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

TL;DR

This paper demonstrates that energetic neutral atoms produced by stellar wind interactions, not radiation pressure, explain the high velocity hydrogen observed around exoplanet HD 209458b, offering a new way to probe stellar wind conditions.

Contribution

It introduces a novel explanation for high velocity hydrogen around HD 209458b based on energetic neutral atoms from stellar wind interactions, not just radiation pressure.

Findings

Energetic neutral atoms explain the observed Ly-$$ absorption.

Stellar wind conditions near HD 209458b are inferred to be slow and hot.

This is the first detection of energetic neutral atoms outside the solar system.

Abstract

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.