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

Absorption in the stellar Lyman-alpha (Ly alpha) 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 planet(1,2). This has been interpreted as hydrogen atoms escaping from the planet's exosphere(1,3), possibly undergoing hydrodynamic blow-off(4), 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 exospheres(5-7), however, and this process also should occur around extrasolar planets. Here we show that the measured transit-associated Ly alpha 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.