Characterizing habitable extrasolar planets using spectral fingerprints

The detection and characterization of an Earth-like planet is approaching rapidly thanks to radial velocity (RV) surveys (e.g. HARPS) and transit searches (Corot, Kepler). A rough characterization of these planets will be already achievable in 2014 with the James Webb Space Telescope, and more detailed spectral studies will be obtained by future large ground based telescopes (ELT, TNT, GMT), and dedicated space-based missions like Darwin, Terrestrial Planet Finder, New World Observer. In this article we discuss how we can read a planet's spectrum to assess its habitability and search for the signatures of a biosphere. Identifying signs of life implies understanding how the observed atmosphere physically and chemically works, and thus gathering information on the planet in addition to observing its spectral fingerprint.