Neptune’s smallest moon Hippocamp may have been created by comets

We’re all made of star stuff, but some things in the universe are created by comets.

Neptune’s recently discovered and smallest moon, Hippocamp, has been confirmed and observed in detail by the Hubble Space Telescope according to new research published in Nature on Wednesday.

Named Hippocamp for the half-horse, half-fish creature from Greek mythology — all of Neptune’s moons are named for Greek and Roman mythological figures — it’s the smallest of the planet’s seven inner moons, with a diameter of approximately 20-21 miles (34 kilometres). 

How have we never met Hippocamp before? The planet’s other six small inner moons were picked up in a 1989 fly-by from the Voyager 2 spacecraft, but Hippocamp was missed. 

Between 2004 and 2009, the Hubble picked up a “white dot” from 150 images, and in 2013, Mark Showalter of California’s SETI Institute officially discovered the moon by analyzing the photographs and plotting its circular orbit. 

Hippocamp was officially confirmed in the study published Wednesday by Showalter alongside Imke de Pater from the University of California, Berkeley, Jack Lissauer of NASA’s Ames Research Center, and R. S. French of SETI.

While there are three Hubble programmes dedicated to studying Neptune’s rings, arcs and small inner moons, the study’s authors had to develop their own specialised image processing techniques to focus on the inner satellites, including Hippocamp, because of their speedy orbits. 

With these new techniques, the team were able to confirm not only that Neptune officially has 14 moons, but how the smallest was likely formed.

Part of another moon?

Hippocamp sits in orbit near Proteus, the largest and outermost of Neptune’s moons. In fact, the study’s authors suggest Hippocamp could be derived from Proteus, as an ancient fragment of it. 

“The first thing we realized was that you wouldn’t expect to find such a tiny moon right next to Neptune’s biggest inner moon,” study author Showalter said on NASA’s blog. 

“In the distant past, given the slow migration outward of the larger moon, Proteus was once where Hippocamp is now.”

This diagram shows the orbits of several moons located close to the planet Neptune.

The inner moons are thought to be younger than Neptune, having formed after the capture (a successful pull into orbit) of Neptune’s largest moon, Triton. 

But according to the study, each inner moon has likely been fragmented by comet impacts, including Proteus, which sports the enormous Pharos crater thought to be unusually large in relation to the size of the moon, and possibly created by a comet.

Astronomers refer to it as “the moon that shouldn’t be there.”

“Based on estimates of comet populations, we know that other moons in the outer solar system have been hit by comets, smashed apart, and re-accreted multiple times,” said Lissauer. 

“This pair of satellites provides a dramatic illustration that moons are sometimes broken apart by comets.”

It’s this type of comet impact that the authors hypothesise could have released debris from the moon, which then settled into orbit and gradually accreted (formed) into Hippocamp. According to NASA, astronomers refer to it as “the moon that shouldn’t be there.”

A pretty violent way to be born, but there it is.