Using numerical simulation, Holmstrom et al. (2008) proposed a plausible alternative explanation of the observed Lyman-alpha absorption that was seen during the transit of HD 209458b (Vidal-Madjar et al. 2003). 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 Vidal-Madjar et al.(2003).
@article{arxiv.0902.3994,
title = {The origin of hydrogen around HD 209458b},
author = {A. Lecavelier des Etangs and A. Vidal-Madjar and J. -M. Desert},
journal= {arXiv preprint arXiv:0902.3994},
year = {2009}
}