Exoplanet HD 209458b (Osiris^*): Evaporation Strengthened

Following reanalysis of Hubble Space Telescope observations of primary transits of the extrasolar planet HD 209458b at Lyα, Ben-Jaffel claims that no sign of evaporation is observed. Here we show that, in fact, this new analysis is consistent with the one of Vidal-Madjar and coworkers, and supports the detection of evaporation. The apparent disagreement is mainly due to the disparate wavelength ranges that are used to derive the transit absorption depth. Vidal-Madjar derives a 15% ± 4% absorption depth during transit over the core of the stellar Lyα line (from –130 to +100 km s⁻¹), and this result agrees with the 8.9% ± 2.1% absorption depth reported by Ben-Jaffel from a slightly expanded data set but over a larger wavelength range (±200 km s⁻¹). These measurements agree also with the 5% ± 2% absorption reported by Vidal-Madjar over the whole Lyα line from independent, lower resolution data. We show that stellar Lyα variability is unlikely to significantly affect those detections. The H I atoms must necessarily have velocities above the escape velocities and/or be outside the Roche lobe, given the lobe shape and orientation. Absorption by H I in HD 209458b's atmosphere has thus been detected with different data sets, and now with independent analyses. All these results strengthen the concept of evaporating hot Jupiters, as well as the modeling of this phenomenon.