Definition for Protoplanet
From Biology Forums Dictionary
Protoplanets are large planetary embryos that originate within protoplanetary discs and have undergone internal melting to produce differentiated interiors. They are believed to form out of kilometer-sized planetesimals that attract each other gravitationally and collide. According to planet-formation theory, protoplanets perturb each other's orbits slightly and thus collide to gradually form the dominant planets.