Wednesday, November 9, 2011

Flares from a Supermassive Black Hole

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 once 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 generated 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. 

Sunday, September 4, 2011

A Pulsar’s Diamond Planet

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 electromagnetic radiation 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.

Saturday, July 30, 2011

Land Planets and Ocean Planets

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.

Friday, July 15, 2011

Tight Stellar Binary

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.

Saturday, June 25, 2011

Faces of Iapetus

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.

Friday, June 3, 2011

Exploding Black Holes

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.

Wednesday, May 18, 2011

The Runaway Giant

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”.

Wednesday, May 11, 2011

Lava-Ocean Planets

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.

Wednesday, May 4, 2011

Ultra-Hot Super-Earth

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.

Wednesday, April 27, 2011

Extrasolar Carbon Planets

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.

Thursday, April 21, 2011

Worlds Like Titan

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.

1. 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.
2. 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.

Friday, April 15, 2011

Dark Matter and Alien Planets

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.

Thursday, April 7, 2011

Black Hole Propelled Starship

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.

Friday, April 1, 2011

Drifting Crust

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.

Friday, March 25, 2011

White Sun

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.

Friday, March 18, 2011

Arriving At Mercury

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 to learn more about MESSENGER and its mission around Mercury.

Thursday, March 17, 2011

Galactic Blast

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.

Wednesday, March 9, 2011

Cool Dwarf

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 paper by Kevin Luhman, et al. (2011) 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).