Absorption in the stellar Lyman-α (Ly-α) line observed during the transit of the extrasolar planet HD 209458b reveals high velocity atomic hydrogen at great distances from the planet 1, 2 . This has been interpreted as hydrogen atoms escaping from the exosphere of the planet 1, 3 , possibly undergoing hydrodynamic blow-off 4 , 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 5,6,7 , and should also occur at extrasolar planets. Here we show that the measured transit-associated Ly-α 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.Energetic neutral atoms (ENAs) are produced wherever energetic ions meet a neutral atmosphere, and solar wind ENAs have been observed at every planet in the solar system where ENA instrumentation has been available -at Earth 5 , at Mars 6 , and at Venus 7 .By energetic we mean that the ions have a much greater velocity than the thermal velocities of the exospheric neutrals. During the charge exchange process, an electron is transferred from the neutral to the ion, resulting in a neutral atom and an ionized neutral. the exospheric neutrals, the momenta of the individual atoms are preserved to a good approximation. Thus, the produced ENAs will have the same velocity distribution as the source population of ions. When first observed (also by their Ly-α signature 8, 9 ), the extended hydrogen coronae of Mars and Venus were assumed to constitute the uppermost layers of an escaping exosphere. The observed densities were used to infer exospheric scale heights and temperatures, which proved to be extremely high compared to theoretical predictions (up to 700 K). In situ spacecraft observations later found exospheric temperatures of ∼210 and ∼270 K 10 . The discrepancy was eventually explained by photochemically-produced energetic particles, and by ENAs, produced by charge exchange between energetic solar wind protons and the planetary exosphere. This mechanism, well-known in the solar system, has however not been considered as a possible origin of the atomic hydrogen corona revealed by HST observations of HD 209458b.HD 209458b is a Jupiter-type gas giant with a mass of ∼0.65 M Jup and a size of ∼1.32 R Jup that orbits around its host star HD 209458 at ∼0.045 AU 12 , which is a solar-like G-type star with an age of about 4 Gyr. The activity of the star can be estimated from its X-ray luminosity measured by the XMMNewton space observatory, and is comparable to ...

HD 209458b is an exoplanet found to transit the disk of its parent star. Observations have revealed a large population of high-velocity atomic hydrogen around the "hot Jupiter" HD 209458b during transit. This work expands on an earlier work studying the production of energetic neutral atoms (ENAs) as a result of the interaction between the stellar wind and the exosphere. We present an improved flow model that, together with stellar wind values similar to the ones in our solar system, further supports that the observed hot hydrogen are ENAs. We also study how the production of ENAs depends on the exospheric parameters, and establish an upper limit for the obstacle standoff distance at approximately 4-10 planetary radii.Finally we compare the results obtained for the obstacle standoff distance with existing exomagnetospheric models, and show how the magnetic moment of HD 209458b can be estimated from ENA observations for different exospheric scenarios.

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[1] The Analyzer of Space Plasma and Energetic Atoms (ASPERA-4) experiment on Venus Express provides the first measurements of energetic neutral atoms (ENAs) from Venus. The results improve our knowledge on the interaction of the solar wind with a nonmagnetized planet and they present an observational constraint to existing plasma models. We characterize the tailward flow of hydrogen ENAs observed on the nightside by providing global images of the ENA intensity. The images show a highly concentrated tailward flow of hydrogen ENAs tangential to the Venus limb around the Sun's direction. No oxygen ENAs above the instrument threshold are detected. The observed ENA intensities are reproduced with a simple ENA model within a factor of 2, indicating that the observed hydrogen ENAs originate from shocked solar wind protons that charge exchange with the neutral hydrogen exosphere.