Clump of material, formed in the early stages of solar system formation, that was the forerunner of the planets we see today.
an object that is in the process of becoming a planet.
Massive object resulting from the coalescence of planetesimals in the solar nebula and destined to become a planet.
Protoplanets are moon-sized planet embryos within protoplanetary discs. 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 in giant impacts to gradually form the real planets.