Accepted for publication in The Astrophysical Journal Preprint typeset using L ATEX style emulateapj v. 10/09/06 THERMAL EMISSION OF EXOPLANET XO-1B

We estimate flux ratios of the extrasolar planet XO-1b to its host star XO-1 at 3.6, 4.5, 5.8, and 8.0 μm with IRAC on the Spitzer Space Telescope to be -->0.00086 ± 0.00007, -->0.00122 ± 0.00009, -->0.00261 ± 0.00031, and -->0.00210 ± 0.00029, respectively. The fluxes are inconsistent with a canonical cloudless model for the thermal emission from a planet and suggest an atmosphere with a thermal inversion layer and a possible stratospheric absorber. A newly emerging correlation between the presence of a thermal inversion layer in the planetary atmosphere and stellar insolation of the planet, as by Burrows and colleagues, is refined. The substellar point flux from the parent star at XO-1b of ~ -->0.49 × 109 erg cm−2 s−1 sets a new lower limit for the occurrence of a thermal inversion in a planetary atmosphere.

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