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.

REFERENCES

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: http://adsabs.harvard.edu/abs/2013A%26A...558A.109A

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 http://www.sciencemag.org/content/early/recent.

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: http://adsabs.harvard.edu/abs/2013MNRAS.431.3444C

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: http://adsabs.harvard.edu/abs/2012ApJ...751..158H 

Hansen BM, Murray N. (2013) Testing in situ assembly with the Kepler planet candidate sample. Astrophysical Journal 775, 53. Abstract: http://adsabs.harvard.edu/abs/2013ApJ...775...53H

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: http://adsabs.harvard.edu/abs/2013ApJ...778...78H

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.