New observations of the extended hydrogen exosphere of the extrasolar planet HD 209458b ^*

¹ Laboratoire d'astrophysique de l'observatoire de Grenoble, Université Joseph Fourier, CNRS (UMR 5571) BP 53, 38041 Grenoble Cedex 9, France e-mail: david.ehrenreich@obs.ujf-grenoble.fr
² Institut d'astrophysique de Paris, Université Pierre & Marie Curie, CNRS (UMR 7095) 98 bis, boulevard Arago 75014 Paris, France
³ Department of Earth and Space Science and Engineering, York University 4700 Keele street, Toronto, ON M3J1P3, Canada
⁴ Department of Atmospheric, Oceanic, and Space Sciences, University of Michigan 2455 Hayward street, Ann Arbor, MI 48109, USA
⁵ Lunar and Planetary Laboratory, University of Arizona 1040 East 4th street, Tucson, AZ 85721-0077, USA

Received: 25 January 2008
Accepted: 21 March 2008

Abstract

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 to 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 of a hydrogen comet-like tail trailing the planet, whose 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 were used to observe two transits of HD 209458b. Transit light curves were obtained by performing photometry of the unresolved stellar Lyman-α (Lyα) emission line during both transits. Absorption signatures of exospheric hydrogen during the transit were compared to light curve models predicting a hydrogen tail.

Results. Transit depths of and were measured on the whole Lyα line in visits 1 and 2, respectively. Averaging data from both visits, we find an absorption depth of , in good agreement with previous studies.

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