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 % \pm 0.004 % at 3.6 micron ; 0.143 % \pm 0.006 % at 4.5 micron ; 0.134 % \pm 0.049 % at 5.8 micron and 0.150 % \pm 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 ( F _ { p } \sim ( 1.9 - 4.2 ) \times 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 inversions and some low insolation planets like XO-1b do have inversions .
Secondary factors such as sulfur chemistry , atmospheric metallicity , amounts of macroscopic mixing in the stratosphere or even dynamical weather effects likely play a role .
Using the secondary eclipse timing centroids we determined the orbital eccentricity of XO-3b as e = 0.277 \pm 0.009 .
The model radius-age trajectories for XO-3b imply that at least some amount of tidal-heating is required to inflate the radius of XO-3b , and the tidal heating parameter of the planet is constrained to Q _ { p } \lesssim 10 ^ { 6 } .