  We present Spitzer Space Telescope time series photometry of the exoplanet
system HD 189733 spanning two times of secondary eclipse, when the planet
passes out of view behind the parent star. We estimate the relative eclipse
depth in 5 distinct bands and find the planet-to-star flux ratio to be 0.256
+/- 0.014% (3.6 microns), 0.214 +/- 0.020% (4.5 microns), 0.310 +/- 0.034% (5.8
microns), 0.391 +/- 0.022% (8.0 microns), and 0.598 +/- 0.038% (24 microns).
For consistency, we re-analyze a previously published time series to deduce a
contrast ratio in an additional band, 0.519 +/- 0.020% (16 microns). Our data
are strongly inconsistent with a Planck spectrum, and we clearly detect
emission near 4 microns as predicted by published theoretical models in which
this feature arises from a corresponding opacity window. Unlike recent results
for the exoplanet HD 209458b, we find that the emergent spectrum from HD
189733b is best matched by models that do not include an atmospheric
temperature inversion. Taken together, these two studies provide initial
observational support for the idea that hot Jupiter atmospheres diverge into
two classes, in which a thermal inversion layer is present for the more
strongly irradiated objects.
