Katja Poppenhager (CfA) : How to X-ray an exoplanet

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 planet's gravitational well, causing substantial mass loss. This mass loss of atomic hydrogen has been observed in UV spectral lines for a handful of 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. I will 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. I will end with a discussion about how present and future X-ray missions can provide new insights into exoplanetary atmospheres that are inaccessible at other wavelengths.