We report the discovery of planet Kepler-12b ( KOI-20 ) , which at 1.695 \pm 0.030 R _ { \mathrm { J } }  is among the handful of planets with super-inflated radii above 1.65 R _ { \mathrm { J } } .
Orbiting its slightly evolved G0 host with a 4.438-day period , this 0.431 \pm 0.041 M _ { \mathrm { J } } Â 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 \pm 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 \pm 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 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 .