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 mu m 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 similar to 0.49; 10(9) erg cm(-2) s(-1) sets a new lower limit for the occurrence of a thermal inversion in a planetary atmosphere.