A large body of matter in orbit around the sun or a star and thought to be developing into a planet.
1940s: from proto- + planet.
Way out in the Oort cloud, there is probably enough matter to form a planet or two, but it is so far from the sun that gravity and rotation aren’t going to make it form into a protoplanet. An occasional comet may come inward from there. Besides, protoplanets happen much earlier in the formation of a star’s system. Our solar system is too sophisticated, “worldly” as they say, to play such juvenile games.