Detection of an Extrasolar Planet Atmosphere

We report high precision spectrophotometric observations of four planetarytransits of HD 209458, in the region of the sodium resonance doublet at 589.3nm. We find that the photometric dimming during transit in a bandpass centeredon the sodium feature is deeper by (2.32 +/- 0.57) x 10^{-4} relative tosimultaneous observations of the transit in adjacent bands. We interpret thisadditional dimming as absorption from sodium in the planetary atmosphere, asrecently predicted from several theoretical modeling efforts. Our model for acloudless planetary atmosphere with a solar abundance of sodium in atomic formpredicts more sodium absorption than we observe. There are severalpossibilities that may account for this reduced amplitude, including reactionof atomic sodium into molecular gases and/or condensates, photoionization ofsodium by the stellar flux, a low primordial abundance of sodium, or thepresence of clouds high in the atmosphere.Comment: 26 pages, 8 figures, accepted by ApJ 2001 November 1