Detection of Thermal Emission from an Extrasolar Planet

Abstract

We present Spitzer Space Telescope infrared photometric time series of the\ntransiting extrasolar planet system TrES-1. The data span a predicted time of\nsecondary eclipse, corresponding to the passage of the planet behind the star.\nIn both bands of our observations, we detect a flux decrement with a timing,\namplitude, and duration as predicted by published parameters of the system.\nThis signal represents the first direct detection of (i.e. the observation of\nphotons emitted by) a planet orbiting another star. The observed eclipse depths\n(in units of relative flux) are 0.00066 +/- 0.00013 at 4.5um and 0.00225 +/-\n0.00036 at 8.0um. These estimates provide the first observational constraints\non models of the thermal emission of hot Jupiters. Assuming that the planet\nemits as a blackbody, we estimate an effective temperature of T_p=1060 +/- 50\nK. Under the additional assumptions that the planet is in thermal equilibrium\nwith the radiation from the star and emits isotropically, we find a Bond albedo\nof A = 0.31 +/- 0.14. This would imply that the planet absorbs the majority of\nstellar radiation incident upon it, a conclusion of significant impact to\natmospheric models of these objects. We compare our data to a\npreviously-published model of the planetary thermal emission, which predicts\nprominent spectral features in our observational bands due to water and carbon\nmonoxide. Based on the time of secondary eclipse, we present an upper limit on\nthe orbital eccentricity that is sufficiently small that we conclude that tidal\ndissipation is unlikely to provide a significant source of energy interior to\nthe planet.(abridged)\n