The largest moon, and the only one big enough to have the shape of a sphere is Triton. Triton was discovered on October 10, 1846 by British astronomer William Lassell. Unlike all other large planetary moons, Triton moves in the other direction than the other moons. This shows the moon was probably captured and maybe was once a Kuiper belt object. It is close enough to Neptune to be locked into a synchronous orbit. It is also slowly moving into Neptune and will one day be torn apart when it passes the Roche limit. Triton is the coldest object that has been measured in the Solar System, with temperatures of −235°C (38 K, −392 °F). 