The origin of hydrogen around HD 209458b

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Arising from: M. Holmström et al. Nature 451, 970–972 (2008)10.1038/nature06600; Holmström et al. reply

Using numerical simulation, Holmström et al.1 proposed a plausible alternative explanation of the observed Lyman-α absorption that was seen during the transit of HD 209458b (ref. 2). They conclude that radiation pressure alone cannot explain the observations and that a peculiar stellar wind is needed. Here we show that radiation pressure alone can in fact produce the observed high-velocity hydrogen atoms. We also emphasize that even if the stellar wind is responsible for the observed hydrogen, to have a sufficient number of atoms for charge exchange with stellar wind, the energetic neutral atom (ENA) model also needs a significant escape from the planet atmosphere of similar amplitude as quoted in ref. 2.

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Figure 1: Comparison of modelled with observed Lyman-α profiles as in Fig. 3 of ref. 1.


  1. 1

    Holmström, M. et al. Energetic neutral atoms as the explanation for the high-velocity hydrogen around HD 209458b. Nature 451, 970–972 (2008)

  2. 2

    Vidal-Madjar, A. et al. An extended upper atmosphere around the extrasolar planet HD209458b. Nature 422, 143–146 (2003)

  3. 3

    Vidal-Madjar, A. et al. Exoplanet HD 209458b (Osiris). Evaporation strengthened. Astrophys. J. 676, L57–L60 (2008)

  4. 4

    Vidal-Madjar, A. & Lecavelier des Etangs, A. “Osiris” (HD209458b), an Evaporating Planet. ASP Conf. Series 321, 152–159 (2004)

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Etangs, A., Vidal-Madjar, A. & Désert, J. The origin of hydrogen around HD 209458b. Nature 456, E1 (2008) doi:10.1038/nature07402

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