1. Institut d'Astrophysique de Paris, CNRS/UPMC, 98bis boulevard Arago, F-75014 Paris, France
2. Lunar and Planetary Laboratory, University of Arizona, 1040 E. 4th St., Rm 901, Tucson, Arizona 85721-0077, USA
3. Observatoire de Genève, CH-1290 Sauverny, Switzerland

Correspondence to: A. Vidal-Madjar¹ Correspondence and requests for materials should be addressed to A.V.-M. (e-mail: Email: alfred@iap.fr).

Abstract

The planet in the system HD209458 is the first one for which repeated transits across the stellar disk have been observed^{1, 2}. Together with radial velocity measurements³, this has led to a determination of the planet's radius and mass, confirming it to be a gas giant. But despite numerous searches for an atmospheric signature^{4, 5, 6}, only the dense lower atmosphere of HD209458b has been observed, through the detection of neutral sodium absorption⁷. Here we report the detection of atomic hydrogen absorption in the stellar Lyman line during three transits of HD209458b. An absorption of 15 4% (1) is observed. Comparison with models shows that this absorption should take place beyond the Roche limit and therefore can be understood in terms of escaping hydrogen atoms.

1. Institut d'Astrophysique de Paris, CNRS/UPMC, 98bis boulevard Arago, F-75014 Paris, France
2. Lunar and Planetary Laboratory, University of Arizona, 1040 E. 4th St., Rm 901, Tucson, Arizona 85721-0077, USA
3. Observatoire de Genève, CH-1290 Sauverny, Switzerland

Correspondence to: A. Vidal-Madjar¹ Correspondence and requests for materials should be addressed to A.V.-M. (e-mail: Email: alfred@iap.fr).