel, Aldebaran, Pollux and Castor are blazingly stunning when viewed through a telescope!

NASA’s Kepler space observatory was designed to find transiting Earth-like planets in the habitable zones of other stars and it was recently announced to have detected two unusual planet-size transiting objects designated KOI-74b and KOI-81b. KOI-74b is in 5.19 day orbit around an A-type star and KOI-81b is in a 23.9 day orbit around a B-type star. KOI-74b and KOI-81b orbit stars with surface temperatures of 9400 degrees Kelvin and 10000 degrees Kelvin respectively, and each of the stars have over twice the mass of the Sun and tens of times the luminosity of the Sun.

For both KOI-74b and KOI-81b, the depth of the occultation lightcurve is deeper than the depth of the transit lightcurve! A transit is an event where an object crosses in front of the star it orbits and an occultation is an event where an object crosses behind the star it orbits. Therefore, for the objects KOI-74b and KOI-081b, a deeper occultation lightcurve will mean that the surfaces of these two planet-size objects are hotter than the surfaces of their stars!

KOI-74b is about 8 to 9 times the diameter of the Earth and its estimated surface temperature is over 12000 degrees Kelvin. On the other hand, KOI-81b is about 25 times the diameter of the Earth and its estimated surface temperature is over 13000 degrees Kelvin. In comparison, the planet Jupiter is 11 times the diameter of the Earth and the Sun is 109 times the diameter of the Earth. KOI-74b and KOI-81b are both very interesting objects, and confirmation of their masses with radial velocity measurements, together with follow up observations as well as continued observations by Kepler, will help to unravel their nature. You can read the paper detailing this discovery at http://arxiv.org/abs/1001.3420v1.

Focused Worlds

2010-01-20T22:14:00.001+08:00

EUCLID is a proposed dark energy probe and microlensing planet hunter that is currently under study by the European Space Agency (ESA). The remarkable synergy between the study of dark energy via cosmic shear measurements and the detection of extrasolar planets via gravitational microlensing enables the use of similar instruments for both fields, that is, a wide-field telescope optimized for infrared observing. EUCLID will be a space-based telescope with a 1.2 meter aperture primary mirror and it will have three instruments - a visible imaging channel, a near infrared photometric channel and a near infrared spectroscopic channel. I shall just describe the planet detection part of this proposed mission.

Of the hundreds of extrasolar planets that have been discovered so far, almost all were found using radial velocity and stellar transits, and only a handful can be attributed to detection via gravitational microlensing. However, gravitational microlensing allows the detection of extrasolar planets in the parameter space that is not easily accessible to radial velocity and stellar transits. Employing gravitational microlensing, EUCLID will be able to detect planets down to 10 percent the mass of the Earth, in all orbits starting from approximately half the Earth-Sun distance to free-floating planets wandering in interstellar space. A significant number of planets are expected to be ejected during the formation of planetary systems to produce free-floating planets that do not orbit any star. EUCLID will also be sensitive to planets orbiting all types of stars, as well as white dwarfs stars, neutron stars and black holes!

Part of the parameter space of EUCLID will overlap with that of the current Kepler mission, where both will be able to detect Earth-like planets in the habitable zones of Sun-like stars. Combined with data from current missions such as Kepler and ground based radial velocity and stellar transits observations, EUCLID will give a comprehensive census of all types of extrasolar planets down to well below the mass of the Earth. A very similar mission to EUCLID is NASA’s proposed Microlensing Planet Finder (MPF).

Orion’s Arm is a website which explores ideas which are regarded as science fiction today and it projects what the future might look like, ranging from near future interplanetary spaceflight to the far future where the galaxy is ruled by vast ascended intelligences. However, the ideas and explanations explored have to be scientifically plausible at every level. Therefore, ground rules such as matter cannot travel faster than light and the conservation of matter and energy are upheld. Even the most fantastic elements within the setting must be provided with logical explanations. The homepage of Orion’s Arm is http://www.orionsarm.com/.

I came across two very interesting concepts which I have researched on a few years ago while browsing through Orion’s Arm. On the website itself, these two concepts are entitled Bank Orbitals and Artificial Planets. The article on Bank Orbitals can be read at http://www.orionsarm.com/eg-article/4845ef5c4ca7c and the article for Artificial Planets can be read at http://www.orionsarm.com/eg-article/49a3ee435bd98.

Here are the titles of some interesting papers which I came across this week - “The Masses and Radii of HD186753B and TYC7096-222-1B - The First M-dwarfs known to Eclipse A-type Stars”, “Episodic Accretion on to Strongly Magnetic Stars”, “Thermal Emission and Tidal Heating of the Heavy and Eccentric Planet XO-3b”, “The Survival of Water within Extrasolar Minor Planets”, “The Microlensing Planet Finder - Completing the Census of Extrasolar Planets in the Milky Way” and “EUCLID - Dark Universe Probe and Microlensing planet Hunter.”

Uncharted Heavens

2010-01-14T19:22:00.002+08:00

Rocket propulsion! This week, I derived 12 pages of equations which describe in detail, the performance and design parameters of rocket nozzles. I also wrote a program and computed the thrust, thrust coefficient, effective exhaust velocity and specific impulse for various rocket nozzle designs as a function of ambient pressure ranging from sea-level to vacuum conditions. This is extremely useful for evaluating and comparing the performance of various rocket nozzle designs!

Dusk crept across the surface of Oceanus Procellarum - a vast basaltic plain located on the western corner of the near side of the Earth’s Moon. During the wee hours of Sunday morning, 10 January 2010, the night sky was exceptionally clear and I brought out my telescope and started observing the lunar landscape. The features of the lunar landscape appeared crispy clear through my telescope’s optics. The Moon is an interesting world with a lot that has yet to be discovered and both the Lunar Reconnaissance Orbiter (LRO) and the Lunar Crater Observation and Sensing Satellite (LCROSS) missions have made spectacular discoveries about the Moon that are reshaping our perspectives of the Moon. Visit http://lunar.gsfc.nasa.gov/ and http://www.nasa.gov/mission_pages/LCROSS/main/ to know more about these missions.

I also explored the region of the sky where the constellations Orion and Gemini are. While looking at Betelgeuse, I placed my palm over the eyepiece of my telescope and focused the starlight from Betelgeuse onto a spot on my palm! The light from Betelgeuse is much “redder” when compared to the other bright stars in Orion. Betelgeuse is a red supergiant star that is 100000 times more luminous than the Sun and it is located over 600 light years away. Hundreds of millions of Suns could fit within the volume of Betelgeuse and if it were placed at the center of our Solar System, its surface would extend out to between the orbits of Mars and Jupiter, engulfing Mercury, Venus, Earth and Mars! I read a newly published paper entitled “Imaging the spotty surface of Betelgeuse in the H band” and it details the discovery of two gigantic bright spots which cover a large fraction of Betelgeuse’s surface. The two bright spots appear to be convective in nature and they are much hotter with respect to the average temperature of the star.

Planets are typically known to form in protoplanetary disks around their newly formed host stars. However, another planetary formation mechanism can occur in evolved binary star systems where mass transferred from the evolved star to its companion star could form a circumstellar disk around the companion star that could resemble a protoplanetary disk in many ways and produce a second generation of planets. In addition, mass transfer in close separation binary star systems can also produce circumbinary disks of material, producing an environment suitable for formation of circumbinary planets.

Such scenarios are likely to have major effects on pre-existing planetary systems, possibly leading to the re-growth or rejuvenation of the pre-existing planets and even creating another epoch of planetary migration. These pre-existing first generation planets can grow to become much more massive than ordinary planets and they can even enter the mass regime of brown dwarfs. Observations of two co-existing misaligned planetary systems or even counter rotating planets in the same system will serve as a good example of second generation planetary formation with pre-existing first generation planets.

Second generation planets are expected to appear much younger than their host stars and their composition is likely to be different relative to that of typical first generation planets. Second generation planets are expected to exist in evolved binary systems, especially in a binary system comprising of a white dwarf star and a main sequence star or a binary system comprising of a pair of white dwarf stars. Further evolution of the stars in evolved binary systems could even lead to subsequent generations of planets! To get a more detailed explanation, visit http://arxiv.org/abs/1001.0581v1 for the paper entitled “Second Generation Planets.”

Unknown Horizons

2010-01-08T20:06:00.001+08:00

Titan Mare Explorer (TiME) is a proposed mission to Saturn’s moon Titan which will explore one of the methane seas located in the northern region of the moon, making it the first exploration of an extra-terrestrial sea. As of now, TiME is still in its conceptual phase and if selected, its intended launch window is scheduled to be in January 2016, with arrival at Titan expected on June 2022. TiME is targeted for “splashdown” on Ligeia Mare, with Kraken Mare as the backup target; both of which are located in the northern region of Titan and they are the largest bodies of liquid identified to date on Titan’s surface. After “splashdown”, the wind is expected to push this buoyant nautical craft around the sea’s surface, since the craft is not self-propelled.

The science objectives of the TiME mission are:

1. Determine the chemistry of seas to constrain Titan's methane cycle, look for patterns in the abundance of constituents in the liquids and analyze noble gases.

2. Determine the depth of the Titan’s seas to determine sea volumes, and thus, organic inventory.

3. Investigate Titan’s lake-related processes by characterizing physical properties of its liquid seas, and how they vary with depth.

4. Determine how the local meteorology over the seas ties to the global cycling of methane on seasonal and longer timescales.

5. Analyze the nature of the sea surface and if possible, shorelines, to determine physical properties of sea liquids and better understand origin, evolution, and subsurface methane/ethane hydrology of Titan lakes and seas.

Titan’s great distance from the Sun and its thick atmosphere rules out the use of solar panels, while batteries would only provide a few hours of power at most. Therefore, TiME is proposed to be powered by an advanced Stirling radioisotope generator (ASRG) which converts heat from radioactive plutonium into mechanical energy with a very high rate of efficiency, over a period of several years. TiME is an extremely interesting proposal and I’ll definitely like to see this mission takeoff and return the first pictures of an extra-terrestrial sea!

NASA’s Kepler mission announced the discovery of 5 new extrasolar planets on 4 January 2010 at the American Astronomical Society meeting in Washington DC. The planets are named Kepler-4b, Kepler-5b, Kepler-6b, Kepler-7b and Kepler-8b respectively and they are all Jupiter-size planets with the exception of Kepler-4b which is a Neptune-size planet. All five extrasolar planets orbit stars hotter and larger than the Earth’s Sun, with orbital periods ranging from 3.3 to 4.9 days. These are the first five planets discovered by the Kepler mission and they were discovered in the first six weeks of the telescope’s operation. Jupiter-size planets in short orbits are expected to be the first planets Kepler could detect and it is only a matter of time before Kepler eventually finds Earth-size planets in orbit around Sun-like stars in their habitable zones.

Planetary Moons

2010-01-02T20:11:00.004+08:00

A partial lunar eclipse occurred on 31 December 2009, with greatest eclipse occurring at 19:22:39 (Universal Time). This phenomenon was visible from all of Africa, Europe, Asia, Middle East and Australia. Between 3 to 4 a.m. on Friday, 1 January 2010 (Singapore Time), I managed to witness the partial lunar eclipse, including when the eclipse was at its maximum. It should be noted that Singapore is 8 hours ahead of Universal Time. Although only a small portion of the Moon entered the Earth’s umbral shadow, the darkening visible over the Moon’s southern region at greatest eclipse was rather distinct. I didn’t manage to observe the eclipse through my telescope as I was out celebrating the start of 2010.

In the hypothetical scenario where the eclipse was observed from the Moon’s surface instead of from the Earth’s surface, it would have been much more spectacular. From the southern region of the Moon that was in the Earth’s umbral shadow, the entire Sun will be eclipsed by the Earth. Elsewhere on the Moon’s dayside, the Sun will be partially eclipsed by the Earth with less of the Sun being eclipsed at more northerly latitudes.

In the film Avatar, Pandora is a fictional Earth-like moon of the gas giant planet Polyphemus, which orbits Alpha Centauri A - one of two stars of the Alpha Centauri binary system. Pandora is one of many moons orbiting the planet Polyphemus and its atmosphere is composed of nitrogen, oxygen, carbon dioxide, xenon, ammonia, methane and hydrogen cyanide. Lush forest covers most of Pandora’s surface and virtually all life on Pandora exhibits bioluminescence. Although Pandora is entirely science fiction, the two stars making up the Alpha Centauri binary system are in reality, the nearest Sun-like stars and they are located 4.37 light years away. The many similarities both stars share with the Sun make them a prime target for the search of extrasolar planets around stars.

Habitable moons like Pandora may soon become science fact as NASA’s Kepler mission has the potential to detect such Earth-sized moons. Kepler looks for planets by detecting the slight dip in the light from a star caused when a planet passes in front of the star, creating a mini-eclipse. If a planet has a sufficiently massive moon, the moon’s gravity will tug on the planet and cause detectable transit duration variations. In fact, the transit of a moon in front of a star might also be detectable by Kepler! If such a moon is found around a nearby star, future space telescopes such as the James Webb Space Telescope (JWST) will be able to study its atmosphere and detect key gases like carbon dioxide, oxygen and water vapor.

New Energy

2009-12-30T23:54:00.001+08:00

This is my last update for the year of 2009 and I’m welcoming 2010! This new decade will definitely bring much surprises and discoveries! Contemplating about telescopes, 2009 has definitely been the year of space telescopes. In March of 2009, we have the launch of NASA’s Kepler space telescope which is designed to detect Earth-like planets orbiting other stars. In May of 2009, we have the fifth and final space shuttle servicing mission to the Hubble Space Telescope (HST) and also the launch of the European Space Agency’s (ESA) Herschel and Planck space observatories. Finally, in December of 2009, we have the launch of NASA’s Wide-Field Infrared Survey Explorer (WISE) which is an infrared space observatory designed to survey the entire sky in infrared wavelengths with unprecedented sensitivity.

During the last two days, I researched on Stirling engines and nuclear energy from direct Brayton cycles. I ran calculations for the three main types of Stirling engines which are basically called alpha, beta and gamma Stirling engines. I computed things like total momentum volume and engine pressure as a function of crank angle, indicated energy per engine cycle, indicated engine power and thermal efficiency of the engine for different Stirling engines.

One of the most interesting aspects of Stirling engines for me is its application as a Stirling radioisotope generator (SRG) because a Stirling engine produces approximately four times as much electric power from the same amount of plutonium fuel than a radioisotope thermoelectric generator (RTG). Such energy sources are used in situations where power is required for durations too long for batteries or fuel cells to provide economically and where solar energy is not viable, such as for long duration missions to the outer solar system.

As for my research on nuclear energy from direct Brayton cycles, I ran calculations for different power plants with net power outputs ranging from a few megawatts to hundreds of megawatts and reactor outlet temperatures ranging from 850 to 1400 degrees Kelvin. I “played around” with the parameter spaces to find optimum solutions for all these different cases. The performance of such a power plant is most sensitive to the compression and expansion efficiencies of the compressor and the turbine respectively. In many cases, I also include a regenerator which transfers heat from the hot exhaust gas leaving the turbine to the high-pressure gas leaving the compressor as long as the temperature of the gas leaving the turbine is higher than the temperature of the gas leaving the compressor.

Generation IV nuclear reactors are a set of reactor designs currently being studied for implementation in the near future. The primary goals are to improve nuclear safety, improve proliferation resistance, minimize waste and natural resource utilization, and to decrease the cost of construction and operation of such plants. The Gas-cooled Fast Reactor (GFR), the Very High Temperature Reactor (VHTR), the Supercritical Water-cooled Reactor (SCWR), the Sodium-cooled Fast Reactor (SFR), the Lead-cooled Fast Reactor (LFR) and the Molten Salt Reactor (MSR) are six reactor systems that have been selected for further evaluation and potential development as generation IV nuclear reactors. During the past couple of days, I researched on each of the reactor systems and focused a lot on their thermodynamic cycles.

Planetary Science! Last week, I carried out extensive research on some remarkable processes and surface features of some of the planetary moons in the Solar System. Some of my areas of focus are - the surface dichotomy of Saturn’s moon Iapetus, the formation and morphology of the Moon’s Near Side Megabasin and its antipode, the global climate of Saturn’s moon Titan and its seasonal variations, the structure of the volcanic plumes of Jupiter’s moon Io, etc…

Recently, the Diviner instrument onboard NASA’s Lunar Reconnaissance Orbiter (LRO) measured amazingly low temperatures within permanently shadowed craters at the moon’s poles. Mid-winter nighttime surface temperatures inside the coldest permanently shadowed craters at the north polar region of the Moon dip down to an incredible minus 248 degrees Celsius or 25 degrees Kelvin! These are some of the lowest temperatures measured anywhere in the Solar System so far and to find surface temperatures this low, you’ll have to travel way out to the Kuiper Belt and beyond. Visit http://lunar.gsfc.nasa.gov/ to find out more about this mission.

NASA’s New Horizon spacecraft crossed another milestone on 29 December 2009 when it became closer to Pluto than it is to the Earth! Closest approach to Pluto will occur on 14 July 2015, which is still well over 5 years away. The halfway point of the long journey to Pluto depends on how you define it. So, when will the New Horizons spacecraft be halfway to Pluto? On 25 February 2010, New Horizons will have covered half the heliocentric distance to Pluto from its launch at Earth. On 20 April 2010, New Horizons will be half as far from the Sun as Pluto will be at the time of the encounter on 14 July 2015. On 17 October 2010, New Horizons will have traveled half the flight time to reach Pluto. Visit http://pluto.jhuapl.edu/ for more information about this far-flung mission to Pluto and the Kuiper Belt. It is also interesting to note that New Horizons will pass Uranus’ orbit on 18 March 2011 and Neptune’s orbit on 24 August 2014.

Kraken Mare

2009-12-19T21:44:00.006+08:00

Titan is the largest moon around Saturn, the second largest moon in the solar system, the only known moon in the solar system to have a dense atmosphere and the only object other than the Earth known to have stable bodies of liquid on its surface. The atmosphere of Titan is largely comprised of molecular nitrogen like the Earth and its surface pressure is about 1.5 times the pressure at sea level on Earth. Titan’s diverse surface morphologies and atmospheric phenomena show great resemblance to those found on the Earth. The Cassini spacecraft is currently orbit around Saturn, studying the planet, its ring system and its numerous moons. Ever since it entered orbit around Saturn in 2004, the Cassini spacecraft and the Huygens probe have made an impressive amount of discoveries and returned numerous breathtaking images.

Over the past few years, the Cassini spacecraft has revealed the presence of numerous bodies of liquid on the surface of Titan. These lakes are located around the north and south poles, with a much greater concentration of lakes around the north pole of Titan. These lakes are filled with liquid hydrocarbons, whereby in the frigid environment of Titan, methane and ethane is on Titan what water is on Earth. The largest known lakes on Titan are Kraken Mare, Ligeia Mare, Punga Mare and Ontario Lacus. Each one of these lakes is hundreds of kilometers across and of these large lakes, only Ontario Lacus is located at the south polar region of Titan. Ontario Lacus is about the same size as Lake Ontario in North America and it was the first lake to be confirmed on Titan.

I recently came across an amazing image showing a glint of sunlight reflected off a lake that is located near the north pole of Titan. This image is the first of its kind and it can be viewed at http://photojournal.jpl.nasa.gov/catalog/PIA12481. The glint was detected by the Visual and Infrared Mapping Spectrometer (VIMS) on the Cassini spacecraft and the reflection has been correlated to be from the southern shoreline of the lake Kraken Mare. Kraken Mare is the largest known body of liquid on the surface of Titan and with a surface area of about 400000 square kilometers, this lake is larger than the Caspian Sea on Earth. The Cassini mission homepage is http://saturn.jpl.nasa.gov/.

During the past couple of days, I read a number of newly published papers reporting the discoveries of ever more extrasolar planets. Two of the more interesting papers entitled “A Super-Earth Transiting a Nearby Low-Mass Star” and “Three Possible Origins for the Gas Layer on GJ 1214b” describe the discovery and properties of a newly discovered super-Earth mass extrasolar planet called Gliese 1214b which was initially detected from its transits across its parent star. Planets in the super-Earth mass range are more massive than the Earth but much less massive than planets like Uranus and Neptune. Currently, Gliese 1214b is the second super-Earth mass extrasolar planet known to transit its parent star.

In addition, the newly announced WASP-19b is currently the extrasolar planet with the shortest known orbital period around its parent star at just under 19 hours! WASP-19b is slightly more massive than Jupiter and its orbits about 24 times closer to its parent star than Mercury is from the Sun! The previous record holder was the transiting super-Earth mass extrasolar planet called COROT-7b. Visit http://exoplanet.eu/ or http://media4.obspm.fr/exoplanets/ to know more about all the extrasolar planets that have been discovered so far.