* Astronomy

Title: Discovery and Atmospheric Characterisation of Giant Planet Kepler-12b: An Inflated Radius Outlier
Authors: Jonathan J. Fortney, Brice-Olivier Demory, Jean-Michel Desert, Jason Rowe, Geoffrey W. Marcy, Howard Isaacson, Lars A. Buchhave, David Ciardi, Thomas N. Gautier, Natalie M. Batalha, Douglas A. Caldwell, Stephen T. Bryson, Philip Nutzman, Jon M. Jenkins, Andrew Howard, David Charbonneau, Heather A. Knutson, Steve B. Howell, Mark Everett, Francois Fressin, Drake Deming, William J. Borucki, Timothy M. Brown, Eric B. Ford, Ronald L. Gilliland, David W. Latham, Neil Miller, Sara Seager, Debra A. Fischer, David Koch, Jack J. Lissauer, Michael R. Haas, Martin Still, Philip Lucas, Michael Gillon, Jessie L. Christiansen, John C. Geary

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 3.7{\sigma} 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 7{\sigma} and 4{\sigma} in the 3.6 and 4.5 {\mu}m bandpasses, respectively. The occultation photometry timing is consistent with a circular orbit, at e < 0.01 (1{\sigma}), and e < 0.09 (3{\sigma}). The occultation detections across the three bands favour 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.

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