Exoplanet transits in X-rays: a new observational window to the exoplanetary atmosphere
Abstract
Exoplanets in short-period orbits are subject to strong irradiation from their host star and can lose mass through evaporation. The main driver for this evaporation is high-energy emission from the host star. However, it is observationally unclear where in the exoplanetary atmosphere the bulk of the high-energy radiation is absorbed, and the energy budget for the evaporation is not well constrained. We have observed seven transits of the Hot Jupiter HD 189733 b in front of its host star, using X-ray observations with Chandra and XMM-Newton. We detect the exoplanetary transit in X-rays for the first time. We find a surprisingly large X-ray transit depth of 6-8%, in stark contrast to an optical transit depth of only 2.4%. We can trace this back to extended outer atmosphere layers of the planet which reach out to 1.75 optical planetary radii in altitude. We are able to derive density and temperature estimates for the outer planetary atmosphere, as well as a revised energy budget for planetary evaporation due to the large X-ray absorbing radius. These observations, together with accepted further programs in the X-ray regime, will allow us to build a comprehensive picture of the atmospheres of strongly irradiated exoplanets.
- Publication:
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American Astronomical Society Meeting Abstracts #223
- Pub Date:
- January 2014
- Bibcode:
- 2014AAS...22320705P