| Summary: | We present secondary eclipse photometry of the extrasolar planet WASP-5b taken in the 3.6 and 4.5 μm bands with the Spitzer Space Telescope's Infrared Array Camera as part of the extended warm mission. By estimating the depth of the secondary eclipse in these two bands we can place constraints on the planet's atmospheric pressure-temperature profile and chemistry. We measure secondary eclipse depths of 0.197% ± 0.028% and 0.237% ± 0.024% in the 3.6 μm and 4.5 μm bands, respectively. For the case of a solar-composition atmosphere and chemistry in local thermal equilibrium, our observations are best matched by models showing a hot dayside and, depending on our choice of model, a weak thermal inversion or no inversion at all. We measure a mean offset from the predicted center of eclipse of 3.7 ± 1.8 minutes, corresponding to ecos ω = 0.0025 ± 0.0012 and consistent with a circular orbit. We conclude that the planet's orbit is unlikely to have been perturbed by interactions with another body in the system as claimed by Fukui et al.
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