An extended upper atmosphere around the extrasolar planet HD209458b

A. Vidal-Madjar (), A. Lecavelier des Etangs, J.-M. Désert, G. E. Ballester, R. Ferlet, G. Hébrard and M. Mayor
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A. Vidal-Madjar: Institut d'Astrophysique de Paris, CNRS/UPMC
A. Lecavelier des Etangs: Institut d'Astrophysique de Paris, CNRS/UPMC
J.-M. Désert: Institut d'Astrophysique de Paris, CNRS/UPMC
G. E. Ballester: University of Arizona
R. Ferlet: Institut d'Astrophysique de Paris, CNRS/UPMC
G. Hébrard: Institut d'Astrophysique de Paris, CNRS/UPMC
M. Mayor: Observatoire de Genève

Nature, 2003, vol. 422, issue 6928, 143-146

Abstract: Abstract 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.

Date: 2003
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DOI: 10.1038/nature01448

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