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Extrasolar Life
Some 4.6 billion years ago, before Earth existed, the Sun was surrounded by a disk of gas and dust, from which countless small bodies were forming. Most of these "planetesimals" coalesced into larger planetary embryos, which grew larger still to become the eight planets of the solar system. Why eight? There is nothing special about the number. Chaotic encounters between planetesimals early on led to a system with enough large bodies to sweep up most of the smaller ones. Computer simulations suggest that such encounters could as readily have ended up with fewer or more planets–”but not too many. The present configuration
of the solar system is filled nearly to capacity, and additional planets would be dynamically unstable. (Artist´ rendering of a hypothetical
planetary system in the making by T. Pyle, courtesy of NASA/JPL-Caltech.)


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