Authors: Pavel Machalek, Peter R. McCullough, Christopher J. Burke, Jeff A. Valenti, Adam Burrows, Joseph L. Hora, Christoper Johns-Krull, Thomas Greene, Drake Deming

We report on the detection of the thermal radiation from 3 unique Hot-Jupiters, discovered by the XO team, using the Infrared Array Camera (IRAC) on the Spitzer Space Telescope in order to obtain broadband spectra in the infrared to constrain the upper atmospheric temperature structure of Hot-Jupiters. The infrared spectral energy distributions of 3 Hot-Jupiters XO-1b, XO-2b and XO-3b have revealed a presence of a thermally inverted stratosphere in all 3 of them. Since the 3 XO Hot-Jupiters occupy a wide range of substellar point fluxes (Fp ~ 0.49 - 4.2 x 109 erg cm-2 s-1) it allows us to set a threshold bolometric substellar point flux on the planet above which the intense optical/UV light from the star drives a thermal inversion - a hot stratosphere.