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Neptune’s newly discovered moon may be the survivor of an ancient collision

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A tiny rock may have been part of a much bigger one

An enhanced color image of Neptune, taken by Voyager 2.
Image: NASA / JPL

A newly discovered small moon of Neptune is coming into clearer focus as astronomers have now pinpointed this tiny rock’s orbit and where it might have come from. The moon’s existence heightens the possibility that there are even more tiny worlds around Neptune that we just haven’t seen yet.

Astronomers first spotted this moon in 2013 by combing through images of Neptune that were taken by NASA’s Hubble Space Telescope. The discoverers have now dubbed the world Hippocamp, the name of a horse-like sea monster from Greek mythology. The title fits in nicely with the theme of Neptune’s 13 other moons, all of which are named after Greek gods of bodies of water.

Hippocamp is incredibly tiny for a moon: it’s just 21 miles across, or about the size of a major metropolitan city. Its minuscule size made this rock super difficult to study from Earth. But with the help of further observations from Hubble, astronomers were able to track this little moon over the last few years, detailing their work in a new paper in Nature. That allowed them to distinguish just how big it is as well as the exact path it takes around Neptune. “We’ve done a full analysis so we know precisely how this object moves,” Mark Showalter, a senior research scientist at the SETI Institute and lead author of the Nature paper who discovered Hippocamp, tells The Verge.

Before this discovery, many of Neptune’s inner moons had been found by NASA’s Voyager 2 spacecraft, which flew by the cold, gassy planet on its tour of the outer Solar System. Voyager snapped images of five new moons that were never before seen at the Neptune system. But the spacecraft ultimately missed Hippocamp, and now Showalter knows why. “We know the orbit now well enough to predict where it was in any of the Voyager images,” he says. “And it turns out there were a few close ones, but it was just never quite in the right place at right time for the Voyager cameras.”

An early image of Hippocamp taken in 2004 by the Hubble Space Telescope.
Image by Mark R. Showalter / SETI Institute

Finding Hippocamp was an unexpected discovery, too. Back in 2013, Showalter was poring over pictures from Hubble in order to study the weird features around Neptune known as arcs. The arcs are thin rings, like those around Saturn, that don’t connect all the way around Neptune. The problem with studying these rings is that they’re very faint and they move very quickly, which makes them hard to capture with cameras from Earth. Fixed, long-exposure images that bring in extra light ultimately blur the arcs out.

So Showalter came up with a neat trick where he basically combined multiple images from Hubble together and tweaked them to compensate for the movement. As a result, it was as if Hubble had been following the arcs during their motion. When he did this, he noticed a tiny dot kept popping up in all the images. After looking at it more closely, he realized it was a moon.

Since then, the goal has been to get a much better understanding of this little rock. Based on its orbit, Showalter and his team now have a pretty good idea of where this moon came from. Hippocamp’s orbit brings the moon very close to a much bigger moon of Neptune called Proteus, which is 130 miles across. And based on their analysis, Showalter believes that Hippocamp is probably a piece of Proteus that was broken off billions of years ago by a passing comet. “Now we see a very real example of what happens when a comet hits a moon,” he says. “In the case of Proteus, it doesn’t quite break it apart but breaks off a piece and there’s the Hippocamp we see today.”

Relative size comparisons of Neptune’s moons, including Hippocamp.
Image by Mark R. Showalter / SETI Institute

It’s suspected that the early Solar System was a very volatile place, with lots of comets zipping around the giant outer planets. It seems plausible that a comet collided with Proteus, chipping off Hippocamp.

This version of Hippocamp’s origin story is backed up by the moons’ positions relative to each other and Neptune. Hippocamp and Proteus aren’t in the same spot now because Proteus is very slowly spiraling away from Neptune. Proteus creates tides on Neptune, just like our Moon does on Earth, and this interaction causes the moon to move outward. So 4 billion years ago, after the birth of the Solar System, Proteus was probably about 10,000 kilometers closer to Neptune than it is now. “If you look at the system today and play it back 4 billion years, suddenly Hippocamp and Proteus are practically on top of each other,” says Showalter.

Based on this discovery, it seems possible that there are maybe even more tiny moons like Hippocamp. It’s possible that rocks have broken off of larger moons or pieces of debris from elsewhere in the Solar System. Showalter is hopeful that we may be able to find more someday, possibly with NASA’s future observatory, the James Webb Space Telescope. That telescope will be the most powerful one humanity has ever sent into space, and it may be able to do some great observations of both Neptune and Uranus. But to really know just how many moons Neptune has, Showalter says we need to send another spacecraft there — one that sticks around longer than Voyager did.

“I suspect when the day comes that NASA and the European Space Agency send a spacecraft out to Neptune and study the system much more closely, there will be quite a few more moons to discover,” says Showalter. “But this is probably the limit of what we can do from Earth.”