Planetary formation. Computer artwork of a protoplanet during the early history of the solar system. Protoplanets are thought to have formed from smaller planetesimals that were gravitationally attracted to each other and collided to eventually form protoplanets. In the history of the solar system, planets formed soon after the Sun, about 4.6 billion years ago. The rocky inner planets formed during a chaotic period when many protoplanets formed and collided with each other to form bigger protoplanets. Eventually, four planets (Mars, Earth, Venus and Mercury) survived, along with numerous asteroids.