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Evaporation of the planet HD189733b observed in HI Lyman-alpha

We observed three transits of the extrasolar planet HD189733b in HI Lyman-alpha and in a few other lines in the ultraviolet with HST/ACS, in the search for atmospheric signatures. We detect a transit signature in the Lyman-alpha light curve with a transit depth of 5.05 +/- 0.75 %. This depth exceeds the occultation depth produced by the planetary disk alone at the 3.5-sigma level (statistical). Other stellar emission lines are less bright, and, taken individually, they do not show the transit signature, while the whole spectra redward of the Lyman-alpha line has enough photons to show a transit signature consistent with the absorption by the planetary disk alone. The transit depth's upper limits in the emission lines are 11.1% for OI at 1305A and 5.5% for CII at 1335A. The presence of an extended exosphere of atomic hydrogen around HD189733b producing 5% absorption of the full unresolved Lyman-alpha line flux shows that the planet is losing gas. The Lyman-alpha light curve is well-fitted by a numerical simulation of escaping hydrogen in which the planetary atoms are pushed by the stellar radiation pressure. We constrain the escape rate of atomic hydrogen to be between 10^9 and 10^{11} g/s and the ionizing extreme UV flux between 2 and 40 times the solar value (1-sigma), with larger escape rates corresponding to larger EUV flux. The best fit is obtained for dM/dt=10^{10} g/s and an EUV flux F_{EUV}=20 times the solar value. HD189733b is the second extrasolar planet for which atmospheric evaporation has been detected.