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!
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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
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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
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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
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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.