Answer:
A planetesimal is an object formed from dust, rock, and other materials. The word has its roots in the concept infinitesimal, which indicates an object too small to see or measure. Planetesimals can be anywhere in size from several meters to hundreds of kilometers.
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.
