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.

The faint moon was discovered in archived images from the Hubble Space Telescope. Mark Showalter of the SETI Institute in Mountain View, California, was poring over pictures of Neptune taken in 2009 to study segments of its rings.

The rings around our outermost planet are too faint to see without taking very long-exposure pictures. However, the rings orbit so fast that taking one long shot would smear them across the frame. Showalter and colleagues gathered multiple shorter-exposure images and developed a technique to digitally rewind the orbits to the same point in time. Then they could stack several images on top of each other to reveal details of the rings.

"I got nice pictures of the arcs, which was my main purpose, but I also got this little extra dot that I was not expecting to see," says Showalter.

Stacking eight to 10 images together allowed the moon to show up plain as day, he says. When he went back and repeated the process using Hubble pictures taken in 2004, the moon was still there and moving as expected.

"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?"

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