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M. Salz

Research: Mass Loss from Exoplanetary Atmospheres

(Michael Salz)

The number of detected exoplanets is rising quickly and the search for habitable planets continues. In this context, we are interested in planets, which are close to their host stars and are exposed to large amounts of ionizing radiation. This radiation heats the planetary atmospheres, similar to HII regions which are heated by O-type stars. The high degree of energy input leads to an expansion of the upper atmosphere up to several planetary radii and, if the heating is strong enough, even to a mass loss. The mass loss of the atmosphere occurs in the form of a hydrodynamic escape. Planetary evolution can be affected by this process, which could even result in a complete evaporation of the planet. Looking at our "sister planet" Venus, there is an obvious depletion of water compared to the Earth with its large oceans. This rises the question, if a similar process could have evaporated Venusí water during an early active phase of our sun and directly links the hydrodynamic escape process to the search for habitable exoplanets. The habitability of an exoplanet is then restricted by the distance from the host star, at which water is not evaporated from the surface of the exoplanet.

My research considers only the case of close in Jupiter-sized planets (hot Jupiters), where the expanded atmospheres have been measured (e.g., Vidal-Madjar et al. 2003). Several hydrodynamic simulations exist, which try to predict the mass loss of a planet by fitting the absorption features of the expanded atmospheres. These simulations take into account strongly simplified mechanisms for the ionization heating in the ultra-violet spectral range. We will especially consider the effect of X-rays, which lead to a different ionization structure of the atmospheres and thereby also to a different distribution of the heat source throughout the planetary atmosphere. The effect of X-rays will only be important for host stars that are more active than our own Sun. An ionization and micro physics code (Cloudy) is used in combination with a hydrodynamic simulation (PLUTO) to compute the planetary wind with higher accuracy. The final goal is to explain the detected absorption features of the extended atmospheres to receive the best results for the mass loss.