A proto-planet is a large planetary body in the early stages of formation. Protoplanets are thought to form from the accumulation of dust and gas in the protoplanetary disk that surrounds a young star. As they grow, they begin to gravitationally interact with one another, which can lead to their eventual coalescence into a single planet.
The first stage in the formation of a proto-planet is the growth of dust grains in the protoplanetary disk. These dust grains collide and stick together to form larger and larger clumps. Once these clumps reach sizes of about 1 kilometer, they are referred to as “planetesimals”.
As planetesimals continue to grow through accretion, they eventually become large enough (1000 kilometers or more) that their gravitational interactions with one another become significant. These interactions can result in mutual collisions and mergers, which lead to the growth of even larger bodies known as “protoplanets”.
It is thought that our own Solar System’s planets formed through this process of accretion and collision among protoplanets over the course of several million years. The end result was a series of planets orbiting our Sun, each with its own unique characteristics.