Neptune moon: Tiny, dark, whizzing space ball captured on film (barely)

Showalter has a proven knack for seeing the invisible. He has already discovered five other moons – Saturn's Pan, Uranus's Mab and Cupid, and Pluto's recently named Kerberos and Styx – as well as wispy gas rings around Jupiter, Uranus, and Neptune.

When it occurred to Showalter to use his software to look beyond Neptune's rings, this little moon appeared. And then again. In less than a week, it showed up ten times. He hasn't yet used this technique to look at other planets, but he plans to. "How much deeper could I go in the Hubble data sets?" he asks. "It seems like a promising question to look into."

Discovering a moon is cool enough. Among other things, it usually gives you naming rights, which Showalter exercised when he proposed the names for Pan, Mab, and Cupid to the International Astronomical Union (IAU). Showalter graciously opened the naming of Pluto's moons to the public, who voted overwhelmingly for Vulcan, though the IAU overruled them and selected the #2 and #3 vote-getters. Showalter hopes to involve the public in giving S/2004 N1 a better name, but he's still working out how. "I will say, I've already received suggestions via email, including some pretty good ones," he says.

And discovering this moon in particular raises some fascinating scientific questions. Astronomers thought they'd figured out the story of Neptune's moons, which focuses on the fact that the smaller moons are close in and the bigger moons are farther away from the gas giant. That story does not include having a small, dark rock, barely 12 miles across, whizzing madly out near the orbit of Neptune's bigger moons.

It might have something to do with Triton, Neptune's biggest – and oddest – moon. Triton is enormous – hundreds of times larger than any other moon of Neptune – and it orbits backward with respect to Neptune's own rotation and the orbit of all the other moons, like a car driving the wrong way up a highway. This tells astronomers that Triton wasn't originally part of Neptune's story, but got poached from somewhere else.

"We can be pretty sure that when Triton arrived, it disrupted whatever system of moons was originally circling Neptune," writes Showalter. "The moons that we see today somehow re-formed after that event. With the discovery of S/2004 N 1, the key points of this story have not changed, but we now have one more piece of the puzzle to fit into place."

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