Did you know that Triton, the big moon of Neptune, was probably captured from an independent orbit around the Sun long ago?
Triton, moon of Neptune (click to enlarge WARNING large image: 2MB)

Normally moons orbit planets in the same sense that the planets spin, but Pluto-sized Triton is odd because it orbits Neptune backwards. Accordingly, it very unlikely that the planet and satellite formed together; instead, Triton probably began life freely orbiting the Sun. One day it wandered too close to Neptune and got captured by the planet's strong gravity. In order to facilitate capture, it is likely that Triton once had a large moon much like Pluto's Charon. When the binary pair wandered too close to Neptune, Triton gave up its freedom so that its companion could go free.

Return to the Astronomy Workshop