A LARGE UV-OPTICAL TELESCOPE FOR CHARACTERIZATION OF THE ATMOSPHERES OF EXTRASOLAR PLANETS AND SATELLITES

摘要

A Large UV-optical Telescope is well within the current or soon-coming technical capabilities. Such a telescope could dedicate a significant fraction of time to spec-troscopic observations of the atmospheres of extrasolar planets, thus providing a powerful tool to characterize the planets which will be discovered in the coming decade. With a large photon collecting area, it can allow the search for bio-markers like ozone or water in a very large number of extrasolar planets and planetary satellites up to hundreds of parsecs away.rnDetections and observations of the atmosphere of an extrasolar planet have already been made. They show that space observations of absorption during planetary transits in the UV-optical wavelength range provide an efficient tool to characterize the atmospheres. Many molecules have strong transitions in the UV-optical to the near-IR domain. In particular, bio-markers like O3 and O2 have very strong transitions in the ultraviolet. The Hartley bands of O3 are the main absorbers at 200-350 nm. H2O has many bands in the 700-2000 nm wavelength range. Finally, escaping atmospheres of evaporating ocean from water rich planets can be detected in Lyman-α at 121.6 nm .rnWe estimate the number of targets as a function of the telescope size for various kinds of planets which can be observed with a large space telescope. Taking into account the transit probabilities, we evaluate the possibilities of searches for bio-markers and other atmospheric constituents in giant and small planets orbiting K, G and F main-sequence stars. We find that planets around K-stars are favoured mainly because the star is small. Moreover they provide a large fraction of potential targets.rnIt appears that 'Venus-like' planets have atmospheres which are the most difficult to probe by such techniques. The atmospheres of thousands of giants planets and of a few dozen Earth-like or small Earth-like planets could be observed with a 10-meter or larger sized telescope provided that these planets are frequent and that the surveys will have good efficiency to detect them. Owing to their large atmospheric scale heights, the easiest targets are the low density planets, such as 'ocean-planets' or 'Titan-like' small volatile-rich bodies.