The emerging field of extrasolar planet search has shown an extraordinary ability to combine research by astrophysics, chemistry, biology and geophysics into a new and exciting interdisciplinary ap-proach to understand our place in the universe. Are there other worlds like ours? How can we characterize those planets and assess if they are habitable? After a decade rich in giant exoplanet detections, observa-tion techniques have now reached the ability to find planets of less than 10 MEarth (so called Super-Earths) that may potentially be habitable. The detection and characterization of Earth-like planet is approach-ing rapidly with dedicated space observatories already in operation (Corot) or in development phase (Kepler, James Webb Space Telescope, Extremely Large Telescope (ELT), Darwin/TPF). Space missions like CoRoT (CNES, Rouan et al. 1998) and Kepler (NASA, Borucki et al. 1997) will give us statistics on the number, size, period and orbital distance of planets, extending to terrestrial planets on the lower mass range end as a first step, while missions like Darwin/TPF are designed to characterize their atmospheres. In this chapter we discuss how we can read a planet's spectral fin-gerprint and characterize if it is potentially habitable. We discuss the first steps to detect a habitable planet and set biomarker detection in context in Section 1. In Section 2 we focus on biomarkers, their signatures at different wavelengths, abiotic sources and cryptic photo-synthesis using Earth as our primary example - the only habitable planet we know of so far. Section 3 concentrates on planets around different stars, and Section 4 summarizes the chapter.
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