Protoplanets

Protoplanets are large planetary embryos that originate within protoplanetary discs and have undergone internal melting to produce differentiated interiors.[1]

Formation Theory
Protoplanets 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. It’s believed that initial collisions would have replaced the first "generation" of embryos with a second generation consisting of fewer but larger embryos. These in their turn would have collided to create a third generation of fewer but even larger embryos. Eventually only a handful of embryos were left, which collided to complete the assembly of the planets proper.

The giant impact hypothesis proposes that Earth's Moon formed from a colossal impact of a hypothetical protoplanet, named Theia, with Earth, early in Sol System's history.

Protoplanets in the Sol System
In the inner Sol System, the three protoplanets to survive more-or-less intact are the asteroids 1 Ceres, 2 Pallas, and 4 Vesta. The asteroid 21 Lutetia also has shown characteristics that resemble a protoplanet. Kuiper-belt dwarf planets have also been referred to as protoplanets. Because iron meteorites have been found on Earth, it is deemed likely that there once were other metal-cored protoplanets in the asteroid belt that since have been disrupted and that are the source of these meteorites.

Modern Use of Planetesmals and Protoplanets
Because planetesmals and protoplanets are usually found in asteroid belts, they are normally the first bodies in them to be exploited. First for metals and other elements, then as bases, industrial sites and finally colonies (as in the case of Ceres, Pallas and Vesta in the Sol System.) At the end of this cycle, group associations such as Sol’s Main Asteroid Belt Association form.