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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.

The tiny addition to Neptune's family is an added shock because it seems too small to have survived the formation of the other moons, according to accepted theories.

Neptune's biggest moon, Triton, is 2705 kilometres wide and orbits backwards – travelling in the opposite direction to the planet's spin. Its large size and wonky orbit led astronomers to believe that Triton was captured by Neptune's gravity about 4 billion years ago and that it destroyed whatever moons the gas giant originally had as it was settling into its new home.

"The Neptune moons we see today were probably broken up and regenerated after the arrival of Triton," says Showalter.

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?"

A more immediate question may be what to call this new and unusual moon. Neptune's other natural satellites are named after minor water deities in Greek and Roman mythology, an official naming convention set up by the International Astronomical Union (IAU). Showalter and colleagues also recently discovered two new moons around Pluto and put their names to a public vote – although the IAU had the final say.

"Compared to work we recently did naming the moons of Pluto, there's not quite as colourful a cast of characters to work with, but there is still an interesting list of sea creatures one can choose from," he says.

For now, Showalter and the discovery team do not have a favourite in mind: "We don't really have a name for it. It's just 'that little moon', because S/2004 N1 does not roll off the tongue."