Radiation Transfer Through an Extended Planetary Atmosphere

K. Schnabel, Ch. Helling, P. Woitke, E. Sedlmayr

AG Tagung München 2001

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Abstract:

A transit of a planet in front of its parent star nowadays provides the best possibility to gain detailed insight into extrasolar planet atmosphere. Experiences with the difficult detection of possible features of an extended atmosphere ("exosphere") indicate the need for theoretical guidance through the wealth of weak absorption lines on top of the parent star spectrum. The work presented here is a first attempt to discuss molecular features possibly observable during an extrasolar planetary transit event. Utilizing the HITRAN (e.g. CO2, O2, CH4, NH4, OH) and the Copenhagen (e.g. CO, TiO, SiO, H2O) data base for 25 molecular species as well as continuum cross sections, a radiative transfer along characteristic rays through a simplified exosphere has been calculated.

The evaluation of the optical depth shows, that the planet atmosphere is optically thick in inner adiabatic layers. The evaluation of number densities and cross sections in the optically thin upper regions identified six molecules (CO, O2, SiO, TiO, CO2, OH) as the main opacity contributers of the planetary gas. The line dominated part of the composed planet-star spectrum is mainly influenced by the four molecules O2, CO2, TiO, and SiO in the range 0.25...1 µm. The absorption bands in this range can be assigned to these molecules. In the range from 1 to 10 µm all six above mentioned molecules show more or less strong and significant absorption bands.