Much Ado About Earth 2

Earth 2 + N: Icons of small transiting exoplanets at their relative sizes, with Earth at the same scale. All these objects are believed to occupy their systems’ habitable zones. To suggest the potential range of morphologies, the extrasolar icons represent a warm ocean planet, an icy ocean planet, a temperate planet with continents and seas, and a Big Mars. Assignment of surface structure and atmosphere is arbitrary, since any of these objects might be an ocean planet or a Big Mars. The three smallest Kepler planets are likely to be tidally locked, so all three appear as Eyeball Earths with standing cyclones around the substellar point. Only Kepler-62f is likely to rotate like Earth and Mars.

--------------------------------------

Eleven days ago, just in time for Twelfth Night, a research team led by Guillermo Torres reported the “Validation of Twelve Small Kepler Transiting Planets in the Habitable Zone.” The cybermedia loved the story, broadcasting many thrilling variations on the theme of Most Earthlike Planet Yet! Now that the piping pipers and dancing ladies have come and gone, I’ve found time to look behind the barrage of news items and unwrap these Gifts of the Magi.

In fact, Torres and colleagues have brought us only two new Earth-like objects (though some popular accounts suggested other numbers, ranging from one to eight to a thousand). Their official names are Kepler-438b and Kepler-442b. Torres’ collaborators, most of whom are associated with the Kepler Mission, started their search for another Earth 2 about a year ago. This was some time after the announcement of Kepler-62f, the first plausibly terrestrial planet orbiting in an extrasolar habitable zone. They began by selecting all Kepler candidates then believed to be in the habitable zone and smaller than 2.5 Earth radii (2.5 Rea). Then they conducted extensive follow-up observations and analyses to derive the most robust parameters possible for each object of interest.

In the end Torres and colleagues were able to validate a dozen of these smallish planets, though the status of the twelfth is less secure than the others. Meanwhile, a subgroup of his collaborators reported one of the most Earth-like candidates (Kepler-186f) separately from the rest. Then another of those candidates (Kepler-296f) was found to orbit a member of a close binary system, making the planet’s characterization more difficult. Of the remainder, three are clearly larger than 2 Rea, ruling out a terrestrial composition, and only two are smaller than 1.5 Rea, widely regarded as the approximate upper boundary for Earth-like composition. Those two objects were at the center of last week’s hoopla. 

Along with Kepler-62f and Kepler-186f, we now have four robust candidates for terrestrial composition and surface water. And yes, the new ones, especially Kepler-438b, compare very favorably with the earlier candidates, as shown in Table 1:

Four Potentially Earth-like Extrasolar Planets

Column 1 represents the star name; column 2 the stellar effective temperature in Kelvin; column 3 the stellar mass in Solar units; column 4 the stellar metallicity; column 5 the stellar age in billions of years; column 6 the distance to the system in parsecs; column 7 the planet name; column 8 the KOI number; column 9 the planet radius in Earth units; column 10 the orbital semimajor axis in astronomical units (Earth’s orbit = 1); column 11 the planet equilibrium temperature in Kelvin; and column 12 the orbital period in days. Data on equilibrium temperatures, and all data on Kepler-62, derive from the Kepler Discoveries Table. Other data follow Torres et al. 2015.

--------------------------------------

If each of these objects had the same iron/silicate composition as Earth, their respective masses would be about 1.5, 1.6, 2.5, and 3.5 Mea (Zeng & Sasselov 2013, Lissauer et al. 2013). Although there is probably an upper mass limit for the habitability of a purely rocky planet, based on the reduced likelihood of plate tectonics at high mass, no consensus has emerged on what the limit might be. My conservative guess is about 3 Mea, disfavoring Kepler-62f.

A somewhat wetter composition – approximately 10% ice, 90% iron/silicate – would reduce the two smaller objects to about 1 Mea and the two larger to about 2 Mea. Such a large watery component, however, would most likely render all these planets uninhabitable, because surface water would be isolated from core metals by an ice layer, and thus would lack the chemical diversity believed necessary for the emergence of life (Alibert 2014).

Regarding planetary climates, the table above presents the equilibrium temperatures (Teq) estimated by Kepler Mission scientists. All planets except Kepler-186f have Teq within the traditional limits (i.e., 185-303 K; Kopparapu et al. 2013). These limits are based on the fact that Earth, with its Teq of 255 K and mean surface temperature of 288 K, is securely parked in a habitable space. However, many studies over the past few years have debated the definition of the habitable zone (e.g., Zsom et al. 2013) as well as the use of Teq in this definition (e.g., Kastings et al. 2014). Some astronomers have argued for extending the habitable zone’s inner limits (Seager 2013), while others have argued against it (Kastings et al. 2014). Amid these debates, all four planets remain promising, even Kepler-186f. Its mass predicts a substantial atmosphere whose greenhouse effect could raise surface temperatures appropriately.

Another key factor in understanding surface conditions on an extrasolar planet is rotation. Three out of four planets in Table 1 orbit within their host star’s tidal locking radius (Selsis et al. 2007). Therefore, their rotation is likely to be “synchronous” with their orbits – in other words, they always turn the same hemisphere toward their suns. A possible outcome of synchronous rotation is loss of atmosphere through freeze-out of volatiles on the permanent night side, rendering the planet uninhabitable. Fortunately, studies old and new (Joshi 2003, Yang et al. 2014) provide scenarios that avoid this outcome.

As Yang and colleagues recently argued, the major variables involved are the synchronous planet’s complement of surface water, its geothermal flux (i.e., volcanism and related processes), and the percentage of its night side covered by land. On an ocean world where geothermal flux is strong and scattered archipelagos are the only land, some sea ice would accumulate on the night side, but liquid water would be abundant everywhere. On a world with oceans, continents, and geothermal flux in the range of Earth values, ice sheets would accumulate on night-side continents, but total freeze-out would not occur and clement conditions would prevail. Only a world with low geothermal flux, limited surface water, and a night side covered by continents would build massive ice sheets and suffer complete loss of liquid water. Thus the odds of a tidally locked planet with livable surface conditions seem favorable.

system architecture and habitability

So far, so good. At least three out of four planets remain plausible candidates for the evolution and survival of life, and the two newest arrivals appear very similar to the old ones. However, Kepler-438b and -442b differ from Kepler-62f and -186f in one important way: each of the new candidates is the only detected planet orbiting its host star. Indeed, their loneliness made it more difficult for Torres’ team to validate them, since validation gets progressively easier as the number of transiting candidates per star increases.

I found this detail surprising and a little disappointing. Each of the older candidates is the outermost planet of five, and in both systems, all orbits are mutually well-aligned and all planets are smaller than 2 Rea. These data tell us that the systems had relatively placid dynamic histories, and that their planets are likely to be rich in refractory elements. They are also consistent with our expectation that planets in the range of 1 to 6 Rea occur in compact systems with neighboring planets of similar size.

So what’s going on with Kepler-438 and Kepler-442? Does each star host only one planet within 0.5 AU? Or does each actually host numerous small planets, except that the Earth-like candidates happen to be significantly misaligned with the rest?

As far as I understand, it’s possible for a planetary system to be “flat” (i.e., co-planar with minimal misalignment), yet our viewing angle can be such that only the inner planets are visible in transit, while the outer planets remain just outside of range. Regardless of viewing angle, however, if an outer planet transits, then mutually aligned inner planets should also transit. At face value, then, Kepler-438b and Kepler-442b do not seem to be members of the compact multiplanet systems we’ve come to know and love. I look forward to future investigations on this point.

superhabitable planets?

Debates aside, all four of these candidates still look promising. Their charms were enhanced by a cover story in Scientific American that appeared on the newsstands when Torres et al. announced their findings: “The Hunt for Planets Better Than Earth.” Written by Rene Heller, this article presents a plain-language summary of Heller & Armstrong’s 2014 study of “Superhabitable Planets” in Astrobiology.

Heller starts by noting that Earth is hardly the “best of all possible worlds,” because its host star will eventually evolve off the main sequence and evaporate all our water. (Only 1.75 billion years to go!) If we want a longer-lived biosphere, we need to find planets that orbit M and K dwarfs, since these types will continue burning on the main sequence for many tens of billions of years. Recognizing that M dwarfs are subject to “powerful stellar flares and other dangerous effects,” Heller singles out K dwarfs in particular as occupants of the “sweet spot of stellar superhabitability.” In addition, he argues that planets more massive than Earth – ideally about 2 Mea – are friendlier to life than planets in the mass range of Venus and Earth. This is because more massive planets will probably have higher levels of geothermal flux, which sustains the carbon cycle and maintains the planetary magnetic field, thereby averting both a CO₂ greenhouse and atmospheric erosion by cosmic rays over multi-billion year time scales.

All the Kepler planets summarized on this page are approximately consistent with Heller’s criteria, although none provide a perfect fit: The only candidate orbiting a K dwarf is Kepler-62f, which must be about 3.5 Mea if it is purely rocky.
  
the problem with M dwarfs

The three best Goldilocks candidates, including the two newest, orbit M dwarfs. These relatively dim stars have masses between about 10% and 60%  Solar (0.1-0.6 Msol). They represent the commonest spectral type in the Galaxy, accounting for 75% of the overall stellar population. They also seem to be rich in small planets, and their habitable zones have much smaller radii than the ones around more massive stars. These criteria mean that habitable planets around M dwarfs have much shorter orbital periods than those around Sun-like stars. Shorter periods, in turn, mean that habitable M dwarf planets are more likely to transit than habitable G dwarf planets, and are easier to detect when they do. 

All these circumstances help to explain the fact that, even though Kepler was specifically designed to study Sun-like stars (spectral types G, early K, and late F), only one of the four candidates discussed here (Kepler-62f) has a reasonably Sun-like host.

Yet M dwarfs still have problems. A brand-new study by Luger & Barnes (2015) provides a convenient rundown:

• M dwarfs typically emit much of their luminosity in X-rays and extreme ultraviolet wavelengths, which can drive atmospheric escape and harm organisms.
• They are subject to brief flaring events in which they emit a much higher energy flux, which can destroy volatiles and erode atmospheres, especially in planets orbiting in their close-in habitable zones.
• They spend a long stretch of their formative years at luminosities one to two orders of magnitude higher than they will be after they settle down on the main sequence. Since planetary systems must form during this epoch, gestating M dwarf planets might develop a runaway greenhouse early on and lose all their volatiles.
• This luminosity evolution also means that the planets we now observe in M dwarf habitable zones probably formed when habitable temperatures were available only at much wider separations from the central star. Such planets would be born bone-dry.

All these concerns cast doubt on the potential of planets orbiting M dwarfs to support Earth-like environments. Instead of Super Earths, M dwarfs might preferentially harbor Super Mercuries and Super Venuses. Three out of four planets in Table 1 would fit the bill.

EPIC 201367065

We can expect to hear about many more small planets orbiting M dwarfs over the next few years. In fact, the successor mission to Kepler, known as K2, has just reported a system of three planets orbiting an M0 star located only about 45 parsecs (147 light years) away, much closer than any of the planets in Table 1 (Crossfield et al. 2015). The host star was evidently missed by the major catalogs of nearby stars (Henry Draper, Gliese, and Hipparcos). As a result, instead of a familiar HD, GJ, or HR designation, it is known by one of the most unmemorable character strings I’ve ever seen: EPIC 201367065. The planets, which received the usual unglamorous designations b, c, and d, have respective radii of 2.14, 1.72, and 1.52 Rea, and respective orbital periods of 10, 25, and 45 days.

EPIC 201367065 System Architecture

Orbits and planets are represented at their relative sizes, according to Crossfield et al. 2015. According to a widely accepted definition of the habitable zone (Kasting et al. 2014), planet d is too hot for liquid water. However, Zsom et al. 2013 provide certain scenarios (which they concede to be rare) in which this planet might be cool enough for habitability.

--------------------------------------

Depending on one’s definition of the habitable zone, planet d is located either just inside the inner edge (Zsom et al. 2013), indicating potential surface water if certain finely tuned conditions are met, or significantly starward of the inner edge (Kasting et al. 2014), implying temperatures far too high for liquid water. In addition, within error margins, this planet’s radius might be as small as 1.32 Rea, putting it in the same ballpark as Kepler-442b. As the authors note, “this planet [is] a very interesting potential super-Venus or super-Earth.” Cheers and applause all around!

I’d been worrying that K2 would find only boring old Hot Jupiters, so this early return is a very pleasant surprise . . . even if it compounds the problem of too many interesting planets orbiting the wrong kind of star.

Nonetheless, I'm still a bit troubled that after more than three years of data collection by Kepler, and more than a year of additional analyses, we have only one oversized candidate for the status of terrestrial planet in the habitable zone of a K or G star. Are they intrinsically rare, or just hard to find?

References

Alibert Y. (2014) On the radius of habitable planets. Astronomy & Astrophysics 561, A41.

Crossfield I, Petigura E, Schlieder J, Howard AW, Fulton BJ, Aller KM, Ciardi DR, Lepine S, Barclay T, et al. (2015) A nearby M star with three transiting Super-Earths discovered by K2. In press.

Heller R. (2015) Better than Earth. Scientific American 312, 32-39.

Heller R, Armstrong J. (2014) Superhabitable Worlds. Astrobiology 14, 50-66.

Joshi MM. (2003) Climate model studies of synchronously rotating planets. Astrobiology 3, 415-427. Abstract: http://adsabs.harvard.edu/abs/2003AsBio...3..415J

Kasting JF, Kopparapu R, Ramirez RM, Harman CE. (2014) Remote life-detection criteria, habitable zone boundaries, and the frequency of Earth-like planets around M and late K stars. Proceedings of the National Academy of Sciences 111, 12641-12646. Abstract: http://adsabs.harvard.edu/abs/2014PNAS..11112641K

Kopparapu R, Ramirez RM, Kasting JF, Eymet V, Robinson TD, Mahadevan S, Terrien RC, Domagal-Goldman S, Meadows V, Deshpande R. (2013) Habitable zones around main-sequence stars: New estimates. Astrophysical Journal 65, 131. Abstract: http://adsabs.harvard.edu/abs/2013ApJ...765..131K

Luger R, Barnes R. (2015) Extreme water loss and abiotic O₂ buildup on planets throughout the habitable zones of M dwarfs. In press.

Torres G, Kipping DM, Fressin F, Caldwell DA, Twicken JD, Ballard S, Batalha NM, Bryson ST, Ciardi DR, Henze CE, Howell SB, Isaacson HT, Jenkins JJ, Muirhead PM, Newton ER, Petigura EA, Barclay T, Borucki WJ, Crepp JR, Everett ME, Horch EP, Howard AW, Kolbl R, Marcy GW, McCauliff S, Quintana EV. (2015) Validation of twelve small Kepler transiting planets in the habitable zone. Astrophysical Journal, in press. Abstract: http://adsabs.harvard.edu/abs/2015arXiv150101101T

Yang J, Liu Y, Hu Y, Abbott DS. (2014) Water trapping on tidally locked terrestrial planets requires special conditions. Astrophysical Journal Letters 796, L2. doi:10.1088/2041-8205/796/2/L22

Zsom A, Seager S, de Wit J, Stamenkovic V. (2013) Toward the minimum inner edge distance of the habitable zone. Astrophysical Journal 778, 109. Abstract: http://adsabs.harvard.edu/abs/2013ApJ...778..109Z

Seager S. (2013) Exoplanet habitability. Science 340, 577-581. Abstract: http://adsabs.harvard.edu/abs/2013Sci...340..577S

 

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

-----------------------

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. 

-----------------------

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.

-----------------------

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

-----------------------

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.

-----------------------

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).

------------------------

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!

------------------------

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.

Izidoro A, Morbidelli A, Raymond SN. (2014) Terrestrial planet formation in the presence of migrating Super-Earths. Astrophysical Journal 794:11. doi:10.1088/0004-637X/794/1/11

Kuppers M, O’Rourke L, Bocklee-Morvan D, et al. (2014) Localized sources of water vapour on the dwarf planet (1)Ceres. Nature 505, 7484: 525-527. Abstract: http://adsabs.harvard.edu/abs/2014Natur.505..525K

Lopez ED, Fortney JJ. (2014) Understanding the mass-radius relation for sub-Neptunes: Radius as a proxy for composition. Astrophysical Journal 792, 1. Abstract: http://adsabs.harvard.edu/abs/2014ApJ...792....1L 

Leger A, Selsis F, Sotin C, et al. (2004) A new family of planets? “Ocean Planets.” Icarus 169, 499-504. Abstract: http://adsabs.harvard.edu/abs/2003ESASP.539..253L

Neveu-VanMalle M, Queloz D, Anderson DR, Charbonnel C, Collier Cameron A, Delrez L, Gillon M, Hellier C, Jehin E, Lendl M, Maxted P, Pepe F, Pollacco D, Ségransan D, Smalley B, Smith A, Southworth J, Triaud A, Udry S, West RG. (2014) WASP-94 A and B planets: hot-Jupiter cousins in a twin-star system. Astronomy & Astrophysics 572, A49.

Raymond SN, Cossou C. (2014) No universal minimum-mass extrasolar nebula: evidence against in situ accretion of systems of hot super-Earths. Monthly Notices of the Royal Astronomical Society 440, L11-L15.Robertson P, Mahadevan S, Endl M, & Roy A. (2014) Stellar activity masquerading as planets in the habitable zone of the M dwarf Gliese 581. Science 345, 6195: 440-444. Abstract: http://adsabs.harvard.edu/abs/2014Sci...345..440R

Robertson P, Mahadevan S. (2014) Disentangling planets and stellar activity for Gliese 667C. Astrophysical Journal Letters 793, L24. Abstract: http://adsabs.harvard.edu/abs/2014ApJ...793L..24R

Schlaufman KC. (2014) Tests of in situ formation scenarios for compact multiplanet systems. Astrophysical Journal 790, 91. doi:10.1088/0004-637X/790/2/91

Schlichting HE. (2014) Formation of close in Super-Earths & Mini-Neptunes: Required disk masses & their implications. Astrophysical Journal Letters 795, L15. Abstract: http://adsabs.harvard.edu/abs/2014ApJ...795L..15S

Schmitt JR, Agol E, Deck KM, Rogers LA, Gazak JZ, Fischer DA, Wang J, Holman MJ, Jek KJ, Margossian C, Omohundro MR, Winarski T, Brewer JM, Giguere MJ, Lintott C, Lynn S, Parrish M, Schawinski K, Schwamb ME, Simpson R, Smith AM. (2014) Planet Hunters VII. Discovery of a new low-mass, low-density planet (PH3 c) orbiting Kepler-289 with mass measurements of two additional planets (PH3 b and d). Astrophysical Journal 795, 167. Abstract: http://adsabs.harvard.edu/abs/2014ApJ...795..167S

Vogt SS, Butler RP, Rivera EJ, Haghighipour N, Henry GW, Williamson MH. (2010) The Lick-Carnegie exoplanet survey: A 3.1 MEA planet in the habitable zone of the nearby M3V star Gliese 581. Astrophysical Journal, 723: 954.