Detection of thermal emission of XO-2b: Evidence for a weak temperature inversion

Abstract

We estimate flux ratios of the extrasolar planet XO-2b to its host star XO-2 at 3.6, 4.5, 5.8, and 8.0 μm with Infrared Array Camera on the Spitzer Space Telescope to be 0.00081 ± 0.00017, 0.00098 ± 0.00020, 0.00167 ± 0.00036, and 0.00133 ± 0.00049, respectively. The fluxes provide tentative evidence for a weak temperature inversion in the upper atmosphere, the precise nature of which would need to be confirmed by longer wavelength observations. XO-2b substellar flux of 0.76 × 109 erg cm-2 s-1 lies in the predicted transition region between atmospheres with and without upper atmospheric temperature inversion. © 2009. The American Astronomical Society. All rights reserved.