Natalie Jones, of Rangi Ruru School, asks :-
Why does Triton, a moon of Neptune, rotate in the opposite direction to Neptune?
Alan Gilmore, an astronomer at the University of Canterbury's Mt John Observatory, responded.
We think this is because Triton is not an original moon of Neptune but an asteroid `captured' by Neptune's gravity. Other planets also have maverick moons. Saturn's satellite Phoebe, and several of Jupiter's small distant moons, move in orbits different from the big moons.
The current idea is that moons formed at the same time as their planet. A cloud of dust and gas gathered together and most of it became the planet. A small amount of it went into a disk spinning around the planet's equator; like Saturn's ring but much bigger. The disk coalesced into moons, all moving in circular orbits in the same plane.
Later, some planets captured asteroid-sized objects that strayed into their neighbourhood. Since these captured moons were not part of the disk their orbits are quite different from the other moons.
There is an old theory that Pluto was once a moon of Neptune till it and Triton got into a gravitational tangle. This resulted in Pluto being kicked into an orbit around the sun while Triton's orbit was reversed. We don't believe so now. Since 1992 we've learned that Pluto is merely the biggest of a belt of icy asteroids that orbit beyond Neptune. Triton was almost certainly one of those asteroids before it was captured.