New moon challenges theory of Neptune’s moon formation

New Scientist : Temporarily designated S/2004 N1, a new moon—the first for more than a decade—has been found orbiting Neptune. Mark Showalter of the SETI Institute in Mountain View, California, and his colleagues made the discovery while examining images taken by the Hubble Space Telescope in 2009 of Neptune’s faint outer rings. Because the rings are very faint, the researchers were looking at short-exposure images and layering them to reveal details. The layering revealed a bright dot that appeared to be a small moon. To confirm the discovery, the team examined a similar set of images taken in 2004 (hence the S/2004 designation) and found the moon present then as well. At 20 km in diameter it has a nearly circular 23-hour orbit halfway between Proteus and Larissa, the two outermost moons other than Neptune’s largest moon, Triton. Because Triton orbits opposite the spin of Neptune, it is believed to have been captured by the planet’s gravity. The capture would have destroyed any moons Neptune had at the time, and the moons it has now would have formed afterward. But how such a small moon could have formed or survived where it did is unclear; theory suggests that it would have been captured by the gravity of Proteus.