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

Abstract

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 ...

The relative role of EUV radiation and X-rays in the heating of hydrogen-rich exoplanet atmospheres

“…A re-analysis of the Vidal-Madjar et al (2003) HST data performed by Ben-Jaffel (2007), although confirming the detection of an expanded atmosphere, found no indications for mass loss rates of the order of 10 10 g/s (see, however, Vidal-Madjar et al 2008;and Ehrenreich et al 2008, for a different interpretation of the data). A very recent study by Holmström et al (2008) presents evidence that the atomic hydrogen absorption in the stellar Lyman-α by the planet HD 209458b detected with HST is affected by energetic neutral atoms originating from the stellar wind interaction with the expanded atmosphere. Since a range of exospheric conditions and atmospheric loss rates are consistent with the observations, Holmström et al (2008) concluded that we have little information on the main exospheric component.…”

“…A very recent study by Holmström et al (2008) presents evidence that the atomic hydrogen absorption in the stellar Lyman-α by the planet HD 209458b detected with HST is affected by energetic neutral atoms originating from the stellar wind interaction with the expanded atmosphere. Since a range of exospheric conditions and atmospheric loss rates are consistent with the observations, Holmström et al (2008) concluded that we have little information on the main exospheric component. Berdyugina et al (2008) used polarized scattered light to gain information about the atmosphere of HD 189733b.…”

Exoplanet Upper Atmosphere Environment Characterization

“…3, we show two modeled hydrogen corona and ENAs as well as the attenuation spectra in comparison with the HST/STIS in and out off transit observations of Vidal-Madjar et al (2003). We apply the same plasma flow, exosphere, ENA production and Lyman-α attenuation model described in detail in Holmström et al (2008) and Ekenbäck et al (2010) to the planetary parameters favored by Koskinen et al (2010). The results shown in Fig.…”

“…Because of the steep decline of the number density of neutral H atoms, the optical depth in the wing of the line is very low. If the amount of atomic H is not adequate to fit the observations, the fact that the transits of HD 189733b in H Lyman−α have been observed imply that external processes, such as the production of energetic neutral atoms (ENAs) as suggested by Holmström et al (2008) and Ekenbäck et al (2010), play an important role. One should also note that even in case the assumption of Koskinen et al (2010) is right, ENAs will be produced if the upper atmosphere interacts with the stellar wind plasma.…”

Constraints on the exosphere of CoRoT-7b

“…While the exosphere of gaseous extrasolar planets has been detected in hydrogen as well as heavier elements (Vidal-Madjar et al 2003;Vidal-Madjar et al 2004;Lecavelier des Etangs et al 2010;Linsky et al 2010;Holmström, et al 2008;Eckenbäck et al 2010;Fossati 2010), no attempts have yet been made to detect the exosphere of a rocky extrasolar planet.…”