Wednesday, January 1, 2014

The Year Kepler Climaxed

“The diversity of the phenomena of nature is so great, and the treasures
hidden in the heavens so rich, precisely in order that the human mind
shall never be lacking in fresh enrichment.”
Johannes Kepler

Apart from a few peaks and one big valley, the past year in exoplanets continued the momentum built by the Kepler Mission over the past three years. These were among the major developments of exoplanetary astronomy in 2013:
  • Kepler Mission scientists finally reported a plausibly terrestrial planet orbiting in the classical habitable zone of a Sun-like star. In fact, over the course of the year they reported three such planets, each transiting and each with a distinct claim to the title of Most Earthlike Exoplanet: Kepler-62e (Kaltenegger et al. 2013), Kepler-62f (Borucki et al. 2013), and Kepler-69c (Barclay et al. 2013). With these discoveries, Kepler can justly say “mission accomplished.”
  • The best candidate of the lot is Kepler-62f, a probable rocky object whose radius is just 1.41 times that of Earth (1.41 Rea). Its host is an orange star of spectral type K2, about 69% as massive as our Sun (0.69 Msol). With an orbital period of 267 days, Kepler-62f traces the outer edge of the system habitable zone. Four more transiting planets, all smaller than 2 Rea, orbit closer to the star. There’s one drawback, though. It hinges on the argument that only rocky objects with a minimal percentage of ices can sustain a carbon cycle or plate tectonics (Alibert 2013). A purely rocky object with the same radius as Kepler-62f will be about 3.5 times as massive as Earth (3.5 Mea). Much research cautions that such a massive object might be unlikely to support plate tectonics, potentially ruling out seas as well as a carbon cycle. Despite its small radius, then, Kepler-62f may still be too big to sustain life. The other two candidates are even worse: Kepler-62e (which orbits just inside Kepler-62f) has a radius of 1.61 Rea, implying a rocky mass of 6 Mea, while Kepler-69c has a radius of 1.70 Rea, implying 8 Mea.
  • Just one month after breaking the happy news about Kepler-62, the Kepler crew reported disaster: the space observatory itself suffered catastrophic equipment failure, ending the mission. The exoplanet community found consolation by reflecting that Kepler managed to complete its originally scheduled scientific mission and has been, by any measure, a revelatory success.
  • And those Kepler discoveries keep rolling in, since the available data are so rich that they will continue to yield new planets for months, if not years, to come. Among the most striking publications of the past few months are a group of mixed-mass systems numbered Kepler-87 through Kepler-90, which collectively illustrate a system architecture barely attested in the past: a closely packed assemblage of low-mass planets with one or more gas giants tucked in their midst. With luck I’ll write about them in 2014.
  • As predicted, the census maintained by the Extrasolar Planets Encyclopaedia surged past 1000 in 2013, and is on course to reach 1100 during 2014. Along with this burgeoning census, the tendency for data to outrun theory continues apace.
  • The general consensus that planets form by accretion and subsequent migration, such that the local ice line plays a major role in system architectures, has been shaken by the Kepler data on compact multiplanet systems. Doubts had been rumbling for the past few years, and in 2013, thanks to some widely discussed publications, the rumbles got louder. The most popular alternative scenario for forming an inner system full of low-mass planets is now in situ assembly (Hansen & Murray 2012, Chiang & Laughlin 2013, Hansen & Murray 2013). More exotic solutions have also been proposed, such as a combination of gravitational instability, “tidal downsizing,” and core accretion (Galvagni & Mayer 2013, Ogihara et al. 2013). But the “classic migrationists” (my neologism) have already begun to strike back (Alibert et al. 2013, Hasegawa & Pudritz 2013) by producing new models that seem capable of accounting for the architectures discovered by Kepler. This clash of theories should provide interesting blog fodder in 2014.
  • Finally, the many controversies surrounding proposed planetary systems in the Sun’s back yard are no nearer to resolution than they were last year. My annual round-up for 2012 noted doubts and arguments about GJ 667C, GJ 676A, and Tau Ceti; later posts discussed Fomalhaut, Epsilon Eridani, and Alpha Centauri B. All the question marks and asterisks remain in place.

National Geographic led its review of the year in science with this artist’s view of Kepler-62f

So then! As the new year begins, we can point to 1055 confirmed exoplanets, including one or two that may have a slim chance of supporting environments that resemble certain places on Earth or Mars. Talk about wow!

Will the coming year be mostly about consolidating the evidence and revising the theories, or will we have pleasant surprises that compare with the circumbinary planets of 2011-2012 or the bona fide Super Earths of 2013? Stay tuned.

Kepler-69c was also proposed as a habitable Super Earth, but it’s probably too hot, too big,
and nowhere near as seductive as this artist’s view.

Alibert Y. (2013) On the radius of habitable planets. Astronomy & Astrophysics, in press.
Alibert Y, Carron F, Fortier A, Pfyffer S, Benz W, Mordasini C, Swoboda D. (2013) Theoretical models of planetary system formation: mass vs semi-major axis. Astronomy & Astrophysics 558, A109. Abstract:
Barclay T, Burke CJ, Howell SB, Rowe JF, Huber D, Isaacson H, et al. (2013) A Super-Earth-sized planet orbiting in or near the habitable zone around a Sun-like star. Astrophysical Journal 768, 101.
Borucki WJ, Agol E, Fressin F, Kaltenegger L, Rowe J, Isaacson H, et al. (2013) Kepler-62: A Five-Planet System with Planets of 1.4 and 1.6 Earth Radii in the Habitable Zone. Science Express 18 April 2013. 10.1126/science.1234702
Chiang E, Laughlin G. (2013) The minimum-mass extrasolar nebula: in situ formation of close-in super-Earths. Monthly Notices of the Royal Astronomical Society 431, 3444-3455. Abstract:
Galvagni M, Mayer L. (2013) Early evolution of clumps formed via gravitational instability in protoplanetary discs: precursors of Hot Jupiters? Monthly Notices of the Royal Astronomical Society, in press.
Hansen BM, Murray N. (2012) Migration then assembly: Formation of Neptune-mass planets inside 1 AU. Astrophysical Journal 751, 158. Abstract: 
Hansen BM, Murray N. (2013) Testing in situ assembly with the Kepler planet candidate sample. Astrophysical Journal 775, 53. Abstract:
Hasegawa Y, Pudritz RE. (2013) Planetary populations in the mass-period diagram: A statistical treatment of exoplanet formation and the role of planet traps. Astrophysical Journal 778, 78. Abstract:
Kaltenegger L, Sasselov D, Rugheimer S. (2013) Water-planets in the habitable zone: Atmospheric chemistry, observable features, and the case of Kepler-62e and -62f. Astrophysical Journal Letters 775, L47.
Ogihara M, Inutsuka S, Kobayashi H. (2013) Crowding out of giants by dwarfs: An origin for the lack of companion planets in Hot Jupiter systems. Astrophysical Journal Letters 778, L9.



  1. What about if Kepler-62f has a more Mars-like composition, with a correspondingly lower density than if it was similar to Earth's density? It's in a Mars-like orbit (42% equivalent solar insolation compared to Mars' 36%). Or would the greater mass compress the planet, giving it a higher density regardless?

    1. You're right, that planet could have a lower density, The mass-radius curves that I saw in 2013 seemed to assume that all terrestrial planets have a composition like Earth's. However, earlier diagrams -- like the one I posted here ( -- included an option for pure perovskite, the lightest possible rocky composition. An object with that composition and a radius of 1.41 Rea would be about 2.75 Mea. That's a little more homey than 3.5 Mea. But I have to confess -- I've been a little too dubious of the possibility of plate tectonics on massive planets. There seems to be a certain mass above which mantle convection and all that jazz are all wrong for tectonics, but I've never seen a specific number for that boundary. Maybe anything under 4 or 5 Earth masses is good to go. Generally, studies consider 10 Mea to be a high mass for a Super Earth; I haven't seen people distinguish between, e.g., 3 Mea and 7 Mea. So I just revised the sentence where I said that plate tectonics is unlikely -- now it "might be" unlikely. :) I plan to blog about this very question in the next month or so.

  2. I'm hoping we'll get an exomoon, though I won't hold my breath.

    I suspect the year will be dominated by RV follow up and an expanded sample of planets where we know mass, radius and density. Similarly, an expanded sample of hot Jupiters with spectroscopic followup.

    I'm also hopeful that K2 gets the go ahead and we start to find transiting planets around brighter nearby stars. It will also be interesting to see how quickly Gaia starts to produce exoplanets - this year feels a bit early ...

    And very nice to see one I've been involved with made your summary (Kepler 90)!

    1. You must be one of the Planet Hunters! Congratulations! I've had to stay on high alert to keep track of all the names that wonderful system has had -- KIC 11442793, KOI-351, and finally Kepler-90. I have a post in the works about K87 through K89, all of which feature at least one gas giant in close proximity to low-mass planets. Ve-ry interesting systems!

      I've also been thinking that lots of RV follow-up would be nice to see -- e.g., the work of Robert Wittenmyer's team in 2013 -- they're the ones who discovered a second planet in many nearby single-planet systems discovered by RV.

      But today's exoplanet news was all about the Gemini Planet Imager and its gorgeous photo of Beta Pictoris b. As one of the folks on that team predicted, 2014 will be the year of Direct Imaging! I suspect it may be so.

      As for exomoons -- even the word is alluring -- I would love to see one confirmed.

    2. Thanks! The ad hoc pipeline that's developed at Planet Hunters is quite interesting - from those who tag transits through to a few of us who can look for contamination and do transit fitting, etc.

      On RV, the HARPS-N list of "protected targets" for this semester is quite interesting - lots of candidates in there

      As SPHERE has just shipped to Chile and first light on UT3 is expected in May, you may be right about this being the year of direct imaging!