AbstractExospheric atomic hydrogen escaping from the planet HD 209458b provides the largest observational signature ever detected for an extrasolar planet atmosphere. We present observations of this transiting planet's extended exosphere with the Advanced Camera for Surveys on board the Hubble Space Telescope. From the two transit light curves obtained at Lyman α, we find an in-transit absorption of (8.0±5.7)%, in good agreement with previous studies. These new constraints on the size of the exosphere strengthens the evaporation scenario. Full details are provided in Ehrenreich et al. (2008).