Context : Atomic hydrogen escaping from the planet HD 209458b provides the largest observational signature ever detected for an extrasolar planet atmosphere .
However , the Space Telescope Imaging Spectrograph ( STIS ) used in previous observational studies is no longer available , whereas additional observations are still needed to better constrain the mechanisms subtending the evaporation process , and determine the evaporation state of other ‘ hot Jupiters ’ .

Aims : Here , we aim to detect the extended hydrogen exosphere of HD 209458b with the Advanced Camera for Surveys ( ACS ) on board the Hubble Space Telescope ( HST ) and to find evidence for a hydrogen comet-like tail trailing the planet , which size would depend on the escape rate and the amount of ionizing radiation emitted by the star .
These observations also provide a benchmark for other transiting planets , in the frame of a comparative study of the evaporation state of close-in giant planets .

Methods : Eight HST orbits are used to observe two transits of HD 209458b .
Transit light curves are obtained by performing photometry of the unresolved stellar Lyman- \alpha ( Ly \alpha ) emission line during both transits .
Absorption signatures of exospheric hydrogen during the transit are compared to light curve models predicting a hydrogen tail .

Results : Transit depths of ( 9.6 \pm 7.0 ) \% and ( 5.3 \pm 10.0 ) \% are measured on the whole Ly \alpha line in visits 1 and 2 , respectively .
Averaging data from both visits , we find an absorption depth of ( 8.0 \pm 5.7 ) \% , in good agreement with previous studies .

Conclusions : The extended size of the exosphere confirms that the planet is likely loosing hydrogen to space .
Yet , the photometric precision achieved does not allow us to better constrain the hydrogen mass loss rate .