Triton’s retrograde orbit
Why does Triton orbit Neptune backwards?
Most moons travel the same way their planet spins, because they formed from the same spinning disc. Triton goes the other way, so it did not form next to Neptune. It was captured, almost certainly from the Kuiper belt. On Neptune’s moons it is the one that runs backwards.
Backwards means captured
If a moon forms beside its planet, it inherits the spin of the disc. Clockwise with clockwise, or the other way around — but matching. Triton does not match. That is the giveaway: it arrived later.
The favoured picture is that Triton was one of a pair, like Pluto and Charon. Neptune grabbed one and flung the other away. The leftover is bound, and going the wrong way.
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It is spiralling in
A backward orbit is also a decaying one. Tides are dragging Triton inward. In some billions of years it will cross Neptune’s Roche limit and break into a ring — unless it hits first. Either way, the system we see is not finished.