Triton Orbits Neptune the “Wrong” Way

Neptune’s largest moon, Triton, orbits backward. It is the only large moon in the solar system that orbits in the opposite direction of its planet’s rotation, a retrograde orbit. That suggests it was not born around Neptune, and scientists think Triton is a Kuiper Belt object captured by Neptune’s gravity. A 2024 modeling study found that tidal heating after capture could have been orders of magnitude greater than Io’s today. Triton is still active: Voyager 2 found geysers on it, and a surface temperature of -235°C.