Early solar system. Artwork showing the collision of rocky bodies (planetesimals) in the early solar system. Our solar system is thought to have formed around 4.5 billion years ago. It started as a disc of gas and dust surrounding the new Sun. Gravitational attraction caused the dust to accrete, forming larger and larger bodies. These continued to collide until one large body dominated its orbit, becoming a planet. As well as the eight planets there is much debris left over from this process, mainly found in the Asteroid Belt between Mars and Jupiter, the Kuiper Belt beyond Neptune and the even more distant Oort Cloud.