Transmission spectroscopy of extrasolar planets

The atmosphere of the Earth is the only known shelter of life in the Universe, but this may not remain so forever. The ongoing search for planets, orbiting other stars than the Sun, has revealed a manifold and ever growing fraction of the population of gaseous and rocky planets in the Galaxy. Yet, little is known about the conditions on their surfaces.
Transiting planets periodically eclipse the bright disks of their host stars whenever they complete an orbit and they play a fundamental role both in the search for exoplanets and the study of their atmospheres. Like the shadow in a galanty show, the detailed analysis of the stellar dimming caused by the passage of the planetary body and its atmosphere, allows to extract information on the object casting the shadow. Transits easily allow to accurately measure their orbital period, and, notably, only for transiting planets, true masses, radii, and therefore densities can be measured, which gives an idea of their composition. Even when these fundamental properties are known, the narrow atmospheric shells decisively influence the the conditions on the surfaces. Clearly, all energy or mass exchanged between the planet and its surroundings must cross the atmosphere. Its crucial role is quite obvious when looking at the Earth, whose surface would become a freezing desert without it. However, only a slight change in its composition might suffice to turn it into a seething greenhouse. The planetary atmosphere is therefore the crucial layer to study to obtain a better understanding of planetary evolution and, finally, habitability.
Our objective is to improve our understanding of planetary atmospheres, by employing the technique of high-resolution transit spectroscopy. At spectral resolutions on the order of 100,000 the characteristic signatures, imprinted on the stellar light by the individual chemical species in the atmosphere during transit, can be distinguished. Thus, the composition and physics of the planetary atmosphere can be probed. This technique has successfully been used to detect, e.g., traces of hydrogen, sodium, and water in the atmospheres of various planets and it can even be used to resolve motion of the material, e.g., caused by winds. A relatively new possibility is to obtain high-resolution spectra in the near-infrared regime. This range covers a triplet transition of the excited helium atom, which has become a main focus of transmission spectroscopy because it is the only known tracer of planetary mass loss observable with ground based instrumentation. The excitation of the triplet is intimately related to stellar extreme-ultraviolet and X-ray emission, which is a result of stellar magnetic activity. Because the latter also produces stochastic variation in the stellar spectra, it is a notorious nuisance in transmission spectroscopy, which has to be tightly controlled. Curiously, also the atmosphere of the Earth with its changing weather patterns is a strong disturbance, needing close attention. Only a comprehensive treatment of these effects enables us to study the atmospheres of other worlds.