Is the exoplanet HD209458b's exosphere in a blowoff state ?

Details Is the exoplanet HD209458b's exosphere in a blowoff state ? Is the exoplanet HD209458b's exosphere in a blowoff state ?

Jul 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003hst..prop10081v&link_type=abstract

HST Proposal ID #10081

Computer Science

Hst Proposal Id #10081

Scientific paper

Using HST Lyman alpha observations, we have detected an extended tail shaped upper atmosphere around the extra-solar planet HD209458b. The Cycle 10 STIS G140M observations revealed an absorption of 15+/-4% of the stellar Lyman alpha emission line during the planetary transit in front of the star {Vidal-Madjar, et al. 2003}. This showed that HI evaporation is taking place within a highly extended upper atmosphere. Very recent studies on the evaporation mechanism are now indicating that instead of ``Jeans escape'' a more probable mechanism is complete atmospheric ``blowoff'' as suffered by the telluric planets in the early solar system. This mecanism could be directly tested with HST by searching for the escape of species slightly heavier than HI. It is easy to show that neither oxygen nor carbon would be detected with HST if only ``Jeans escape'' takes place while both species, under ``blowoff'' conditions, should reach the Roche limit of the planet and escape as hydrogen. If this is the case an absorption during transit of about 15% should be observed for both. Twelve orbits with the STIS G140L spectrograph will allow us to detect the presence of these species during three transits with a better than 5% accuracy, needed to detect an object of the size of the planetary Roche lobe. Simultaneously these observations will confirm and give more precision about the extention of the HI cometary like tail expected to occult the star well after the end of the visible transit. This proposed investigation is a unique opportunity to further study hot giant planets upper atmospheres, reveal the precise evaporation mechanism and give new constraints on the ``hot Jupiters'' lifetime. This could explain the lack of planets discovered so far at less than 0.04 AU from their star. We ask for HST observations under Director Discretionnary time to have access to the coming transits observable in the early fall of 2003.

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