Detecting life-bearing extrasolar planets with space telescopes

One of the promising methods to search for life on extrasolar planets (exoplanets) is to detect its signature in the chemical disequilibrium of exoplanet atmospheres. Spectra at the modest resolutions needed to search for methane, oxygen, carbon dioxide, or water will demand large collecting areas and large diameters to capture and isolate the light from planets in the habitable zones around the stars. Single telescopes with coronagraphs to isolate the light from the planet will have to be 8 m or more in diameter to generate sample sizes with a reasonable probability of finding at least one life-bearing planet; interferometers using smaller telescopes can overcome some of the limitations but will still need similarly large collecting areas. Even larger telescopes will be needed to detect atmospheric signatures in transiting planets. In all cases, the sample sizes increase at the third power of telescope diameter. Direct observation using coronagraphs or interferometers are most sensitive to planets around stars with masses similar to that of the Sun, whereas transit observations favor low-mass stars near the nuclear burning limit. If the technical difficulties of constructing very large space telescopes can be overcome, they will be able to observe planets near hundreds to thousands of stars with adequate resolution and sensitivity to look for the signatures of life.