Characterization of extrasolar terrestrial planets from diurnal photometric variability

The detection of massive planets orbiting nearby stars has become almost ro utine(1,2), but current techniques are as yet unable to detect terrestrial planets with masses comparable to the Earth's. Future space-based observato ries to detect Earth-like planets are being planned. Terrestrial planets or biting in the habitable zones of stars-where planetary surface conditions a re compatible with the presence of liquid water-are of enormous interest be cause they might have global environments similar to Earth's and even harbo ur life. The light scattered by such a planet will vary in intensity and co lour as the planet rotates; the resulting light curve will contain informat ion about the planet's surface and atmospheric properties. Here we report a model that predicts features that should be discernible in the light curve obtained by low-precision photometry. For extrasolar planets similar to Ea rth, we expect daily flux variations of up to hundreds of per cent, dependi ng sensitively on ice and cloud cover as well as seasonal variations. This suggests that the meteorological variability, composition of the surface (f or example, ocean versus land fraction) and rotation period of an Earth-lik e planet could be derived from photometric observations. Even signatures of Earth-like plant life could be constrained or possibly, with further study , even uniquely determined.