WHEN TRITON CRASHED THE PARTY AT NEPTUNE

Neptune’s original satellites may have been destroyed when its largest moon, Triton, entered the picture. The massive moon may have tossed some of the original satellites into the ice giant, kicked others out of orbit and swallowed up the rest, creating a new family that doesn’t look much like those of other giant planets.

For years scientists have suspected that Triton wasn’t part of Neptune’s original collection of moons. The massive moon has a backward orbit and makes up over 99 per cent of all the mass orbiting the planet. They think it’s a captured object whose orbit was circularised by debris discs created by impacts.

The moons of Jupiter, Saturn and Uranus are all well-behaved compared with Neptune’s. The other three gas giants have a wealth of satellites – Jupiter has 79 to Neptune’s 14 – travelling in nearly circular paths around their equators. While Triton’s path is circular, it travels backwards compared with Neptune’s rotation, and spins backwards too.