Neptune has a new moon, and its existence is an enigma. The object, known for now as S/2004 N1, is the first Neptunian moon to be found in a decade. Its diminutive size raises questions as to how it survived the chaos thought to have created the giant planet’s other moons.
S/2004 N1 is about 20 kilometres across, and it has a nearly circular orbit that takes it around Neptune in 23 hours. Its orbit is squarely between Proteus, the outermost moon aside from Triton, and Larissa. These moons are 400 and 200 kilometres across, respectively. But in the post-Triton chaos, such a small rock should have been swept up to become part of Proteus, or broken up by interloping asteroids sometime after the system settled down. “How you can have a 20-kilometre object around Neptune is a little bit of a puzzle,” says Showalter. “It’s far enough away that its orbit is stable. Once you put it there it will stay there. The question is, how did it get there?”
There are currently no proposed names for the new moon.