A&A 448, 379-393 (2006)
DOI: 10.1051/0004-6361:20053861

The transmission spectrum of Earth-size transiting planets

D. Ehrenreich¹, G. Tinetti², A. Lecavelier des Etangs¹, A. Vidal-Madjar¹ and F. Selsis<sup>3

1</sup>  Institut d'Astrophysique de Paris, CNRS (UMR 7095) - Université Pierre & Marie Curie, 98bis boulevard Arago, 75014 Paris, France
    e-mail: ehrenreich@iap.fr
²  NASA Astrobiology Institute, California Institute of Technology, IPAC, MS 220-6, 1200 E. California, Pasadena, 91125 (CA), USA
³  Centre de Recherche Astronomique de Lyon, École Normale Supérieure, 47 allée d'Italie, 69364 Lyon Cedex 7, France

(Received 19 July 2005 / Accepted 29 September 2005)

Abstract
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 (H₂O), carbon dioxide (CO₂), ozone (O₃), or molecular oxygen (O₂). 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.

Key words: planets and satellites: general

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