We report the spectroscopic detection of mid-infrared emission from the transiting exoplanet HD 209458b .
Using archive data taken with the Spitzer/IRS instrument , we have determined the spectrum of HD 209458b between 7.46 and 15.25 \mu m. We have used two independent methods to determine the planet spectrum , one differential in wavelength and one absolute , and find the results are in good agreement .
Over much of this spectral range , the planet spectrum is consistent with featureless thermal emission .
Between 7.5 and 8.5 \mu m , we find evidence for an unidentified spectral feature .
If this spectral modulation is due to absorption , it implies that the dayside vertical temperature profile of the planetary atmosphere is not entirely isothermal .
Using the IRS data , we have determined the broad-band eclipse depth to be 0.00315 \pm 0.000315 , implying significant redistribution of heat from the dayside to the nightside .
This work required development of improved methods for Spitzer/IRS data calibration that increase the achievable absolute calibration precision and dynamic range for observations of bright point sources .