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An extended upper atmosphere around the extrasolar planet HD209458b

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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.

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Figure 1: The HD209458 Lyman α emission line.
Figure 2: The HD209458 Lyman α profile observed with the G140M grating.
Figure 4: Spectra and ratios with no geocoronal correction.
Figure 3: Relative flux of Lyman α as a function of the HD209458's system phase.

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Acknowledgements

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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Authors and Affiliations

  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, Arizona, Tucson, 85721-0077, USA

    G. E. Ballester

  3. Observatoire de Genève, CH-1290, Sauverny, Switzerland

    M. Mayor

Authors
  1. A. Vidal-Madjar
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  2. A. Lecavelier des Etangs
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  3. J.-M. Désert
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  4. G. E. Ballester
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  5. R. Ferlet
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  6. G. Hébrard
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  7. M. Mayor
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Correspondence to A. Vidal-Madjar.

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Vidal-Madjar, A., des Etangs, A., Désert, JM. et al. An extended upper atmosphere around the extrasolar planet HD209458b. Nature 422, 143–146 (2003). https://doi.org/10.1038/nature01448

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