Optical Phase Curves of Kepler Exoplanets

Abstract

We have conducted a comprehensive search for optical phase variations of all planet candidates with tight orbits in fifteen quarters of data from the Kepler space telescope. After correcting for systematics, we found eight systems that appear to show secondary eclipses as well as phase variations. Of these, five (Kepler-5, Kepler-6, Kepler-8, KOI-64 and KOI-2133) are new and three (TrES-2, HAT-P-7 and KOI-13) have previously published phase curves, albeit with many fewer observations. We model the full phase curve of each planet candidate, including the primary and secondary transits, and derive their albedos, day- and night-side temperatures, ellipsoidal variations and Doppler beaming. We find that KOI-64 and KOI-2133 have night-side temperatures well above their equilibrium values (while KOI-2133 also has an albedo >1), so we conclude that they are likely to be self-luminous objects rather than planets. The characteristics of the six other candidates are consistent with their being planets with low geometric albedos (<0.3). For TrES-2 and KOI-13, the Kepler bandpass appears to probe atmospheric layers hotter than the planet's equilibrium temperature. For KOI-13, we detect a never-before-seen third cosine harmonic with an amplitude of 6.7+/-0.3 ppm and a phase shift of -1.1+/-0.1 radians in the phase curve residual, which could be due to its spin-orbit misalignment. We report derived planetary parameters for all six planets, including masses from ellipsoidal variations and Doppler beaming, and compare our results to published values when available. Our results nearly double the number of Kepler exoplanets with measured phase curve variations, thus providing valuable new constraints on the properties of close-in hot Jupiters.