Protoplanet

n. cloud of cosmic gas and dust that orbits a star and is in the process of condensing to form a planet (Astronomy)

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.
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The name given to a planet in the process of formation. As yet, no conclusive observations have been made of one.