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's 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's 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.