The transmission spectrum of Earth-size transiting planets

A variety of terrestrial planets with different physical parameters and exotic atmospheres might plausibly exist outside our Solar System, waiting to be detected by the next generation of space-exploration missions. Some of these planets might be transiting their parent star. We present here a detailed study of the atmospheric signatures of transiting Earth-size exoplanets. We focus on a limited number of significant examples, for which we discuss the detectability of some of the possible molecules present in their atmospheres, such as water (H_2O), carbon dioxide (CO_2), ozone (O_3), or molecular oxygen (O_2). To this purpose, we developed a model to simulate transmission spectra of Earth-size exoplanets from the ultraviolet (UV) to the near infrared (NIR). According to our calculations, the signatures of planetary atmospheres represent an absorption of a few parts-per-million (ppm) in the stellar flux. The atmospheres of a few Earth-like planets can be detected with a 30-40 m telescope. The detection of the extensive atmospheres of tens of small satellites of giant exoplanets and hundreds of hypothetical ocean-planets can be achieved with 20-30 m and 10-20 m instruments, respectively, provided all these planets are frequent and they are efficiently surveyed. We also found that planets around K stars are favored, mainly because these stars are more numerous and smaller compared to G or F stars. While not addressed in this study, limitations might come from stellar photometric micro-variability.