Observed atomic hydrogen absorption in the stellar Lyman-alpha line by the extrasolar planet HD 209458b shows hydrogen with high velocity at great distances from the planet. This has been interpreted as exospheric hydrogen atoms undergoing hydrodynamic escape.
We show by computer simulations that an alternative, and more likely, explanation is that the observed hydrogen are energetic neutral atoms (ENAs) from the interaction of the stellar wind with the extrasolar planet's exosphere. This presents a new way of observing stellar wind conditions such as velocity and temperature.
Created 2007-09-06 09:24:43 by Mats HolmstrÃ¶m Last changed 2007-09-06 09:25:42 by Mats HolmstrÃ¶m