An extended upper atmosphere around the extrasolar planet HD209458b

A. Vidal-Madjar, A. Lecaveller des Etangs, J. M. Désert, G. E. Ballester, R. Ferlet, G. Hébrard, M. Mayor

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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 signaturen4-6, 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.

Original languageEnglish (US)
Pages (from-to)143-146
Number of pages4
Issue number6928
StatePublished - Mar 13 2003
Externally publishedYes

ASJC Scopus subject areas

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