HST-COS observations of the transiting extrasolar planetary system HD 209458b

Abstract

We summarize results from deep spectroscopic observations of the HD 209458 planetary system, carried out with the Hubble SpaceTelescope—Cosmic Origins Spectrograph. Orbitally resolved observations are used to show that hot gas emission lines, arising only in the stellar atmosphere, are not variable, while lower ionizations species found in the upper atmosphere of the hot Jupiter HD 209458b absorb stellar photons during transit. For both C II and Si III, we find mean transit attenuation of ∼8%. The firm detection of silicon is in direct conflict with previous low-resolution studies, which we attribute to long-term variability in the system. We also use these observations to search for auroral emission from the planet, detecting a statistically significant emission feature at 1582 Å that is consistent with H₂ photoexcited by stellar O I photons.