Physics
Scientific paper
Jul 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005hst..prop10718v&link_type=abstract
HST Proposal ID #10718
Physics
1
Hst Proposal Id #10718 Solar System
Scientific paper
We propose to use HST to directly detect the atmosphere and extended envelope {exosphere} of a recently discovered planet that transits the relatively bright star HD 149026. Absorption of background stellar Lyman-alpha emission is an extremely sensitive exospheric diagnostic that only HST can measure. If the HD 149026 planet is evaporating, then hydrogen in the exosphere should fill or overfill the Roche lobe, which has a size 4 times the diameter of Jupiter. If we do not detect a decrease in Lyman-alpha flux during several transits, then the hydrogen exosphere does not fill the Roche lobe of the planet and significant evaporation is unlikely. This would be a surprise, given that Vidal-Madjar et al. {2003} detected the exosphere of the planet that transits HD 209458, and the models of Lecavelier des Etangs et al. {2004} predict an order of magnitude more evaporation for the planet around HD 149026, due to its lower mass and smaller orbital distance. The newly discovered planet has 3 times the mean density and at least 3 times the core mass of the planet that transits HD 209458, which is consistent with significant exospheric evaporation in the past. We will also use ACS/PR200L observations to measure stellar limb darkening in the NUV, to detect Mg in the lower atmosphere of the planet, and to search for evidence of a moon or planetary rings.
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