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WASP-8b: Characterization of a Cool and Eccentric Exoplanet with Spitzer

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 {\microns} Infrared Array Camera bands. The eclipse depths are $0.113\pm 0.018$%, $0.069\pm 0.007$%, and $0.093\pm 0.023$%, 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-{\microns} brightness temperature (1552 K); by modeling the eccentricity-caused thermal variation, we found that this temperature is plausible for radiative time scales less than $\sim 10^{2}$ hours. However, as no model spectra fit all three data points well, the temperature discrepancy remains as an open question.