Say Hello to Hippocamp! The New Moon Discovered at Neptune, Which Could Have Broken off from the Larger Moon Proteus

Moons have the coolest names, don’t they? Proteus, Titan, and Callisto. Phobos, Deimos, and Encephalitis. But not Io. That’s a stupid name for a moon. There’s only two ways to pronounce it and we still get it wrong. Anyway, now we have another cool one: Hippocamp!

Okay, maybe the new name isn’t that cool. It sounds like a summer camp for overweight artiodactyls. But whatever. It’s not every day our Solar System gets a new moon.

Hippocamp is tiny, only 34 km (20 miles) across. It’s only about 1/1000th the mass of Proteus, its enormous neighbour. So it’s one of Neptune’s smallest moons.

A team of astronomers have figured out where this tiny Neptunian moon came from. The team hails from NASA’s Ames Research Center, from the SETI Institute, and from the University of California, and they discovered the moon in archival data from the Hubble Space Telescope. They think it’s the direct result of a collision between another, larger moon, and a comet.

When originally discovered, Hippocamp was called S/2004 N 1. It was discovered in 2013 when Mark Showalter analyzed over 150 archival Neptune photographs taken by Hubble from 2004 to 2009. Image Credit: By NASA, ESA, and M. Showalter (SETI Institute) –, Public Domain,

The most interesting thing about this newly-discovered moon is not its name, but its origin. It appears to have broken off the much larger Neptunian moon Proteus. And it all may have been the result of a little drama involving a comet and a collision.

Hippocamp’s origin story starts a long time ago. Astronomers call it the ‘moon that shouldn’t be there,’ and that’s a strong hint at the twists and turns that led to its existence. It’s so small and so close to Proteus that Proteus should have devoured it or swept it aside. But that hasn’t happened.

“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,” said Mark Showalter of the SETI Institute in Mountain View, California, lead author of the paper outlining Hippocamp’s origins.

Hippocamp’s story starts in the early days of the Solar System, when the gas giants coalesced out of the disk of material around the Sun. There’s a lot of detail involved, but essentially after the gas giants formed, they migrated through the Solar System. They weren’t always in the positions we see them in now.

Neptune’s largest moon Triton photographed on August 25, 1989 by Voyager 2. It’s a captured Kuiper Belt Object, and its capture started a chain of events that led to Hippocamp, the little moon that shouldn’t be there. Credit: NASA

These migrations changed the gravitational situation in the Solar System, and at some point, it looks like Neptune captured its moon Triton from the Kuiper Belt. So Triton isn’t a moon that was accreted from rocky material left over from the inner Solar System. It’s a captured Kuiper Belt Object. Other moons, like Saturn’s Phoebe, may also be captured Kuiper Belt Objects.

Triton is so large that its gravity would have torn up Neptune’s original system of moons. Then once Triton settled down into an orbit, moons like Proteus coalesced out of the debris. So Proteus is like a second-generation moon. And Proteus spawned Hippocamp.

Proteus wasn’t discovered until 1989, when Voyager 2 passed by Neptune and discovered it. It took pictures of Proteus and found a massive impact crater on the surface of the moon. And that’s where the new tiny moon Hippocamp first appears in our story.

“In 1989, we thought the crater was the end of the story,” said Showalter.

A Voyager2 image of Proteus. It’s not a great image, because Voyager2’s cameras were designed for more brightly-lit objects, but a large crater is visible on the upper right, partly in shadow. Image Credit: NASA/Voyager2.

Hippocamp wasn’t discovered until 2013, when Hubble’s keen eye spotted it. “With Hubble, now we know that a little piece of Proteus got left behind and we see it today as Hippocamp,” said Showalter. The orbits of the two moons are now 7,500 miles (about 12,070 kilometers) apart.

The team of astronomers behind the new paper explaining Hippocamp’s origin say that a long time ago, a comet collided with the moon Proteus. The collision created the big impact crater, and Hippocamp, the little moon that shouldn’t be there. So from Triton being captured, to the destruction then creation of Proteus, to the impact that created Hippocamp, the tiny moon is like a third-generation moon.

“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,” noted Jack Lissauer of NASA’s Ames Research Center in California’s Silicon Valley, a coauthor on the new research. “This pair of satellites provides a dramatic illustration that moons are sometimes broken apart by comets.”

This diagram shows the orbits of several moons located close to the planet Neptune. All of them were discovered in 1989 by NASA’s Voyager 2 spacecraft, with the exception of Hippocamp (S/2004 N 1), which was discovered in archival Hubble Space Telescope images taken from 2004 to 2009. The moons all follow prograde orbits and are nestled among Neptune’s rings (not shown). The outer moon Triton was discovered in 1846 – the same year the planet itself was discovered. Triton’s orbit is retrograde, suggesting it is a captured Kuiper Belt object and therefore a distant cousin of Pluto. The inner moons may have formed after Triton’s capture several billion years ago. Credit: NASA, ESA, and A. Feild (STScI)

Hippocamp’s origin story is full of twists and turns, of comet collisions, planet migrations, and captured Kuiper Belt Objects. For an object with such a fascinating history, it has a pretty lame name. But there’s a reason for it.

By agreement, Neptune’s moons are all named after Greek and Roman water gods, or beings and minor deities associated with water gods. That makes sense, because Neptune was God of the Sea. But it seems like we’re starting to scrape the bottom of the barrel with Hippocamp.

Maybe we should just give newly-discovered moons better names, and then start naming any new gods after them, to turn the tables on the whole thing?

But that’ll never happen; we’re stuck with Hippocamp, the moon that shouldn’t be there.

(And I was just joking about Encephalitis.)


Evan Gough

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