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.

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.

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.

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.

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”.

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.

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.

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.

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.

Sources:
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.

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.