Retrograde orbit

In Astronomy a retrograde orbit is an orbit in the "wrong direction".

Stellar systems, like the Solar system, all begin as vast tenuous clouds of spinning interstellar gas. If the cloud is massive enough its own gravity will lead to it collapsing, with a smaller Denser core. Denser portions within the contracting cloud coalesce to form stars, planets, and smaller bodies. As that material falls to the centre of the cloud, or to the centre of whorls within the cloud, angular momentum keeps them spinning in the same direction of the original cloud, only more quickly. The rotational speed increases, as the distance drops, just like ice skaters increase the speed of their rotation, by drawing in their arms.

This means there should be an exceptional explanation when an object with a retrograde orbit is discovered. Triton, the largest moon of Planet Neptune, has a retrograde orbit, and the explanation is that it was not originally one of Neptune's moons, and was captured by Neptune.