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Observations of extrasolar planets, which now number close to 200, help to constrain theories of planet formation. In general, recent findings support the core-accretion model, whereby small solid particles in a protostellar disk agglomerate to form planetesimals, which can grow large enough to attract surrounding gas gravitationally. The result is a gas giant similar to Jupiter or Saturn. But the attributes of some of the extrasolar planets recently discovered, and the stars around which they orbit, suggest that current core-accretion theory is still in need of refinement.
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