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An extended upper atmosphere around the extrasolar planet HD209458b

Nature volume 422, pages 143146 (13 March 2003) | Download Citation



The planet in the system HD209458 is the first one for which repeated transits across the stellar disk have been observed1,2. 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 signature4,5,6, 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 α line during three transits of HD209458b. An absorption of 15 ± 4% (1σ) 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.

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This work is based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc. We thank M. Lemoine, L. Ben Jaffel, C. Emerich, P. D. Feldman and J. McConnell for comments, J. Herbert and W. Landsman for conversations on STIS data reduction, and J. Valenti for help in preparing the observations.

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  1. *Institut d'Astrophysique de Paris, CNRS/UPMC, 98bis boulevard Arago, F-75014 Paris, France

    • A. Vidal-Madjar
    • , A. Lecavelier des Etangs
    • , J.-M. Désert
    • , R. Ferlet
    •  & G. Hébrard
  2. †Lunar and Planetary Laboratory, University of Arizona, 1040 E. 4th St., Rm 901, Tucson, Arizona 85721-0077, USA

    • G. E. Ballester
  3. ‡Observatoire de Genève, CH-1290 Sauverny, Switzerland

    • M. Mayor


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The authors declare that they have no competing financial interests.

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Correspondence to A. Vidal-Madjar.

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