Discovery and Atmospheric Characterization of Giant Planet Kepler-12b: an Inflated Radius Outlier

We report the discovery of planet Kepler-12b (KOI-20), which at 1.695± 0.030 RJ is among the handful of planets with super-inflated radii above 1.65 RJ. Orbiting its slightly evolved G0 host with a 4.438-day period, this 0.431± 0.041 MJ planet is the least-irradiated within this largest-planet-radius group, which has important implications for planetary physics. The planet’s inflated radius and low mass lead to a very low density of 0.111± 0.010 g cm−3. We detect the occultation of the planet at a significance of 3.7σ in the Kepler bandpass. This yields a geometric albedo of 0.14±0.04; the planetary flux is due to a combination of scattered light and emitted thermal flux. We use multiple observations with Warm Spitzer to detect the occultation at 7σ and 4σ in the 3.6 and 4.5 μm bandpasses, respectively. The occultation photometry timing is consistent with a circular orbit, at e < 0.01 (1σ), and e < 0.09 (3σ). The occultation detections across the three bands favor an atmospheric model with no dayside temperature inversion. The Kepler occultation detection provides significant leverage, but conclusions regarding temperature structure are preliminary, given our ignorance of opacity sources at optical wavelengths in hot Jupiter atmospheres. If Kepler-12b and HD 209458b, which intercept similar incident stellar fluxes, have the same heavy element masses, the interior energy source needed to explain the large radius of Kepler-12b is three times larger than that of HD 209458b. This may suggest that more than one radius-inflation mechanism is at work for Kepler-12b, or that it is less heavy-element rich than other transiting planets. Subject headings: planetary systems; stars: individual: (Kepler-12, KOI-20, KIC 11804465), planets and satellites: atmospheres, techniques: spectroscopic 1 Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064; [email protected] 2 Massachusetts Institute of Technology, Cambridge, MA 02139, USA 3 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 4 SETI Institute/NASA Ames Research Center, Moffett Field, CA 94035 5 Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA 6 Niels Bohr Institute & StarPlan, University of Copenhagen, Denmark 7 NASA Exoplanet Science Institute, Caltech, MS 100-22, 770 South Wilson Avenue, Pasadena, CA 91125, USA 8 Jet Propulsion Laboratory/California Institute of Technology, Pasadena, CA 91109, USA 9 San Jose State University, San Jose, CA 95192, USA 10 NASA Ames Research Center, Moffett Field, CA 94035 11 National Optical Astronomy Observatories, Tucson, AZ 85719 12 Department of Astronomy, University of Maryland at College Park, College Park, MD 20742 13 Las Cumbres Observatory Global Telescope, Goleta, CA 93117 14 University of Florida, Gainesville, FL 32611 15 Space Telescope Science Institute, Baltimore, MD 21218 16 Yale University, New Haven, CT 06520 US 17 Bay Area Environmental Research Institute/NASA Ames Research Center, Moffett Field, CA 94035, USA 18 Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield, AL10 9AB, England 19 Institut d’Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août 17, Bat. B5C, 4000 Liège, Belgium 20 Observatoire de Genève, Université de Genève, 51 Chemin des Maillettes, 1290 Sauverny, Switzerland 21 Alfred P. Sloan Research Fellow