Neptune’s been hiding a secret: A tiny moon

A new moon was discovered orbiting Neptune, joining the other 13 we already knew about, scientists announced in a study Wednesday.

The moon, named Hippocamp after the sea creature of Greek mythology, is Neptune’s smallest moon, with a diameter of only 21 miles. Hippocamp orbits close to Proteus, the largest and outermost of the planets’ inner moons.

Compared with the planet’s other inner moons, Hippocamp is tiny, “which suggests a violent history for the region,” Anne Verbiscer, a University of Virginia astronomer, said in a companion article in Nature.

The moon may have formed from ejected fragments of the larger moon after it was hit by a comet billions of years ago, according to the study. This supports the idea that all of Neptune’s seven inner moons were formed and shaped by powerful collisions with comets.

“You wouldn’t expect to find such a tiny moon right next to Neptune’s biggest inner moon,” said study lead author Mark Showalter, a SETI astronomer. “In the distant past, given the slow migration outward of the larger moon, Proteus was once where Hippocamp is now.”

It’s so small that it wasn’t detected in 1989 during the Voyager 2 flyby of Neptune. Hippocamp was spotted by astronomers using the Hubble Space Telescope. Showalter and his colleagues discovered it using a special image-processing technique that made Hubble’s cameras extra-sensitive.

“Applying the techniques that were used to find (Hippocamp) might result in the detection of other small moons around giant planets, or even planets that orbit distant stars,” Verbiscer said.