Solar system formation. Artwork of planetesimals (bodies forming into planets) in the disc of dust surrounding the Sun during the formation of the solar system. The Sun was formed as gas coalesced until it was dense and hot enough to begin the nuclear reactions which sustain a star. The larger planetesimals are seen glowing orange as they are continuously impacted upon by a rain of smaller particles. It is thought the planets formed around 4.6 billion years ago as the dust and gas left over from the Sun's formation (the solar nebula) coalesced under gravitational attraction.