Protoplanet
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.
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.
The
giant impact theory proposes that Earth's
Moon formed from a colossal impact of a hypothetical protoplanet, named
Theia, with Earth early in the solar system's history.
