  The transiting exoplanet WASP-18b was discovered in 2008 by the Wide Angle
Search for Planets (WASP) project. The Spitzer Exoplanet Target of Opportunity
Program observed secondary eclipses of WASP-18b using Spitzer's Infrared Array
Camera (IRAC) in the 3.6 micron and 5.8 micron bands on 2008 December 20, and
in the 4.5 micron and 8.0 micron 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.
