The Mid‐Infrared Spectrum of the Transiting Exoplanet HD 209458b

We report the spectroscopic detection of mid-infrared emission from thetransiting exoplanet HD 209458b. Using archive data taken with the Spitzer/IRSinstrument, we have determined the spectrum of HD 209458b between 7.46 and15.25 microns. We have used two independent methods to determine the planetspectrum, one differential in wavelength and one absolute, and find the resultsare in good agreement. Over much of this spectral range, the planet spectrum isconsistent with featureless thermal emission. Between 7.5 and 8.5 microns, wefind evidence for an unidentified spectral feature. If this spectral modulationis due to absorption, it implies that the dayside vertical temperature profileof the planetary atmosphere is not entirely isothermal. Using the IRS data, wehave determined the broad-band eclipse depth to be 0.00315 +/- 0.000315,implying significant redistribution of heat from the dayside to the nightside.This work required development of improved methods for Spitzer/IRS datacalibration that increase the achievable absolute calibration precision anddynamic range for observations of bright point sources.Comment: 35 pages, 12 figures, revised version accepted by the Astrophysical Journa