Spitzer Secondary Eclipses of Wasp-18b

The transiting exoplanet WASP-18b was discovered in 2008 by the Wide Angle Search for Planets project. The Spitzer Exoplanet Target of Opportunity Program observed secondary eclipses of WASP-18b using Spitzer\u27s Infrared Array Camera in the 3.6 mu m and 5.8 mu m bands on 2008 December 20, and in the 4.5 mu m and 8.0 mu m bands on 2008 December 24. We report eclipse depths of 0.30% +/- 0.02%, 0.39% +/- 0.02%, 0.37% +/- 0.03%, 0.41% +/- 0.02%, and brightness temperatures of 3100 +/- 90, 3310 +/- 130, 3080 +/- 140, and 3120 +/- 110 K in order of increasing wavelength. WASP-18b is one of the hottest planets yet discovered-as hot as an M-class star. The planet\u27s pressure-temperature profile most likely features a thermal inversion. The observations also require WASP-18b to have near-zero albedo and almost no redistribution of energy from the day side to the night side of the planet