  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.
