  We estimate the strength of the bandpass-integrated thermal emission from the
extrasolar planet HD 209458b at 3.6, 4.5, 5.8, and 8.0 microns using the
Infrared Array Camera (IRAC) on the Spitzer Space Telescope. We observe a
single secondary eclipse simultaneously in all four bandpasses and find
relative eclipse depths of 0.00094 +/- 0.00009, 0.00213 +/- 0.00015, 0.00301
+/- 0.00043, and 0.00240 +/- 0.00026, respectively. These eclipse depths reveal
that the shape of the inferred emission spectrum for the planet differs
significantly from the predictions of standard atmosphere models; instead the
most plausible explanation would require the presence of an inversion layer
high in the atmosphere leading to significant water emission in the 4.5 and 5.8
micron bandpasses. This is the first clear indication of such a temperature
inversion in the atmosphere of a hot Jupiter, as previous observations of other
planets appeared to be in reasonably good agreement with the predictions of
models without such an inversion layer.
