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Studying the Atmosphere of Terrestrial Exoplanets

Of the 161 planets so far detected around other stars, eight have been discovered by the transit method as they moved between that star and the line of sight to Earth. Such transits, effective as planet finders in themselves, are also useful because they allow scientists to study the properties of the atmospheres around these worlds. The first planet found by transit methods orbits the star HD 209458 and is the object of intense atmospheric study.

Can such methods be applied to transiting Earth-size planets? A new paper studies the question in terms of the kind of signatures that might be expected, and the near-term technologies that could make such detections possible. The paper focuses on terrestrial worlds orbiting K, G or F-type stars, and notes that the best targets will be K-type stars, which are in any case more abundant than the other types as well as smaller. According to the analysis, the strongest signatures in the atmosphere of such worlds could be water, ozone and carbon dioxide.

And note this conclusion, based on a model of the spectra of Earth-like worlds developed in the paper:

The presence of an atmosphere around hundreds of hypothetical ‘ocean-planets’ … could be detected with a 10–20 m telescope. The atmospheres of tens of giant exoplanet satellites … could be in the range of a 20–30 m instrument. A 30–40 m telescope would be required to probe Earth-like atmospheres around Earth-like planets …. These numbers suppose that Earth-size planets are frequent and are efficiently detected by surveys.

The paper, the first to my knowledge to examine transits of Earth-like worlds in terms of atmospheric study, is Ehrenreich, Tinetti at al., “The transmission spectrum of Earth-size transiting planets,” now available at the arXiv site, and awaiting publication in Astronomy & Astrophysics. It goes without saying that Centauri Dreams remains all but awestruck at the fact that within ten short years, we have moved from finding the first exoplanets to actually studying the atmospheres of distant worlds.