Half-Frozen Half-Scorched Worlds

Red dwarf stars are by far the most abundant stars in the galaxy. A terrestrial planet orbiting in the habitable zone of a red dwarf star is expected to be tidally-locked with one side of the planet permanently facing the star. This is because a red dwarf star is much dimmer than the Sun and a planet has to orbit much nearer to it in order to receive sufficient warmth for liquid water to exist on its surface. Being so close to its parent star, tidal evolution quickly causes the planet to become tidally-locked with one hemisphere of the planet permanently pointed towards the red dwarf star, just like the same hemisphere of the Moon always faces the Earth. The outcome of this is a permanent dayside and permanent nightside on the planet.

Figure 1: Habitable zone of the Sun compared to the habitable zone around Gliese 581 - a red dwarf star known to have planets around it.

Figure 2: A tidally-locked planet with a frozen nightside and a scorched dayside. This planet is located in a star system consisting 2 or more stars since part of the planet’s nightside is faintly illuminated by light from a more distant star in the same star system. An ice layer covers the entire nightside of the planet.

With permanent day-night hemispheres, a terrestrial planet in the habitable zone of a red dwarf star is expected to have an exotic climatic system that is very different from the Earth’s. Such a planet will have a hot dayside and a cold nightside, with atmospheric circulation and possibly oceanic circulation connecting both hemispheres. The habitability of this planet is likely to depend on the amount of water it possesses. This is because the cold nightside hemisphere of the planet can act as a cold trap where water gets deposited and forms an ice layer. If the planet has too little water, all of its water can become trapped within the ice layer on the planet’s nightside, leaving the dayside of the planet very dry.

Depending on the abundance of surface water on the planet and the temperature of the planet’s nightside, it appears that the accumulation of ice on the nightside of a tidally-locked planet can be vaguely classed into 3 possible configurations - ice shelf configuration, ice sheet configuration and water-trapped configuration.

The ice shelf configuration involves a tidally-locked Earth-analogue planet which possesses a very large quantity of surface water such that the whole planet is completely covered by a deep global ocean of water. Warmer nightside temperatures can also contribute to an ice shelf configuration since it results in a thinner ice layer. Here, water precipitates as snow and forms an ice layer on the planet’s nightside. The thickness of the ice layer is limited by basal melting because ice begins to melt at high pressures found at the base of the ice layer. As a result, the ice layer can be no more than a few kilometres thick. The presence of a deep global ocean means that the ice layer on the planet’s night side floats as an ice shelf overlying a sub-surface ocean that is directly connected to the ocean on the planet’s dayside. Furthermore, the limited thickness of the ice shelf and the deep ocean keeps the base of the ice shelf from being “grounded” against the ocean floor. The ice shelf configuration follows a mass balance which depends on the balance between surface accumulation of snow and basal melting.

For the same tidally-locked Earth-analogue planet but this time with less surface water and/or colder nightside temperatures, an ice sheet configuration becomes the likely outcome. Here, the ice layer becomes grounded and is referred to as an ice sheet instead of an ice shelf. This occurs because the ocean depth is too shallow to accommodate the thickness of the ice layer and/or the colder nightside temperatures allow for a thicker ice layer. The mass balance of the ice sheet configuration depends on the balance between ice accumulation from snowfall and the flow of ice away from the boundary of the ice sheet.

Finally, the water trapped configuration is the third configuration and it has the potential to strongly affect the habitability of a tidally-locked planet in the habitable zone around a red dwarf star. The water trapped configuration is basically an extreme version of the ice sheet configuration. This configuration occurs for a tidally-locked Earth-analogue planet with very little surface water and/or very cold nightside temperatures. In this case, almost all of the planet’s surface water is frozen and trapped in an ice sheet on the planet’s nightside. This leaves the dayside of the planet extremely dry. Assuming that the planet receives as much insolation as Earth receives from the Sun, it turns out that if the planet has less than a quarter of Earth’s surface water inventory, it could find itself in a water trapped configuration. In reality, the minimum amount of surface water required on a planet before a water trapped configuration is likely to occur depends also on a number of other factors such as the amount of insolation received by the planet, surface topography, global-scale surface weathering, etc.

Figure 3: Schematic plot of three idealized configurations envisioned for the surface water inventory on a tidally-locked terrestrial planet. A view from the pole is shown, with the nightside up and the dayside down. Exaggerated thicknesses are adopted for the ocean (blue) and ice (white) layers. Planets with very little surface water and/or very cold nightside temperatures are more likely to be in the water-trapped configuration. Credit: Kristen Menou (2013)

A water trapped planet with most of its water frozen within an ice sheet on the planet’s nightside is not all bad news for habitability. Abe et al. (2011) show that a drier planet can remain habitable over a slightly wider range of distances from its parent star than a planet with a greater abundance of surface water. Habitable regions on the surface of a water trapped world are likely to be more strongly localized, straddling the “Goldilocks zone” between the very dry dayside and the frigid nightside. It is reasonable to imagine the existence of life on the day-night terminator of the planet as melt water from the edge of the ice sheet flows towards the planet’s dayside.

References:

- Kristen Menou (2013), “Water-Trapped Worlds”, arXiv:1304.6472 [astro-ph.EP]

- Abe et al., “Habitable Zone Limits for Dry Planets”, Astrobiology 2011 June; 11(5): 443-60.

Attacks from Extraterrestrial Civilizations are Unlikely

People have expressed concern regarding the broadcast of powerful electromagnetic signals into space and deliberate attempts to contact extraterrestrial intelligences. The reason for such concern is that extraterrestrial intelligences could be hostile to humanity as they might see Earth as an attractive exploitable resource or the growth of human technology might be perceived as a threat to their future safety. British theoretical physicist Dr. Stephen Hawking states: “We only have to look at ourselves to see how intelligent life might develop into something we wouldn’t want to meet. I imagine they might exist in massive ships, having used up all the resources from their home planet. Such advanced aliens would perhaps become nomads, looking to conquer and colonize whatever planets they can reach… If aliens ever visit us, I think the outcome would be much as when Christopher Columbus first landed in America, which didn’t turn out very well for the Native Americans.”

In recent years, the technologies used to detect new planets and humanity’s ability to find alien life has improved dramatically. As the possibility of finding extraterrestrial life catches up with science fiction, the question of whether extraterrestrial intelligences are likely to be “friend or foe” seems increasingly pertinent. In a paper written by Janne M. Korhonen from Aalto University in Finland, the author suggests that conflicts between extraterrestrial intelligences are very unlikely and also examines a number of factors that would deter such conflicts from taking place. The author assumes that extraterrestrial intelligences are rational and have a concept of risks and benefits.

A major problem for any extraterrestrial civilization that might launch an attack involves knowing what to attack in the first place. Gathering information about another civilization over interstellar distances is going to be extremely difficult and full of uncertainty. Due to the finite speed of light, it takes many years for electromagnetic radiation to travel the enormous interstellar distances that separate individual planetary systems. As a result, whatever information that is collected by a potential aggressor is likely to be outdated by the time the attacking civilization arrives at the target civilization. If the Milky Way galaxy contains one million randomly distributed extraterrestrial civilizations, then the average minimum distance separating one civilization from another is ~200 light years. This creates an intelligence lag of at least 400 years because it takes 200 year for signals from the target civilization to arrive at the attacking civilization and another 200 years or more for the attacking civilization to travel at near the speed of light to the target civilization. The huge lag time can provide ample opportunity for the target civilization to develop some retaliatory capabilities and also allows the target civilization to establish contacts with other civilizations.

The rise of human civilization is only a very recent phenomenon in Earth’s history. Given that the galaxy is billions of years old and considering how much human civilization has progressed in just the last century alone, any extraterrestrial civilization is very unlikely to be at the same level of technology as human civilization. Instead, any extraterrestrial civilization is going to be significantly more advance. Assuming an exponential growth of technology, it is hard to imagine a civilization that is a billion, a million or even a thousand years ahead. A civilization that is technologically able to detect signs of another civilization for the first time will deduce that the detected civilization is already far ahead of theirs.

Detectable signs of an extraterrestrial civilization such as electromagnetic transmissions and signatures of mega-scale engineering projects cannot be reliably used to gauge that civilization’s level of technological development. This is because a hyper-advanced civilization can still use old and obsolete technologies simply for recreational or educational purposes. Furthermore, if a hyper-advanced civilization believes that hostile civilizations can develop, it can create “baits” that mimic less advance civilizations to lure out hostile civilizations and pre-emptively attack them.

If an aggressive civilization were to launch an attack on another civilization, it will be nearly impossible for the aggressor to be certain that it will not face retaliation. Survival of some form of the victim civilization is very likely, especially if the victim civilization has self-sustaining off-world colonies that can easily evaded detection. Just a few survivors from the victim civilization can grow to billions of individuals in a few hundred years assuming a growth rate of one percent per year by human standards. It is uncertain if survivors from the victim civilization will subsequently launch a retaliatory strike. Nevertheless, as long as there are survivors, the attacking civilization cannot completely discount the possibility of a subsequent retaliatory strike.

Even if the attacking civilization completely eliminates the victim civilization, there is always a possibility that the attack may be noticed by other extraterrestrial civilizations. These other civilizations can also include off-shoot colonies of the victim civilization and even the attacking civilization itself. A civilization that launches an unprovoked attack on another civilization is likely to be seen as a danger by other civilizations as well. One or more of these other civilizations may launch a pre-emptive strike on the attacking civilization to eliminate the danger it poses.

Attacking another extraterrestrial civilization for the purpose of resource exploitation is also very unlikely to occur. The reason is that distances involved in interstellar spaceflight are so enormous that the resources required to travel to another planetary system is likely to vastly outweigh any returns. For example, transporting a 1000 ton payload across interstellar space at just 10 percent the speed of light would require at least as much energy as the current annual energy consumption of the entire human civilization. Long before achieving the overwhelmingly more difficult feat of interstellar spaceflight, an extraterrestrial civilization is likely to already have technologies that allow full recycling of its resources which alleviates the need for any further exploitation. As a result, an extraterrestrial civilization with a mastery of interstellar spaceflight is unlikely to attack another civilization since the gain of all resources from a single planet would be trivial. The risk of future retaliation from survivors or witnesses is also expected to greatly outweigh any benefit.

It seems that any attack launched by an extraterrestrial civilization on another is a dangerous gamble for the attacker. An extraterrestrial civilization is unlikely to attack another civilization because the benefit of not attacking is expected to outweigh any benefit derived from an attack. Studies also show that “pacifistic” civilizations tend to perform better than aggressive ones in the long run. Unfortunately, the arguments put forth here are obviously limited since humanity’s own history serves as the only source of reasoning. Any extraterrestrial intelligence is going to have a reasoning process which differs from ours. Besides, extraterrestrial intelligences that are far ahead of human civilization may be entirely incomprehensible.

Even though the threat of an attack by an extraterrestrial civilization appears irrational and very unlikely, a certain amount of caution should still be taken. The design and operation of an interstellar spacecraft can potentially be seen as a threat by another extraterrestrial civilization. For example, a planetary system can be home to an advance civilization and still appear uninhabited because the civilization chooses to remain undetectable. An interstellar spacecraft sent from Earth may accidentally crash into one of the civilization’s planets and cause massive damage. That is not hard to imagine because a 100 ton interstellar spacecraft travelling at just 10 percent the speed of light can release a comparable amount of energy upon impact as humanity’s entire nuclear arsenal. Such an incident can cause the extraterrestrial civilization to strike back at Earth to prevent further “attacks”. As a result, any mission which involves sending an interstellar spacecraft to explore another planetary system must ensure that the planetary system is uninhabited or that the spacecraft will not appear to pose any danger to an extraterrestrial civilization that may reside in the planetary system.

Reference:

Janne M.Korhonen, “MAD with aliens? Interstellar deterrence and its implications”, Acta Astronautica Volume 86, May-June 2013, Pages 201-210

Iron Stars at Eternity’s End

Stars generate energy and fuse lighter elements into heavier ones through nuclear fusion. The majority of stars produce energy by fusing hydrogen into helium. More massive stars can continue to fuse helium into carbon, carbon into oxygen, oxygen into silicon and silicon into iron. Once iron is produced, it signals the last step in any stable nuclear fusion reaction because the creation of elements heavier than iron consume more energy than can be produced. In general, most stars are not massive enough to fuse carbon and other heavier elements. These stars die off as stellar remnants known as white dwarfs which gradually cool over eons and settle as black dwarfs. The present universe is way too young for black dwarfs to exist. Stars massive enough to fuse carbon and other heavier elements typically explode as supernovae.

Currently, the universe is estimated to be 13.8 billion years old. The last stars in the universe are expected to fizzle out ~10¹⁴ years from now and the largest supermassive black holes are expected to evaporate completely via Hawking Radiation ~10¹⁰⁰ years from now. From this time period onwards, most of the ordinary matter in the universe is predicted to be in the form of black dwarfs, brown dwarfs and free-floating planets. With no free hydrogen to from new stars, objects simply float around in a dark and empty universe. Nevertheless, quantum mechanics can still cause a number of interesting phenomena to occur.

If a ball is thrown at a wall, it would always hit the wall and bounce back. However, according to quantum mechanics, there is a very small chance the ball could hit the wall and pass right through it. This process is called quantum tunnelling and it can allow lighter elements to spontaneously fuse into heavier elements at zero temperature. Even though the probability of even a single event like this is incredibly small, over a sufficiently long period of time, an event with an infinitesimal probability of happening is bound to eventually happen.

Initially, a stellar remnant such as a black dwarf is mostly made of elements such as hydrogen, helium, carbon and oxygen. Given enough time, nuclear fusion via quantum tunnelling will cause all matter to fuse into iron. Likewise, elements heavier than iron should also decay into iron by nuclear fission and alpha particle emission. Technically, it is accurate to say that over sufficiently long timescales, a stellar remnant such as a black dwarf is still generating energy through nuclear processes via quantum tunnelling. An iron star is produced once all matter has been converted into iron. Such an object is basically a cold sphere consisting almost entirely of pure iron. The timescale required for ordinary matter in the universe to fuse into iron via quantum tunnelling is estimated to be ~10¹⁵⁰⁰ years. From here on out, all black dwarfs, brown dwarfs and free-floating planets would have turned into cold spheres of iron.

While an iron star can no longer undergo any form of nuclear fusion or fission, it is by no means in its state of lowest energy. Over an immensely longer timescale, an iron star could collapse via quantum tunnelling into a dense sphere of neutrons or a black hole. The timescale estimated for an iron star to collapse into a ball of neutrons is 10^10⁷⁶ years (1 followed by 10⁷⁶ zeroes) and the timescale for collapse into a black hole is 10^10²⁶ years (1 followed by 10²⁶ zeroes).

The collapse of an iron star into a neutron star would produce a huge outburst of energy over a very short period of time. Whether such an outburst constitutes a supernova explosion remains uncertain. Nevertheless, if a supernova does occur, a fraction of the iron is expected to be reconverted back into elements both heavier and lighter than iron. Some of the ejecta could fall back and form a disk of material around the newly born neutron star where planets could form out from the disk’s material. This is not impossible to conceive since planets are known to exist around neutron stars in the present universe.

Conditions favourable for carbon-based life might even be present on some of these planets since they contain elements both heavier and lighter than iron. Heavy elements such as uranium and thorium are radioactive while light elements such as hydrogen, carbon and oxygen are necessary for life. A planet formed out from the debris of such a supernova can produce its own internal heat for many billions of years by radioactive decay. As a result, it is possible to imagine a planet with a frozen icy surface and beneath it is an ocean of water kept liquid by heat from radioactive decay. It sure is intriguing to consider that carbon-based life could exist in a future so incomprehensibly distant that it might just as well be separated from the present universe by an eternity. Although life might exist for billions of years on such a planet, it is still no more than a fleeting instant in a future without end.

It should be understood that predictions regarding the far future of the universe should not be taken with certainty because our currently understanding of the universe is by no means sufficient for such predictions and new knowledge is added all the time. Finally, the possible existence of iron stars assumes that protons are stable and do not decay. This is a reasonable assumption because experiments thus far have not shown any evidence that protons decay.

Reference:

Dyson, Freeman J., “Time Without End: Physics and Biology in an Open Universe”, Reviews of Modein Physics, Vol. 51, No. 3, July 1979

A Stray Blue Supergiant Star

The Virgo Cluster is the nearest massive cluster of galaxies (~50 million light years away) and it contains well over a thousand member galaxies. Such a massive cluster of galaxies creates a huge gravitational potential well around it and galaxies from outside do occasionally fall in at very high speeds. One such galaxy is IC 3418 - a dwarf irregular galaxy that is falling into the Virgo Cluster at a very high speed of ~1000 km/s. IC 3418 features a long trail of material measuring over 50,000 light years in length. This trail is believed to have formed behind IC 3418 due to ram pressure stripping of material from IC 3418 as it plows through the intergalactic medium on its plunge into the Virgo Cluster.

Signs of star formation are evident within the long trail of material behind IC 3418. This is a unique environment for star formation because stars that form here are technically in intergalactic space where the density of gas and dust is a lot sparser than in a galaxy. In contrasts, traditional star formation occurs within the denser environment of a galaxy. Spectroscopic observations of the star-forming trail of IC 3418 have revealed the presence of a compact and luminous source of optical emission located near the far end of the trail. This compact source is catalogued as SDSSJ1229+1122 and it is spectroscopically consistant with either a collection of 50 to 200 B-type stars or a single blue supergiant star. In turns out that SDSSJ1229+1122 is more likely to be a single blue supergiant star because a collection of so many B-type stars requires a very unlikely star formation scenario. Additionally, spectroscopic observations have revealed possible hints of a powerful stellar wind that is consistant with a single blue supergiant star.

Image: Artist’s impression of Rigel - a blue supergiant star. Credit: Guillermo Krieger

Image: Size comparison of Gamma Orionis (a typical blue giant star) with Algol B (left) and the Sun (bottom). The single blue supergiant star in SDSSJ1229+1122 would appear many times larger and more luminous than Gamma Orionis.

As a single blue supergiant star, SDSSJ1229+1122 could only have formed close to where it currently is, which is within the trail of material behind IC 3418. This is because such a massive star has a lifespan of only several million years and that is too short a time for it to have formed within the galactic environment of IC 3418 before moving to its current location. Instead, the blue supergiant star probably formed through a process of turbulence-driven star formation. Turbulence in the trail of material behind IC 3418 creates eddies that aggregate clouds of gas and dust into dense clumps which collapse under their own gravity to form stars. As a result, turbulence-driven star formation can explain the presence of a short-lived blue supergiant star that is on its own in intergalactic space, thousands of light years from the nearest galaxy.

Reference:

Youichi Ohyama and Ananda Hota (2013), “Discovery of a possibly single blue supergiant star in the intra-cluster region of Virgo cluster of galaxies”, arXiv:1304.2560 [astro-ph.CO]

Nearest Stars: Past, Present and Future

At present, the nearest known star is Proxima Centauri which lies at a distance of 4.24 light years from the Sun. Proxima Centauri is a red dwarf star and it is way to faint to be visible with unaided eyes from Earth. Nevertheless, Proxima Centauri is likely to be part of a triple star system with Alpha Centauri - a pair of sun-like stars located slightly further away at 4.37 light years. Although Alpha Centauri is a binary star system, it appears to the unaided eyes as a single star with a combined visual magnitude of -0.27, making it the 3rd brightest star in the night sky after Sirius and Canopus. Alpha Centauri and Proxima Centauri have not always been the nearest stars to the Sun. Over the course of time, a number of stars have been predicted to have come or will come much closer to the Sun.

Figure 1: Distances of the nearest stars from 20,000 years ago until 80,000 years in the future. Credit: Matthews, R. A. J. (1994)

Alpha Centauri and Proxima Centauri are predicted to come as close as 3 light years from the Sun about 27,000 years from now, before receding away. They will continue to remain as the nearest stars until about 33,000 years from now. Beyond that, a red dwarf star named Ross 248 will become the closest star to the Sun between 33,000 to 42,000 years from now.

Looking further into the future, a star named Gliese 710 is particularly interesting since it is travelling nearly head-on towards our Sun. Even though this star is currently about 64 light years away, it is estimated to have a high probability of approaching within a mere one light year from the Sun at approximately 1.4 million years from now. As a result, Gliese 710 is likely to perturb the Oort cloud and send an influx of comets into the inner solar system. However, the increase in impact rates in the inner solar system due to the influx of comets is expected to be very small.

About 7.3 million years ago, a triple star system named Algol passed within 9.8 light years of the Sun. Although it may not seem like a very close approach, Algol has a combined mass that is 5.8 times the mass of the Sun. In addition, the combined luminosity of Algol is a whopping 100 times the luminosity of the Sun. At closest approach, the gravity from Algol might have been sufficient to perturb the Oort cloud. An observer on Earth at that time would have seen Algol shining as a brilliant star with a visual magnitude of about -2.8. That is over 3 times brighter than Sirius appears at present. Currently, Algol is about 93 light years away.

Figure 2: An image of Sirius - the brightest star in the present night sky. Credit: Greg Parker, New Forest Observatory

Zeta Leporis is another star which came relatively close to the Sun in the past. Bobylev & Vadim V. (2010) estimated a closest approach of 4.16 light years from the Sun about 861,000 years ago while García-Sánchez et al. (2001) estimated a closest approach of 5.34 light years from the Sun about one million years ago. Zeta Leporis is 14 times more luminous than our Sun and if it had came as close as 4.16 light years, it would have appeared as a very bright star in the night sky with a visual magnitude of about -2.5.

On 11 March 2013, the discovery of a binary brown dwarf system located at a mere 6.5 light years away was published in a paper by Kevin Luhman, an associate professor of astronomy and astrophysics at Pennsylvania State University. The last time a stellar /substellar object was found to be this close to the Sun occurred nearly a hundred years ago in 1916 when Barnard’s star was discovered at 6.0 light years from the Sun. This binary brown dwarf system is designated WISE 1049-5319 since it was detected by NASA’s Wide-field Infrared Survey Explorer (WISE). Such a discovery shows there are still objects left to be found even within those few light years of space that define the Sun’s neighbourhood. The discoverer of WISE 1049-5319 said: “There are billions of infrared points of light across the sky, and the mystery is which one - if any of them - could be a star that is very close to our solar system.”

References:

- Matthews, R. A. J. (1994), “The Close Approach of Stars in the Solar Neighbourhood”, Quarterly Journal of the Royal Astronomical Society Volume 35: 1-9

- Bobylev, Vadim V. (2010), “Searching for Stars Closely Encountering with the Solar System”. Astronomy Letters Volume 36 (3): 220-226

- García-Sánchez, J.; Weissman, P. R.; Preston, R. A.; Jones, D. L.; Lestrade, J.-F.; Latham, D. W.; Stefanik, R. P.; Paredes, J. M. (2001), “Stellar Encounters with the Solar System”, Astronomy and Astrophysics Volume 379 (2): 634-659

- Garcia-Sanchez, J.; Preston, R. A.; Jones, D. L.; Lestrade, J.-F.; Weissman, P. R.; Latham, D. W. (1997), “A Search for Stars Passing Close to the Sun”, The First Results of Hipparcos and Tycho, 23rd meeting of the IAU, Joint Discussion 14

- K. L. Luhman (2013), “Discovery of a Binary Brown Dwarf at 2 Parsecs from the Sun”, arXiv:1303.2401 [astro-ph.GA]

Activity on Phaethon

Phaethon is an asteroid with an unusual orbit that brings it very close to the Sun. For any object orbiting the Sun, the point in its orbit where it is closest to the Sun is known as perihelion. At perihelion, Phaethon comes as close as 0.14 AU from the Sun and its surface gets intensely heated to a temperature of ~1000 K. In Greek mythology, Phaethon was the name of the son of the sun god Helios. The asteroid Phaethon measures about 5 km in diameter and is believed to be the parent body of the Geminid meteoroid stream. On Earth, meteors from the Geminid meteoroid stream usually peak around the 13th to 14th December.

NASA’s STEREO spacecraft detected anomalous optical brightening of Phaethon each time when the asteroid was at perihelion in 2009 and 2012. At perihelion, Phaethon brightened by a factor of ~6 and the brightening lasted for a duration of about 2 days. Such a brightening is far too large and too long-lived to be attributed simply to the variation in brightness caused by rotation of the irregularly-shaped asteroid every ~3.6 hours. The longevity of the brightening also means that it cannot be attributed to reflection from a mirror-like patch on the asteroid’s surface. Furthermore, the sublimation of water ice cannot be the cause of brightening because surface temperatures on Phaethon are too high for water ice to exist in the first place. Even the estimated interior temperature of Phaethon is too high for deeply buried water ice to survive for long.

Image: Artist’s representation of P/2012 F5 - an asteroid discovered in March 2012 from the Mount Lemmon Observatory in Arizona (USA). It has a trail of dust particles caused by internal rupture or collision with another asteroid. Credit: Servicio de Información y Noticias Científicas (SINC)

A plausible explanation for the anomalous optical brightening of Phaethon at perihelion is the presence of ejected dust particles. For this to occur, the combined cross-sectional area of all ejected dust particles has to be larger than the cross-sectional area of Phaethon itself so as to scatter enough sunlight to account for the brightening of the asteroid. If Phaethon has a density of 3000 kg/m³ and the average grain size is 1 mm, the required mass of dust is ~4×10⁸ kg. This amount of dust is negligible in comparison to the estimated mass of Phaethon at ~2×10¹⁴ kg. Thermal fracture and thermal decomposition of surface minerals serve as possible mechanisms for the ejection of dust grains from Phaethon at perihelion.

As a result, Phaethon is like a “rock comet” where a coma of ejected dust grains forms around the asteroid during each perihelion passage. The subsequent decline in brightness can be explained as the illuminated side of Phaethon and its coma of dust grains gradually face away from STEREO. In addition, sublimation or disintegration of dust grains can also aid in the decline in brightness. The ejection of dust from Phaethon during each perihelion passage is likely to contribute somewhat to the Geminid meteoroid stream.

Reference:

Jing Li and David Jewitt (2013), “Recurrent Perihelion Activity in (3200) Phaethon”, arXiv:1304.1430 [astro-ph.EP]

Ice Sheets on Hot Desert Planets

For a planet like the Earth, water plays a fundamental role in the global climate system. Today, the Earth is in radiative equilibrium balance where the insolation it receives from the Sun is balanced by outgoing infrared emission from the planet’s surface. This gives the Earth an average surface temperature of 288 K or 15 degrees Centigrade. If the Earth were closer to the Sun, it will receive a greater amount of insolation from the Sun and this will heat up the planet, causing more water vapour to be released into the atmosphere. The increase in atmospheric water vapour brings about a stronger greenhouse effect. This causes the Earth to give off more infrared radiation to balance the larger insolation it is getting from the Sun. As a result, if the Earth were nearer to the Sun, it will settle into a new radiative equilibrium balance with a somewhat higher mean surface temperature.

Nevertheless, there is a minimum distance a planet like the Earth can be from its parent star before the amount of insolation received by the planet becomes so great that radiative equilibrium balance can never be achieved. Any closer than this minimum distance, the planet undergoes what is known as a runaway greenhouse effect. As more water vapour is released into the planet’s atmosphere, the greenhouse effect becomes stronger. This causes the planet to become hotter and release yet more water vapour and so on. Eventually, all water on the planet’s surface is vaporised and the surface temperature everywhere on the planet settles at a scorching temperature of ~1400 K, rendering the planet uninhabitable for life. Due to the onset of runaway greenhouse effect, the minimum distance a habitable Earth-like planet can be from its parent star denotes the inner edge of the habitable zone around the star. Any nearer, a runaway greenhouse effect occurs and makes the planet too hot to be habitable.

Although a runaway greenhouse effect seems like a certainty for an Earth-like planet that is too close to its parent star, it may not always be the case since it assumes that there is a sufficient reservoir of water everywhere on the planet’s surface. In fact, a planet whose surface is predominately land with only small areas of surface water can escape a runaway greenhouse effect entirely. A planet like this is referred to as a land planet. In contrast, the Earth is commonly referred to as an ocean planet. On a land planet, the rate at which water is delivered to the atmosphere is too slow to trigger a runaway greenhouse effect. Water that evaporates from the hot regions simply get transported and deposited in permanent cold traps on the planet. The atmosphere of the land planet settles into a collapsed state with most of its water inventory being captured in permanent cold traps. Potential permanent cold traps include the planet’s night side or the planet’s poles. It is important to note that the night side of a land planet can only serve as a cold trap if the planet is tidally-locked where one hemisphere of the planet constantly faces its parent star while the other hemisphere is in perpetual night.

Figure 1: Artist’s impression of a desert planet with polar ice caps.

Figure 2: Artist’s impression of a desert planet.

The cold trapping rate depends very much on the thickness of the planet’s atmosphere. A thicker atmosphere will lead to less efficient cold trapping as the permanent cold traps are warmer. The outcome is that a land planet with an atmosphere that is not too thick can settle into a collapsed state by accumulating most of its water inventory in permanent cold traps at a rate that is sufficiently high to keep the planet from entering a runaway greenhouse state. At these permanent cold traps, the low temperatures allow water to accumulate as ice to form ice sheets. For a land planet that is not tidally-locked and whose axial tilt is not too large, ice sheets can exist over the poles. Similarly, a land planet that is tidally-locked can have ice sheets on its permanent nightside hemisphere. The low temperatures at the permanent cold traps suggest that all accumulated water is likely to be in the form of solid ice. Nevertheless, it is worth considering if there are any processes that can sustain bodies of liquid water for extended periods of time since liquid water is an essential prerequisite for habitability.

As water accumulates as ice on an ice sheet, the ice sheet thickens and gravity drives the flow of ice within the ice sheet. This has the effect of transporting ice away from the cold traps, towards warmer regions where the ice can potentially melt to produce liquid water. On Earth, ice sheets that are just a couple of kilometres thick can transport ice over distances of several hundred kilometres. As a result, one can imagine a land planet with an ice sheet over one of its poles or over its nightside hemisphere. Gravity driven ice flows bring the edge of the ice sheet towards regions where the temperature is warm enough for melting to occur. This leads to liquid water being present along the edge of the ice sheet, akin to pools of melt water at the end of a glacier. Here, water evaporates and precipitates back onto the ice sheet. In addition to liquid water at the edge of the ice sheet, bodies of liquid water can also exist within the ice sheet itself if the ice sheet happens to be over regions of sufficiently high geothermal heat flux.

Gliese 581c is a super-Earth orbiting the red dwarf star Gliese 581. This exoplanet has at least 5.6 times the mass of Earth and it orbits at the inner edge of the habitable zone around its parent star. The close proximity to its parent star means that Gliese 581c is likely to be tidally-locked, with a permanent nightside and a permanent dayside. Assuming that Gliese 581c is a land planet with a small inventory of water and its atmosphere is in a collapsed state where water is captured in permanent cold traps, a simulation was preformed by Leconte et al. (2013) to determine its properties. If Gliese 581c has a surface atmospheric pressure of 200 mbar, the temperature on the permanent nightside hemisphere can be as low as -100 degrees Centigrade. In contrast, the temperature on the planet’s permanent dayside can get higher than 200 degrees Centigrade around the substellar point. Most of the dayside hemisphere of Gliese 581c is simply a blistering hot desert.

Figure 3: Schematic diagram of Gliese 581c showing a dry dayside hemisphere and an ice sheet over the nightside hemisphere. Credit: Leconte et al. (2013)

A land planet with a limited water inventory can escape a runaway greenhouse state and settle into a collapsed state with most of its water being accumulated in ice sheets over its coldest regions. This is an interesting scenario because such a planet can still remain somewhat habitable even though it is closer to its parent star than the inner edge of the star’s classical habitable zone. Thermal emission spectrum and reflection spectrum from a planet can be used to differentiate whether the planet is in a runaway greenhouse state or if the planet is in a collapsed state with most of its water locked in ice sheets. If the planet is in a runaway greenhouse state, its thermal emission spectrum will shift toward shorter wavelengths due to hotter temperatures and its reflection spectrum will feature prominent water absorption bands due to the large abundance of water vapour in the atmosphere. If the planet is in a collapsed state where water is captured in permanent cold traps, its thermal emission spectrum will shift toward longer wavelengths due to cooler temperatures and its reflection spectrum will be more uniform due to the low abundance of water vapour in the atmosphere.

Figure 4: Synthetic reflection spectrum (top) and thermal emission spectrum (bottom) of Gliese 581c assuming the planet has a 200 mbar atmosphere. The dash lines show the runaway greenhouse state while the solid lines show the collapsed state. Credit: Leconte et al. (2013)

Reference:

Leconte et al. (2013), “3D climate modeling of close-in land planets: Circulation patterns, climate moist bistability and habitability”, arXiv:1303.7079 [astro-ph.EP]

Black Hole Rips Giant Planet

In January 2011, the European Space Agency’s INTEGRAL space observatory detected an X-ray flare from a nearby galaxy called NGC 4845. The X-ray flare is designated IGR J12580+0134 and follow-up observations were conducted using XMM-Newton (a space based X-ray observatory), Swift (a space observatory designed to study gamma-ray bursts) and MAXI (an X-ray monitoring device aboard the International Space Station). The light curve of J12580+0134 shows a rise to a maximum in a few weeks, followed by a gradual decrease over a year or so. A supernova explosion is unlikely to produce an X-ray flare like J12580+0134. This is because the peak X-ray luminosity of J12580+0134 is ~100 times larger than from a typical supernova and its subsequent decline in luminosity is too rapid to be consistent with a supernova.

Figure 1: Artist’s impression of a star being tidally disrupted by a black hole.

Tidal disruptions of objects by black holes have been extensively modeled and the decline in emission following peak luminosity tends to follow a power law with a characteristic slope of -5/3. It turns out that J12580+0134 indeed shows such a characteristic and is consistent with a tidal disruption event. As tidally disrupted matter plunges violently into the titanic gravitational well of the black hole, it causes rapid variability of the X-ray emission. Measurements of the fast X-ray variability observed near the peak of the X-ray flare provided an estimate of the black hole’s mass to be no more than 1,000,000 times the mass of the Sun. That would easily place this black hole into the category of supermassive black holes.

Based on the light curve characteristics of the X-ray flare, the tidally disrupted object involved in this event is likely to be a 14 to 30 Jupiter-mass sub-stellar object with about 10 percent of its mass being tidally ripped off and accreted by the black hole. In that mass regime, the object is either a free-floating giant planet or a brown dwarf. A more massive object such as a star is unlikely to be the object responsible for this tidal disruption event because the tidal disruption of a star would show a more rapid decrease in emission following peak luminosity.

Figure 2: The light curve of IGR J12580+0134 observed in the 17.3-80 keV energy band. Red squared points refer to INTEGRAL data; blue crosses show Swift and XMM-Newton observations. The solid line shows a model with a characteristic slope of -5/3, as expected for fallback of material after a tidal disruption event. The long-short dash and dotted lines indicate the predictions of simulations for the disruption of a sub-stellar object (γ = 5/3;

β = 0.7) and, respectively of a star (γ = 4/3; β = 0.65). (Credit: Guillochon, J. & Ramirez-Ruiz, E. 2013, ApJ, 767, 25)

Supermassive black holes can tidally disrupt stars and also less massive sub-stellar objects that happen to venture too close. In a galaxy, the population of sub-stellar objects is expected to be at least as large as the population of stars. For example, the population of free-floating Jupiter-mass objects is estimated by Sumi et al. (2011) to be about twice as common as stars. As a consequence, the tidal disruption of sub-stellar objects could be at least as frequent as the tidal disruption of stars. The detection of tidal disruption events in the vicinity of supermassive black holes may serve as a means to estimate the population of sub-stellar objects.

References:

Marek Nikolajuk and Roland Walter (2013), “Tidal disruption of a super-Jupiter by a massive black hole”, arXiv:1304.0397 [astro-ph.HE]

Sumi, T., Kamiya, K., Bennett, D. P., et al. (2011), “Unbound or distant planetary mass population detected by gravitational microlensing”, Nature 473, 349-352