  Of the over 200 known extrasolar planets, 14 exhibit transits in front of
their parent stars as seen from Earth. Spectroscopic observations of the
transiting planets can probe the physical conditions of their atmospheres. One
such technique can be used to derive the planetary spectrum by subtracting the
stellar spectrum measured during eclipse (planet hidden behind star) from the
combined-light spectrum measured outside eclipse (star + planet). Although
several attempts have been made from Earth-based observatories, no spectrum has
yet been measured for any of the established extrasolar planets. Here we report
a measurement of the infrared spectrum (7.5--13.2 micron) of the transiting
extrasolar planet HD209458b. Our observations reveal a hot thermal continuum
for the planetary spectrum, with approximately constant ratio to the stellar
flux over this wavelength range. Superposed on this continuum is a broad
emission peak centered near 9.65 micron that we attribute to emission by
silicate clouds. We also find a narrow, unidentified emission feature at 7.78
micron. Models of these ``hot Jupiter'' planets predict a flux peak near 10
micron, where thermal emission from the deep atmosphere emerges relatively
unimpeded by water absorption, but models dominated by water fit the observed
spectrum poorly.
