The direct detection of terrestrial extra-solar planned should be possible in the next 10 or 20 years. It is the main purpose of two space missions: Darwin (ESA) and TPF (NASA). Moreover, assuming a detection of such a planet, these telescopes should also be able to perform low-resolution spectra of this new world and therefore to infer the presence of atmospheric compounds. I will discuss the exobiological implications of these extrasolar spectroscopic studies: