  Absorption in the stellar Lyman-$\alpha$ (Ly-$\alpha$) line observed during
the transit of the extrasolar planet HD 209458b reveals high velocity atomic
hydrogen at great distances from the planet. This has been interpreted as
hydrogen atoms escaping from the exosphere of the planet, possibly undergoing
hydrodynamic blow-off, being accelerated by stellar radiation pressure.
However, around solar system planets the production of energetic neutral atoms
from charge exchange between solar wind protons and neutral hydrogen from the
exospheres has been observed, and should also occur at 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 observation.
This is the first observation of energetic neutral atoms outside the solar
system. Since the stellar wind protons are the source of the observed energetic
neutral atoms, this provides a completely new method of probing stellar wind
conditions, and our model suggests a slow and hot stellar wind near HD 209458b
at the time of the observation.
