JEAN-FRANCOIS PODEVIN / SCIENCE PHOTO LIBRARY JEAN-FRANCOIS PODEVIN / SCIENCE PHOTO LIBRARY
Extrasolar planetary orbits. Computer artwork of a stellar system with an Earth-like planet in its nearest orbit, four other planetary orbits, an orbiting asteroid and an orbiting comet. An alien (or extrasolar) planet has to fit a narrow range of sizes and distances from its star if it is to have a surface temperature suitable for liquid water, and to retain an atmosphere. Many planets have already been discovered orbiting nearby stars.
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