Infrared Spectroscopy of an Extrasolar Planet

We have used the Spitzer Space Telescope to observe the transiting extrasolar planet system HD 209458 at the time of secondary eclipse, when the planet disappears behind the star and later reappears. Using the Infrared Spectrograph (IRS), our technique subtracts the spectrum of the star (when the planet is eclipsed) from the combined light spectrum (when the planet is out of eclipse), to isolate the 7--14 micron spectrum of the planet. This process has required decorrelation of systematic effects, due to detector response drift and telescope pointing jitter. The observations, spanning two secondary eclipse events, have sufficient signal-to- noise to reveal the planet's continuum spectrum, with some sensitivity to line structure. We discuss the implications for the structure and composition of the planet's atmosphere.