Protoplanet

From Palaeos

A "planetary embryo" at the formation of a solar system. Over time, protoplanets are thought to have accreted through collision to form the current planets. For instance, the asteroids within the current asteroid belt between Mars and Jupiter are thought to represent protoplanets that have failed to accrete into a planet, probably due to the gravitational influence of Jupiter. See Wikipedia on Protoplanets.