The origin of hydrogen around HD 209458b

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. 3). A mechanism is needed to produce hydrogen atoms at these high velocities exceeding the planet escape velocity. We previously proposed that hydrogen atoms in the exosphere are naturally accelerated by the stellar radiation pressure 2,4 ; however, Holmström et al. 1 concluded that radiation pressure alone cannot explain the observation. Nonetheless, in their work, the strength of the radiation pressure has been artificially reduced to a value 2 to 5 times lower than the solar value, whereas the observed Lyman-α line strength and profile shows that it is significantly larger than the solar value. The low radiation pressure assumed by Holmström et al. 1 is valid only at high radial velocity. However, if low radiation pressure is assumed, high velocities are not reached, which therefore explains the different conclusion reached by Holmström et al. A correct treatment of the link between the radiation pressure and radial velocity is needed.

To show that the radiation pressure can explain the observed spectrum, we calculated the modelled Lyman-α profile with radiation pressure alone, in the same way as done in Fig. 3 of ref. 1 for the ENA model. This calculation is done taking into account the strength and profile of the Lyman-α line, and the corresponding variation of radiation pressure as a function of radial velocity. Planetary and stellar gravities are also included. These differences explain the different results obtained with radiation pressure alone in the two models. The result plotted in Fig. 1 shows that the resulting profiles are similar in the two models (radiation pressure alone and ENA with reduced radiation pressure), and neither possible model can be favoured. Radiation pressure cannot be excluded as an explanation of the observed spectrum.

Although we agree that the ENA model is a plausible scenario, we do not believe that ENAs can explain the observations better than a classical scenario with radiation pressure. The ENA model requires extraordinary conditions for the wind parameters (high temperature and low velocity) which are not constrained by any other observations, whereas the radiation pressure as measured in the Lyman-α spectrum can self-consistently explain the observations.

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