Protostars and Planets VI, Heidelberg, July 15-20, 2013
Exoplanet transits in X-rays - a new observational window to exoplanetary atmospheres
Poppenhaeger, K. (Harvard-Smithsonian Center for Astrophysics)
Schmitt, J. H. M. M. (Hamburger Sternwarte)
Wolk, S. J. (Harvard-Smithsonian Center for Astrophysics)
Many exoplanets orbit their host stars at close distances, with orbital periods of only a few days. The incident stellar flux can deposit sufficient energy in those planetary atmosphere to lift parts of it out of the planetís gravitational well, causing substantial mass loss. And indeed, mass loss of atomic hydrogen has been observed in UV spectral lines for several planets. However, at the temperatures thought to be present in the planetary outer atmospheres, hydrogen is mostly ionized, so that these measurements lose their sensitivity at higher planetary altitudes. Here we present the first X-ray detection of an exoplanetary transit in front of its host star; we find a surprisingly deep X-ray transit with three times the optical transit depth. This can be traced back to thin outer atmosphere layers of the planet, which are transparent at optical wavelengths but opaque to X-ray photons. The planetary atmosphere is thus X-ray opaque out to radii of 1.75 times the optical radius. Due to the larger energy input of X-ray photons into the planetary atmosphere, we derive a twice as large planetary mass loss rate than thought before. Further observations will allow us to detect individual element species in the outer planetary atmosphere, using transit profiles in different X-ray energy bands.
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