WASP-8b: Characterization of a cool and eccentric exoplanet with spitzer

Abstract

WASP-8b has 2.18 times Jupiter's mass and is on an eccentric (e = 0.31) 8.16 day orbit. With a time-averaged equilibrium temperature of 948 K, it is one of the least-irradiated hot Jupiters observed with the Spitzer Space Telescope. We have analyzed six photometric light curves of WASP-8b during secondary eclipse observed in the 3.6, 4.5, and 8.0 μm Infrared Array Camera bands. The eclipse depths are 0.113% ± 0.018%, 0.069% ± 0.007%, and 0.093% ± 0.023%, respectively, giving respective brightness temperatures of 1552, 1131, and 938 K. We characterized the atmospheric thermal profile and composition of the planet using a line-by-line radiative transfer code and a Markov-chain Monte Carlo sampler. The data indicated no thermal inversion, independently of any assumption about chemical composition. We noted an anomalously high 3.6 μm brightness temperature (1552 K); by modeling the eccentricity-caused thermal variation, we found that this temperature is plausible for radiative timescales less than ∼102 hr. However, as no model spectra fit all three data points well, the temperature discrepancy remains as an open question. © 2013. The American Astronomical Society. All rights reserved.