Protoplanets are moon-sized planets, or larger 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 dominant planets.
Early protoplanets had more radioactive elements, the quantity of which has been reduced over time due to radioactive decay. Heating due to radioactivity, impact, and gravitational pressure melted parts of protoplanets as they grew toward being planets. In melted zones their heavier elements sank to the center; while lighter elements rose to the surface; such a process is known as planetary differentiation. Composition of some meteorites show that differentiation took place in some asteroids.