PLANETESIMAL

one of a class of hypothetical bodies that eventually coalesced to form the planets after condensing from gaseous matter early in the history of the solar system. According to the nebular hypothesis, an interstellar cloud of dust and gas underwent gravitational collapse to eventually form a primeval solar nebula. Clumps of interstellar dust left behind in the midplane of the solar disk as it contracted toward its centre would themselves eventually coalesce, through a process of accretion, to form pebbles, boulders, and then planetesimals measuring a few kilometres to several hundred kilometres across. These planetesimals then combined under the force of gravity to form protoplanets, which were the precursors of the current planets of the solar system. See also protoplanet.