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Figure 2. One theory for how planets form...Click to Enlarge Image

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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