We determined the flux ratios of the heavy and eccentric planet XO-3b to its parent star in the four IRAC bands of the Spitzer Space Telescope: 0.101% +- 0.004% at 3.6 micron; 0.143% +- 0.006% at 4.5 micron; 0.134% +- 0.049% at 5.8 micron and 0.150%
+- 0.036% at 8.0 micron. The flux ratios are within [-2.2,0.3, -0.8, -1.7]-sigma of the model of XO-3b with a thermally inverted stratosphere in the 3.6 micron, 4.5 micron, 5.8 micron and 8.0 micron channels, respectively. XO-3b has a high illumination from its parent star (Fp ~(1.9 - 4.2) x 10^9 ergs cm^-2 s^-1) and is thus expected to have a thermal inversion, which we indeed observe. When combined with existing data for other planets, the correlation between the presence of an atmospheric temperature inversion and the substellar flux is insufficient to explain why some high insolation planets like TrES-3 do not have stratospheric
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 microns with the 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, re
spectively. 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 (Burrows et al. 2007b) is refined. The sub-stellar point flux from the parent star at XO-1b of ~ 0.49 x 10^9 erg cm^-2 s^-1 sets a new lower limit for the occurrence of a thermal inversion in a planetary atmosphere.
