One of a class of hypothetical bodies that joined to form the solar system .
According to the nebular hypothesis, a rotating cloud of interstellar dust and gas underwent gravitational contraction, eventually forming a solar nebula comprising a disk of matter with a central hub. As the disk contracted further, clumps of dust 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 the current planets.