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Formation of the Solar System. Artwork showing four stages in the formation of the Solar System. At top, the young Sun has just ignited after coalescing under gravity from a cloud of gas and dust. The gas became denser and hotter until nuclear reactions began. Below this, the remaining gas and dust (the protoplanetary nebula) form a disc due to the rotation of the cloud. Gas and dust in the cloud coalesce to form planetesimals (third stage). These collide and agglomerate to form larger protoplanets, which attract all the remaining matter, leaving just the major planets. It is thought that the Solar System formed around 4.5 billion years ago. (KEYSTONE/SCIENCE PHOTO LIBRARY/MARK GARLICK)