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 real planets.

In the case of the Solar System it is thought that the collisions of planetesimals created a few hundred planetary embryos. Such embryos had masses of about 10²² to 10²³ kg and were a few thousand kilometres in diameter. Over the course of about 100 Ma they were involved in giant impacts with one another. The exact sequence whereby planetary embryos collided to assemble the planets is not known, but it is thought 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.^[1]

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.

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