Planetary formation. Artwork of Earth-like (rocky) planets forming in the early solar system. It is thought that the planets formed from a rotating disc of material that formed around, and at the same time as, the Sun (centre), around 4. 6 billion years ago. The rocky inner planets formed during a chaotic period when many protoplanets formed by gathering up surrounding material (such as the rocky debris seen) by gravitational attraction. Eventually, four planets (Mars, Earth, Venus and Mercury) survived, along with numerous asteroids. These small rocky planets contrast with the outer giant planets that consist mostly of gas and ice.