Solar system formation. Artwork of the primordial nebula (solar nebula) surrounding the newborn Sun (centre). The Sun was formed as gas coalesced until it was dense enough and hot enough to begin the nuclear reactions that sustain a star. The solar nebula, originally a sphere of gas, dust and rock, became a disc that eventually, under gravity, coalesced into planets. Some planets are already seen forming in the disc (red glows), caving out distinct gaps as they attract material in their orbits. It is thought the planets formed around 4.6 billion years ago.