Identification of absorption features in an extrasolar planet atmosphere

Water absorption is identified in the atmosphere of HD 209458b by comparing models for the planet's transmitted spectrum to recent, multiwavelength, eclipse-depth measurements (from 0.3 to 1 mu m) published by Knutson et al. A cloud-free model that includes solar abundances, rainout of condensates, and photoionization of sodium and potassium is in good agreement with the entire set of eclipse-depth measurements from the ultraviolet to near-infrared. Constraints are placed on condensate removal by gravitational settling, the bulk metallicity, and the redistribution of absorbed stellar flux. Comparisons are also made to the Charbonneau et al. sodium measurements.