The detection of massive planets orbiting nearby stars has become almost routine1,2, but current techniques are as yet unable to detect terrestrial planets with masses comparable to the Earth's. Future space-based observatories to detect Earth-like planets are being planned. Terrestrial planets orbiting in the habitable zones of stars—where planetary surface conditions are compatible with the presence of liquid water—are of enormous interest because they might have global environments similar to Earth's and even harbour life. The light scattered by such a planet will vary in intensity and colour as the planet rotates; the resulting light curve will contain information 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 Earth, we expect daily flux variations of up to hundreds of per cent, depending sensitively on ice and cloud cover as well as seasonal variations. This suggests that the meteorological variability, composition of the surface (for example, ocean versus land fraction) and rotation period of an Earth-like 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.
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We thank the members of the Ball Aerospace Terrestrial Planet Finder team, B. Soden, A. Broccoli and G. Williams for discussions, and B. Soden for assistance with the ISCCP database. We wish to thank W. B. Rossow, the Goddard Institute for Space Studies, the Goddard Space Flight Center, and NASA for the production and distribution of this data set. E.B.F. would like to acknowledge support under a National Science Foundation Graduate Fellowship and S.S. is supported by the W. M. Keck Foundation.
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Ford, E., Seager, S. & Turner, E. Characterization of extrasolar terrestrial planets from diurnal photometric variability. Nature 412, 885–887 (2001). https://doi.org/10.1038/35091009