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Energetic neutral atoms as the explanation for the high-velocity hydrogen around HD 209458b

Nature volume 451, pages 970–972 (2008)Cite this article

Abstract

Absorption in the stellar Lyman-α (Lyα) line observed during the transit of the extrasolar planet HD 209458b in front of its host star reveals high-velocity atomic hydrogen at great distances from the planet1,2. This has been interpreted as hydrogen atoms escaping from the planet’s exosphere1,3, possibly undergoing hydrodynamic blow-off4, and being accelerated by stellar radiation pressure. Energetic neutral atoms around Solar System planets have been observed to form from charge exchange between solar wind protons and neutral hydrogen from the planetary exospheres5,6,7, however, and this process also should occur around extrasolar planets. Here we show that the measured transit-associated Lyα absorption can be explained by the interaction between the exosphere of HD 209458b and the stellar wind, and that radiation pressure alone cannot explain the observations. As the stellar wind protons are the source of the observed energetic neutral atoms, this provides a way of probing stellar wind conditions, and our model suggests a slow and hot stellar wind near HD 209458b at the time of the observations.

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Figure 1: The hydrogen cloud around the planet.
Figure 2: Velocities of the hydrogen atoms.
Figure 3: Comparison of modelled with observed Lyα profiles.

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Acknowledgements

This study was carried out within the framework of the International Space Science Institute team “Evolution of Exoplanet Atmospheres and their Characterization”. T.P. is supported by the Marie Curie Fellowship project ISHERPA and the host institution INAF—Osservatorio Astronomico di Palermo. H.L. thanks ASA for funding the CoRoT project. The research used the resources of HPC2N, Umeå University, and LUNARC, Lund University, Sweden. The software was in part developed by the DOE-supported ASC/Alliance Center for Astrophysical Thermonuclear Flashes at the University of Chicago.

Author Contributions A.E. wrote an initial version of the simulation code. F.S. helped in modelling the observation. T.P. provided knowledge on the atmospheres of extrasolar planets. H.L. suggested that the HST observation could be due to ENAs. P.W. contributed expertise in ENA processes

Author information

Authors and Affiliations

  1. Swedish Institute of Space Physics, PO Box 812, SE-98128 Kiruna, Sweden,

    M. Holmström & A. Ekenbäck

  2. Laboratoire d'Astrophysique de Bordeaux (CNRS, Université Bordeaux 1), BP 89, F-33270 Floirac, France,

    F. Selsis

  3. Centre de Recherche Astrophysique de Lyon (CNRS, Université de Lyon, Ecole Normale Supérieure de Lyon), 46 Allée d'Italie, F-69007 Lyon, France,

    F. Selsis

  4. INAF—Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, I-90134 Palermo, Italy,

    T. Penz

  5. Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, A-8042 Graz, Austria,

    H. Lammer

  6. Physikalisches Institut, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland,

    P. Wurz

Authors
  1. M. Holmström
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  2. A. Ekenbäck
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  3. F. Selsis
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  4. T. Penz
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  5. H. Lammer
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  6. P. Wurz
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Corresponding author

Correspondence to M. Holmström.

Supplementary information

Supplementary Information

This file contains Supplementary Notes with Supplementary Figures 1-7 and Supplementary Tables 1-2. It describes the algorithms used to simulate HD 209458b's hydrogen exosphere, the detailed setup used in the numerical experiments, and finally it investigates the effects of changing some model parameters. (PDF 446 kb)

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Holmström, M., Ekenbäck, A., Selsis, F. et al. Energetic neutral atoms as the explanation for the high-velocity hydrogen around HD 209458b. Nature 451, 970–972 (2008). https://doi.org/10.1038/nature06600

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