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One of a class of bodies that are theorized to have formed the planets after condensing from diffuse matter early in the history of the solar system. According to the nebular hypothesis, part of an interstellar cloud of dust and gas underwent gravitational contraction to form a solar nebula comprising a disk of matter with a central hub. As the disk contracted further, clumps of matter left behind in its midplane coalesced successively into bodies the size of pebbles and boulders and then into planetesimals up to several hundred miles across. These then combined under the force of gravity to form protoplanets, precursors of most of the current planets.
This entry comes from Encyclopædia Britannica Concise. For the full entry on planetesimal, visit Britannica.com.