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A Search for CO Absorption in the Transmission Spectrum of HD 209458b

We observed one transit of the extrasolar planet HD 209458b with the NIRSPEC spectrograph on the Keck II telescope. Using time series of low‐noise observations in the wavelength range 2.0–2.5 μm, we searched for extra absorption from the first‐overtone rotation‐vibration band of CO near 2.3 μm. This was not detected with a detection limit that fails to test simple models of the planetary atmosphere by a factor of about 3. Great improvements in the detectability of the CO spectrum features could be realized by observing a transit that is centered near stellar meridian passage, and in better weather. Since it appears that similar observations taken under better circumstances might succeed, we describe our analysis procedures in detail.