Asked by: Derek Snape, Cardiff
Pluto has been in the Solar System as long as the planets. The whole lot started out as a rotating cloud of matter that began to collapse in on itself under its own gravity about 4.6 billion years ago. The centre drew in the most mass and formed the Sun. The remainder of the spinning material is thought to have flattened into a disc, out of which the planets coalesced. Dust and gas orbiting some planets then formed systems of orbiting moons, of which one of the largest was Neptune’s moon Triton. It was once thought Pluto was another moon of Neptune but that theory has since been largely rejected. Pluto inhabits the Kuiper Belt, a region of similar bodies beyond the orbit of Neptune and in 2006 was demoted to the status of lowly ‘dwarf planet’.