astronomical body protoplanet planetary embryo State that this is derived from a collection of planetesimals. An astronomical object that forms within a protoplanetary disc when planetesimals and other solid debris aggregate and, due to gravitational forces, fuse and initiate internal melting to produce a differentiated interior. Protoplanets are thought to form out of kilometer-sized planetesimals that gravitationally perturb each other's orbits and collide, gradually coalescing into the dominant planets. The exact sequence of events which lead to planetary formation from protoplanets is not fully understood. envoAstro