Clouds on the Dark Side of an Extrasolar Hot-Jupiter: Detailed Transit Analysis of Atmospheric Sodium in HD209458b

We present a re-analysis of HST STIS transit data of HD209458b, including both medium and low resolution, to fully probe exoplanetary atmospheric sodium. By fitting the observed Na line profile, we recover the profile of the atmosphere as well as the Na abundance. We find a Na mixing ration of 4.0(-2.1)(+3.6) x 10(-7) in the middle atmosphere and 4.7(-1.3)(+2.0) x 10(-6) in the lower atmosphere, corresponding to 0.3 and 3 times solar abundance. The observed depletion of atomic Na is only at altitudes above the sodium sulfide condensation level, inferring Na2S clouds on the night side of HD209458b. Sodium sulfide clouds naturally explain other non-detections of previously expected atomic and molecular species, most notably potassium, as condensation of sodium alters the local chemistry.