Daydream Destinations, Part 1

Figure 1. Retro Venus, the ultimate daydream vacation. This artistic take on Venus as a lush jungle planet reflects the pulp fiction of C.L. Moore in the early 1930s (“Black Thirst”) and Leigh Brackett in the late 1940s (“The Moon That Vanished”). Image by Steve Thomas. Prints are available for purchase here

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Science fiction was the gateway drug for many of us who now share a passion for exoplanetary astronomy. I suspect we’re hoping – consciously or unconsciously – that science will eventually find worlds like the ones we grew to love through speculative novels, comics, pulps, movies, and television.

This two-part posting indulges those collective hopes with some lucid daydreaming about popular extrasolar destinations. Part 1 looks at systems with multiple stars, including circumbinary and conjoined systems. Part 2 looks at exomoons, tidally locked planets, and monoform worlds.

circumbinary systems

You can be sure you’re on an alien planet if you can see two suns in the sky. If you’re lucky you might even see three! (Figure 2) Ever since Star Wars, a multiplicity of host stars has defined mass cultural expectations for extrasolar planets. Thanks to George Lucas, objects in orbit around a pair of stars are now frequently known as Tatooine planets, not just among publicists but even in astronomical circles. The more technical modifier for such a planet is “circumbinary,” meaning that it orbits both members of the binary rather than just one.

Figure 2. Sunset over Trisol

The planet Trisol orbits a triple star system (types G, K, and M) located in the Forbidden Zone of the Galaxy of Terror. (Futurama, Season 1: episode “My Three Suns,” originally aired May 4, 1999)

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In the full stellar population, the likelihood of stellar multiplicity increases with mass. As the least massive stars, M dwarfs tend to be single, while virtually all of the most massive stars – types O and B – occur in multiple systems. Binaries are the most common type of multiple star, with larger assemblies becoming progressively less frequent. Among star systems of all spectral types inside 10 parsecs, the RECONS survey reported 185 single stars (71%), 55 binary systems (21%), 15 triples (6%), 3 quadruples (1%), and 1 quintuple (less than 1%).

Because our Sun is well above the median stellar mass, 44% of the Sun-like stars (spectral types F6-K3) located inside 25 parsecs occur in multiple star systems (Raghavan et al. 2010). Within that volume, 33% of Sun-like stars occur in binaries, 8% in triple systems, and 3% in quadruple systems or higher. Raghavan & colleagues noted that higher-order multiples tend to be younger than binaries, suggesting that systems with three or more stars in their youth typically lose stars over time.

Even before the Kepler mission, many exoplanetary host stars had known binary companions. Among non-Kepler binaries with planets, the separation between the two stars ranges from about 20 AU (similar to the distance of Uranus from our Sun) to 10,000 AU (Raghavan et al. 2010). Until very recently, only one star in each planetic binary was known to host planets. Now we know of at least two conjoined systems in which each star hosts its own planetary system (XO-2 and WASP-94).

These days, however, discussions of binary host stars often center on circumbinary planets. Although theory predicted that very close binaries could jointly host a single planetary system, Kepler was the first telescope actually to detect such a configuration. These systems are rare: only nine have been identified to date (Table 1). Eight out of nine host single planets, which include puffy gas dwarfs, baby gas giants, and one Jupiter-sized giant (KOI-2939b). Except for the latter planet, the radii of these singletons range from 4 to 8.6 Rea, staking out a poorly populated area of the overall Kepler distribution. The remaining system (Kepler-47) hosts a family of three small planets, all of low density. Two-thirds of the parent binaries in these nine systems comprise a Sun-like star plus an M dwarf; one-third comprise a pair of Sun-like stars of similar mass. Habitable Earth-size planets might be possible in systems like these, but they are clearly more difficult to observe than larger planets, and they may also be less frequent. Notably, 40% of the known binary planets occupy their system habitable zones (Kostov 2015). One of them is KOI-2939b, which is also an excellent candidate for hosting large exomoons.

Table 1. Circumbinary systems as of April 2016

Tags: Msol = star mass in Solar units; period = period in days; a = semimajor axis in Earth units; e = orbital eccentricity (0 = circular); Rea = planet radius in Earth units; Mea = planet mass in Earth units. Distance = distance in parsecs

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Theory and observation alike show that stable multiple star systems are built from binary units arranged in a strict hierarchy (Figure 3). Given the progressively increasing scarcity of a) triple or higher-order multiples of any spectral type, b) Earth-like planets, and c) circumbinary planetary systems, the following prediction seems safe: Habitable planets or moons orbiting all members of a multiple star system are much more likely to have binary hosts than hosts that are triple (single + double) or quadruple (double + double).

Figure 3. Orbital configurations of selected quadruple and quintuple star systems

These three systems are located within 25 parsecs. They illustrate the hierarchy of binary orbits that constitute higher-order multiple star systems. Based on Figures 23 and 24 of Raghavan et al. 2010.

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wide binaries

Although I’m sensitive to their rarity and appeal, I’m not as fascinated by circumbinary planets as the mass media expect me to be. Planets in relatively wide binaries are not only vastly more common, but vastly more interesting – not least because of their science fictional possibilities.

At least two potential scenarios are available. The first involves climatic fluctuations induced by an eccentric binary orbit.

Imagine an inhabited planet whose parent star is an early K dwarf with a brighter, hotter binary companion of spectral type F7. The binary orbit is wide and extremely eccentric, and its period is longer than a thousand Earth years. When the two stars are at periastron (i.e., their closest approach, which would last a few decades) the planet heats up. For several years during periastron passage, the day/night cycle alternates between two seasons: One features hot, bright days with both stars visible in the sky, followed by dark nights with neither star. The other season sees endless light, with the K star shining by day and the F star turning the nights white. Then the two stars would separate, the white star would shrink and grow dimmer, and temperatures would cool again. Assuming that the planet has polar caps and oceans, at periastron the ice would shrink, sea levels would rise, and weather would become unusually volatile. Moving toward apastron (the two stars’ widest separation) the reverse would happen. The effects of this cycle on geology, climate, organisms, psychologies, and cultures would be momentous.

Figure 4. The Helliconia Trilogy

The overall impact would depend on the planet’s baseline climate near apastron, where it would spend most of its orbit. If the baseline is temperate, periastron might occasion fiery disasters, while if the baseline is glacial, periastron might herald a brief, benign epoch of abundance. Brian Aldiss has explored such an eccentricity-induced climate in his Helliconia Trilogy (1982-1985; Figure 4), and George Martin seems to be playing with similar ideas in his ongoing Song of Ice and Fire series (1996-?). However, as Martin has warned us, the irregular climate of Westeros has more to do with magic and dragons than with astrophysics (see also Kostov et al. 2013).

conjoined systems

The second binary scenario that floats my boat involves conjoined planetary systems. The separations between the two stars in such a configuration will be measured in astronomical units (AU) instead of parsecs. Their proximity will enable interstellar travel within a human lifetime through the use of propulsion technologies currently in development. Imagine a binary system comprising one G2 star like our Sun and a cooler G8 star separated by 150 AU. Each system has a different architecture, but both feature at least one inhabited planet. The inhabitants might be indigenous to each system, or indigenous to one and pioneers in the other, or colonists on both from a far-future version of Earth. Conjoined systems plus space travel will likely give rise to a federation of planets, rather like those envisioned in the space operas of the 1930s and 1940s and later updated by both iterations of Battlestar Galactica.

Figure 5. Detail of the Cyrannus quadruple system, home of the Twelve Colonies

Here I’m thinking of C.L. Moore’s tales of Northwest Smith, an outlaw rocketeer whose wanderings take him to Venus, Mars, and an unnamed moon of Jupiter, all of which have indigenous life. Moore’s vision of a densely inhabited Solar System, as presented in her fiction of the early 1930s, was resurrected a decade later in dozens of stories by her friend Leigh Brackett. The latter created a similar outlaw hero, Eric John Stark, an Earthling who was raised from infancy – much like Tarzan – by a tribe of hairy Mercurians. His adventures regularly bring him to Mars and Venus, which as in Moore’s stories are envisioned respectively as a decadent desert planet and a savage jungle planet.

The creators of Battlestar Galactica modernized this many-worlds setting by leaving our Solar System altogether and situating the human race in an extrasolar locale called the Twelve Colonies. These are twelve inhabited worlds in the conjoined planetary systems orbiting a quadruple star system. Google searches returned several maps of this remarkable construct; Figure 5 shows a detail of one. A similar approach was adopted by the people behind the short-lived but much-loved Firefly series when they created their wild extrasolar frontier. Known as the Verse, this locale appears to be a quintuple star system that seriously violates the laws of binary hierarchy (a characteristic shared with Serenity’s crew). Figure 6 shows the official map, which highlights inhabited exomoons and exoplanets.

Figure 6. The Verse, scene of the action in Firefly and Serenity

Next to these impossible wonders, WASP-94 and XO-2 offer thin gruel indeed. But at least they’re real. And the odds are that somewhere in our Galaxy, there’s at least one pair of conjoined systems with a habitable planet or moon between them.

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REFERENCES

Kostov V, Allan D, Hartman N, Guzewich S, Rogers J. (2013) Winter is Coming. Submitted for publication in Oldtown Journal of Evil Omens. Abstract: 2013arXiv1304.0445K

Kostov V, Orosz JA, Welsh WF, Doyle LR, Fabrycky DC, Haghighipour N, et al. (2015) KOI-2939b : the largest and longest-period Kepler transiting circumbinary planet. Astrophysical Journal, in press. Abstract: 2015arXiv151200189K

Raghavan D, McAlister HA, Henry TJ, Latham DW, Marcy GW, Mason BD, Gies DR, White RJ, ten Brummelaar TA. (2010) A survey of stellar families: Multiplicity of Solar-type stars. Astrophysical Journal Supplement Series 190, 1-42. Abstract: 2010ApJS..190....1R

Welsh WF, Orosz JA, Short DR, Cochran WD, Endl M, Brugamyer E, et al. (2015) Kepler 453 b – The 10th Kepler transiting circumbinary planet. Astrophysical Journal 809, 26. 2015ApJ...809...26W

Welsh WF, Orosz J, Quarles B, Haghighipour N. (2015) Kepler-47: A Three-Planet Circumbinary System. American Astronomical Society, ESS Meeting #3, id.402.01. Abstract: 2015ESS.....340201W

A Year of Small Wonders

Figure 1. Artist’s view of the upper atmosphere of HAT-P-11b, a transiting Hot Neptune, where water molecules were recently reported. This was the first detection of water on a low-mass extrasolar planet. Credit: NASA/JPL-CalTech

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Some years bring spectacular extrasolar discoveries (like the first circumbinary planet) or dramatic events (like the breakdown of a beloved space telescope). Others just bring more of what we’ve already seen. But 2014 fits neither description.

Although I didn’t notice any Earth-shattering news last year, I did see a dozen or more smaller discoveries of great significance, scattered across various sub-specialties of planetary and exoplanetary astronomy. Some of them have already been reported in this blog, but most came at times when I was way too busy to write about them.

So here are my top ten news items from outer space in 2014. I’ve grouped them by affinity: first the transiting exoplanets, then the radial velocity exoplanets, then a directly imaged exoplanetary system, and finally some interesting developments in – of all places! – our own Solar System.

1. Water vapor over HAT-P-11b

For the first time, water vapor was detected in the atmosphere of a low-mass extrasolar planet, HAT-P-11b (Fraine et al. 2014). Although water had already been found in several transiting Hot Jupiters, previous observations of HAT-P-11b, other Hot Neptunes, and Hot Super Earths all returned null results. This absence of evidence was puzzling, because recent models of planet formation predicted that the bulk composition of many low-mass planets would include a substantial proportion of ices. Nevertheless, after several different teams reported featureless spectra for such promising targets as GJ 1214 (the nearest transiting Super Earth) and GJ 436 (the nearest transiting Hot Neptune), theorists were motivated to develop novel models in which these planets have deep atmospheres of hydrogen and helium (H/He) directly on top of rock/metal cores (e.g., Lopez & Fortney 2014).

Now we see that these models aren’t a good fit for HAT-P-11b. Instead, according to the discovery paper, this planet most likely has significant fractions of water and other heavy molecules mixed in a deep atmosphere dominated by H/He. It remains uncertain whether HAT-P-11b might support a mantle of high-pressure ice over a rocky core, but it’s definitely a working hypothesis.

This detection breathes new life into older models of Water Planets and Ice Dwarfs (e.g., Leger et al. 2004), and promises to reframe our current perceptions of low-mass planets. With such a watery stratosphere, HAT-P-11b clearly did not form on its present orbit, which whizzes the planet around its K-type parent star in just five days. Instead, it must have accreted much farther away from the star, in a region of the primordial nebula where icy planetesimals were available. Hence this object’s composition provides a meaningful constraint on future work in planet formation and structure. My impression is that this result disfavors the hypothesis of the “minimum mass extrasolar nebula” proposed by Chiang & Laughlin (2013), which was also critiqued on other grounds in several articles last year (e.g., Cossou et al. 2014, Inamdar & Schlichting 2014, Izidoro et al. 2014, Raymond & Cossou 2014, Schlaufman 2014, Schlichting 2014).

2. Massive Kepler data dump

Although the primary mission of the Kepler space telescope was terminated by equipment failure in 2013, the transit data collected during three-plus years of operation will fuel new discoveries for a long time to come. That point was driven home in March, when Kepler scientists confirmed more than 700 planets in one fell swoop, raising the exoplanetary census from less than 1000 to almost 1800 planets in a single day. (By now that number has grown still further, to 1855.) This vastly enlarged sample sharpens our view of the exoplanetary cosmos. Instead of the gas giants that previously dominated the census, the current sample is about 60% low-mass planets, and for the first time transit discoveries outnumber radial velocity discoveries. Much richer data are now available to test theories of planet structure and atmospheric composition, not to mention system architectures and evolution.

[Here are some definitions, in case you need them: A gas giant is a planet more massive than 17% of Jupiter (0.17 Mjup) with a bulk composition more than 50% H/He. A low-mass planet has a mass smaller than 0.17 Mjup (i.e., less than 55 Earth masses or 55 Mea) and a composition more than 50% rock/metal. Transiting planets are detected when they cross the face of their parent stars, such that their shadows, as seen from the perspective of Earthly observers, dim the stars’ light. Radial velocity planets are detected by color shifts in their parent stars’ optical spectra, which happen as the planets’ gravity tugs the stars closer or farther away from us. Transiting planets that are massive enough or located close enough to Earth (usually gas giants) can also be detected by radial velocity observations.]

Figure 2. Kepler-186f is probably tidally locked and possibly terrestrial. If so, it might resemble the Eyeball Earth pictured in this artist's view. The planet imagined here is wet and fairly cold, with its icy hemisphere extending into the sunlit side. At the sub-stellar point, its atmosphere churns in a more or less permanent cyclone. 

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3. Kepler-186f, another Earth-like planet

Kepler scientists found their Holy Grail in 2013 with the confirmation of Kepler-62f, the first potentially rocky planet identified in a circumstellar habitable zone. They followed up with another one last year: Kepler-186f. I wrote extensively about that system at the time of the announcement, so I refer you to my post for full details. Notably, these two candidates for the status of Earth 2 bear a family resemblance. Both orbit stars that are smaller and cooler than our Sun: Kepler-62 is a K dwarf, Kepler-186 is an M dwarf. Both planets have radii between 1.1 and 1.4 times Earth (1.1-1.4 Rea), implying masses between 1 and 4 Mea. And both are predicted to have equilibrium temperatures even lower than on Earth – quite a marvel, considering that the vast majority of low-mass exoplanets, even in the newly enlarged census, are very hot indeed.

Figure 3. Architecture of the Kepler-289 system, with all three transiting planets rendered at their relative sizes. The masses of planets c and d were determined by analyzing transit timing variations; however, that method did not yield an unambiguous mass for the inner planet. Note that, at the time of writing, the designations of these three planets are not set in stone. The object denoted as “d” in this figure was previously designated “c” by Kepler Mission scientists, while the Planet Hunters elected to use their own terminology for all three objects, naming them PH b, PH c, and PH d. In a mash-up of the Solar System and Kepler-289, Mercury’s orbit would lie between planets c and d.

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4. Kepler-289, another mixed-mass system

Last year I blogged about several new examples of a rare planetary system architecture, one that includes at least one gas giant and at least two low-mass planets. This year, the Planet Hunters group reported another of these mixed-mass systems: Kepler-289, which centers on a Sun-like host star located 700 parsecs away (Schmitt et al. 2014). All three detected planets are mutually well-aligned, since all are seen in transit, and two of them (the middle and the outer) have robust masses determined by transit timing variations. The inner and middle planets have radii between 2 and 3 Rea and masses below 10 Mea, while the outer giant has a radius very close to Jupiter’s, despite a mass of only 0.42 Mjup (about 30% more than Saturn’s). Although this system isn’t a likely candidate for habitable worlds, since none of the known planets could support liquid water, it adds to an extremely valuable sample of exoplanets. At my reckoning, we now have seven confirmed Kepler systems with at least one high-mass and two low-mass planets. In addition to Kepler-289, they are Kepler-30, -48, -68, -87, -89, and -90. We also have three radial velocity systems that fulfill the same criteria: GJ 676A, GJ 876, and HD 10180. To exploit a well-flogged metaphor, I believe this ensemble of twelve systems is a cosmic Rosetta Stone holding the secrets of creation. So I’m hoping some clever astronomers will develop a model to explain the formation of all twelve. If they do, I think such a model will also go a long way toward explaining the formation of planetary systems in general.

5. Celebrity planets exposed as phantoms

Now we come to the saddest news of the year, a daunting reminder that almost all the lovely exoplanetary data so assiduously collected by astronomers over the past two decades remain tentative. Paul Robertson and Suvrath Mahadevan set their sights on two nearby systems popular with the astrobiological set, GJ 581 and GJ 667C (Robertson et al. 2014, Robertson & Mahadevan 2014). Considerable controversy has surrounded the number and nature of the planets in each system (as reported here and here). So Robertson, Mahadevan, and their collaborators reanalyzed existing radial velocity data on each one and reached surprising conclusions: Instead of the six planets proposed by Vogt et al. (2010) or the four claimed by Forveille et al. (2011), GJ 581 has only three planets, and none of them orbit in the habitable zone. Instead of the six planets reported by Anglada-Escude et al. (2013), GJ 667C has only two, and neither is a candidate for habitability. (Click the preceding links for full blog posts on each study.) According to Robertson and Mahadevan, the data interpreted as evidence for many of those phantom planets were simply artifacts of the rotation periods of their respective host stars.

And so, after a few years of great optimism, when it looked like Earth had at least two potentially habitable counterparts within 25 light years, we’ve returned to the perspective of 2006: none of the planets returned by radial velocity searches have masses and temperatures consistent with surface bodies of liquid water. (Of course, it’s a different story for transit searches; see #3 above.)

Figure 4. 30 and 31 Cygni, shown here, comprise an especially photogenic binary star system. On closer look this pair is actually a triple, since the yellowish star is itself binary. Credit: Wikimedia

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6. Hot Jupiters in conjoined planetary systems

Many exoplanets are known to orbit stars in binary systems, but until a few years ago, we knew of no binaries in which both members of the pair supported their own families of planets. That seemed odd, because there seemed no reason why one star would be “planetic” while its sibling wasn’t. Then, in 2011, an unpublished manuscript by Michel Mayor and colleagues reported two low-mass planets in orbit around the G-type star HD 20781. This is the binary companion of another G star, HD 20782, which was already known to harbor a gas giant planet. During the year just ended, two more such systems were reported, this time in peer-reviewed journals: XO-2NS (Desidera et al. 2014) and WASP-94 AB (Neveu-VanMalle et al. 2014). The announcement of the latter received the biggest fanfare of the lot, probably because both systems have exactly the same architecture. Each one harbors a Hot Jupiter without any sign of companions on cooler orbits.

Robust confirmation of the reality of planet-rich binaries is significant for two reasons: 1) Most Solar-type stars, which are probably the most likely to harbor habitable planets, occur in binaries. Now that we know both members of a binary can host planets, we can be confident that the sample of potentially habitable systems is huge. 2) From a purely science fictional viewpoint, a pair of such systems with at least one planet supporting a high-tech civilization could become the scene of an interstellar empire - and its citizens would have no need for faster than light travel! They could cruise from one star to the other in ships powered by xenon ion thrusters, or similar technologies known even among primitive species like Earthlings.

A final consideration is what to call these planet-bearing stellar couples. We have “binary” to denote a gravitationally bound pair of stars, and “circumbinary” to denote a planet that orbits both members of a binary. But what about systems like WASP-94AB and XO-2NS? Calling them “binary planetary systems” is insufficient to convey the notion that both stars host their own planets. Therefore, consistent with the biological flavor of common descriptors for binary stars (twins, siblings), I propose the term “conjoined planetary systems.” I concede that “Siamese systems” might be more catchy (though it’s probably culturally inappropriate), and if you prefer a non-biological alternative, I offer “double planetary systems.” But my money’s on “conjoined.”

Figure 5. The dusty protoplanetary disk around HL Tauri was photographed in spectacular detail by ALMA. To provide a scale for this figure, green dots mark the orbits of planets and dwarf planets in the only two systems known to support well-constrained gas giants at wide separations: HR 8799 and our Solar System. Note that the protoplanetary disk around our Sun was much smaller than HL Tauri’s disk. According to the consensus view, the present orbit of Neptune traces its original outer edge. A full-size image of the HL Tauri disk in all its glory, without text, is available here.

7. Spectacular snapshot of the birth of planets

In September, researchers using the Atacama Large Millimeter/submillimeter Array (ALMA), an installation of telescopes in Chile’s Atacama Desert, released the most detailed image ever obtained of a nascent planetary system. The central star is HL Tauri, a pre-main sequence object of Solar mass that has been under intensive study for the past 40 years. The image (available here in various sizes) depicts a large dusty structure with a radius of about 80 AU, almost triple the radius inferred for our Sun’s planet-forming nebula. The dust is visible at submillimeter wavelengths as a bright disk interrupted by a series of eight darker bands that resemble the gaps in the rings of Saturn. ALMA astronomers identify each gap as a likely site for planet formation, hinting that eight large planets are under construction with periods substantially longer than Jupiter’s. As Catherine Vlahakis, an ALMA scientist, stated: “This one image alone will revolutionize theories of planet formation.”

HL Tauri is located at a distance of about 140 parsecs (450 light years). Estimates of its age vary; ALMA scientists characterize the star as less than one million years old. For such a young star, the detailed structures visible in the protoplanetary disk, even at semimajor axes far wider than the orbit of Pluto, came as a big surprise. Astronomers had not expected planetary accretion to have reached such an advanced stage so far from a star.

This spectacular photograph was the first to be obtained in ALMA’s most powerful mode, which enables imaging at resolutions even higher than the Hubble Space Telescope. According to the researchers, we can expect many more brilliant images from ALMA in the next few years.

Figure 6. Enceladus, moon of Saturn, represented at the same scale as Earth. The large body of water on the lower right is Lake Superior, whose volume might be similar to the subsurface lake recently identified on Enceladus.

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8. Subsurface lake on Enceladus

As the sixth-largest moon of Saturn, Enceladus has a diameter of only 505 kilometers (314 miles). It consists of a rocky core supporting a thick mantle of ice. In 2005, the Cassini spacecraft confirmed that this little moon regularly vents fountains of ice, which have been captured in photographs. The leading explanation for this activity has been a hypothetical sea of liquid water beneath the icy mantle.

And so it is! This past year, Iess et al. (2014) used Cassini data on the quadrupole gravity field of Enceladus to identify a subsurface lake or “regional sea” beneath the moon’s south pole. This is the same area where venting is observed above the surface features known as tiger stripes. The research team emphasized the limited extent of this lake, ruling out a global ocean. They found that the water is located 30 or 40 km beneath the ice, and that its depth is about 10 km. News reports compared its volume to Lake Superior, a large body of water in North America (though the maximum depth of Superior is only 406 meters). The Enceladic lake vents vapor and ice particles through fissures reaching to the surface.

This important discovery raises the probability that other icy bodies in the Solar System – notably Europa and Ceres – have similar morphology. It also provides a real-life example of the structures proposed for some icy extrasolar Super Earths (Leger et al. 2004). According to many astrobiologists, subsurface water is a potential environment for the evolution of life.

Figure 7. Artist’s view of watery emissions from the dwarf planet Ceres. Credit: Ron Miller

9. Ceres emits water

Do other small bodies in the Solar System also produce clouds of water and ice? Observations with the Herschel Space Telescope identified emissions of water vapor from Ceres, which is both a dwarf planet and the largest object in the Asteroid Belt. Herschel scientists suggested that the water came from localized sources in Ceres’ mid latitudes, but they could not be sure if it resulted from sublimation of surface ice or from cryovolcanoes similar to those on Enceladus (Kuppers et al. 2014).

NASA’s Dawn spacecraft, which has already visited Vesta, will soon enter orbit around Ceres and hopefully send back our first close-up view of this remnant from the Solar System’s remote past. Ceres is a sphere with a diameter of about 950 km (590 mi), so it should present an excellent subject for robo-photography. With Dawn’s data we should get a better understanding of its internal structure and formation history. Ironically, we know much more about the small moons of Saturn, which are only a fraction of Ceres’ size and far more distant from Earth, than we do about Ceres, entirely because of the various spacecraft that have visited Saturn’s photogenic entourage of rings and moons.

10. Chariklo, the ringed Centaur

Chariklo is a Centaur orbiting between Saturn and Uranus. No, it’s not a mythical amalgam of a horse and a man; in astronomical parlance, a Centaur is a small object resembling an asteroid (or a comet or a Kuiper Belt object) orbiting outside the semimajor axis of Jupiter. With an approximate diameter of 250 km (155 miles), Chariklo is the largest known Centaur. As an asteroid it would be quite big, but as a Kuiper Belt object it would be dinky. During a recent stellar occultation by Chariklo, several “secondary events” were observed, likely evidence of a ring system around the central object (Braga-Ribas et al. 2014). Although Chariklo is unlikely to be spherical, its ring system is circular, consisting of two flat, concentric rings with an outer radius of about 405 km (250 miles). By comparison, the Saturnian ring system has an edge-to-edge diameter similar to the separation between the Moon and Earth (240,000 miles).

The discovery team speculated that Chariklo’s rings might be a transient phenomenon; Saturn’s rings have inspired similar speculations, with no consensus yet. Regarding Chariklo, the team noted that some other Centaurs are known to have tiny moons whose disintegration could create rings. If this view is accurate, it boosts the likelihood that rings are a common feature of exoplanetary systems (where Earthly artists already love to depict them anyway).

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And that was the year in outer space! Lots of news on small planets, small moons, and big asteroids, but best of all: water, water, everywhere!

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REFERENCES

Anglada-Escudé G, Tuomi M, Gerlach E, Barnes R, Heller R, Jenkins JS, Wende S, Vogt SS, Butler RP, Reiners A, Jones HRA. (2013) A dynamically-packed planetary system around GJ 667C with three super-Earths in its habitable zone. Astronomy & Astrophysics 556, A126. Abstract: http://adsabs.harvard.edu/abs/2013A%26A...556A.126A

Braga-Ribas F, Sicardy B, Ortiz JL, Snodgrass C, Roques F, Vieira-Martins R, Camargo JIB, Assafin M, Duffard R, Jehin E, Pollock J, Leiva R, Emilio M, Machado DI, Colazo C, Lellouch E, Skottfelt J, Gillon M, Ligier N, Maquet L, Benedetti-Rossi G, Gomes AR, Kervella P, Monteiro H, Sfair R, Moutamid ME, Tancredi G, Spagnotto J, Maury A, et al. (2014) A ring system detected around the Centaur (10199) Chariklo. Nature 508, 72-75. doi:10.1038/nature13155

Cossou C, Raymond SN, Hersant F, Pierens A. (2014) Hot super-Earths and giant planet cores from different migration histories. Astronomy & Astrophysics 569, A56.

Desidera S, Bonomo AS, Claudi RU, Damasso M, Biazzo K, Sozzetti S, Marzari F, et al. (2014) The GAPS programme with HARPS-N@TNG IV: A planetary system around XO-2S. Astronomy & Astrophysics 567, L6. Abstract: http://adsabs.harvard.edu/abs/2014A%26A...567L...6D

Forveille T, Bonfils X, Delfosse X, et al. (2011) The HARPS search for southern extra-solar planets XXII. Only 4 planets in the Gl 581 system. Unpublished manuscript. Abstract: http://adsabs.harvard.edu/abs/2011arXiv1109.2505F

Fraine J, Deming D, Benneke B, Knutson H, Jordan A, Espinoza N, Madhusudhan N, Wilkins A, Todorov K. (2014) Water vapour absorption in the clear atmosphere of a Neptune-sized exoplanet. Nature 513, 526-529.

Iess L, Stevenson DJ, Parisi M, Hemingway D, Jacobson RA, Lunine JI, Nimmo F, Armstrong JW, Asmar SW, Ducci M, Tortora P. (2014) The gravity field and interior structure of Enceladus. Science 344, 78-80.

Inamdar NK, Schlichting HE. (2014) The formation of Super-Earths and Mini-Neptunes with giant impacts. Monthly Notices of the Royal Astronomical Society, in press.

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