A City-Size Moon Was Hiding Around Neptune

Even to the most powerful telescopes, little Hippocamp appears only as a fuzzy dot in photos.

A blue Neptune with white clouds

The planets may get top billing in the solar system, but they are far outnumbered by the moons. There are hundreds, and they come in all kinds of varieties, like a cosmic tray of assorted chocolates. Our own moon is a barren, rocky world coated with craters. Enceladus, of Saturn, and Europa, of Jupiter, are frozen, shrouded in a thick layer of ice with a liquid-ocean center. Io, another moon of Jupiter, is molten, its surface constantly redrawn by flowing lava.

Astronomers have observed these and other moons for centuries, first with homemade telescopes and now with million-dollar spacecraft. Many moons are well studied, their features documented in astounding detail. They feel as familiar as the planets they orbit. But there is still more to discover.

In 2013, astronomers found something new around Neptune, in a manner that would make Marie Kondo proud. Mark Showalter, a scientist at the SETI Institute, noticed a mysterious white dot in observations of Neptune taken by the Hubble Space Telescope, NASA’s best eyes on the solar system and beyond. To investigate, he dove into a cache of photographs, five years’ worth, examining each one to see if it had that spark of intrigue. The dot showed up time and time again. It was a moon.

Now the moon has a formal name: Hippocamp, after the mythological sea creatures that galloped alongside the Roman god Neptune.

Hippocamp is one of 14 moons orbiting Neptune. Showalter and his colleagues reported new observations of the moon, published Wednesday in Nature, that provide a better measure of its size and orbit. At 21 miles, or 34 kilometers, across, Hippocamp is about the size of a metropolitan city and the smallest of Neptune’s moons. It orbits close to its moon sibling Proteus, another Neptunian moon, and completes one trip around Neptune about every 23 hours.

The astronomers suspect that Hippocamp formed as some other moons in the solar system did, including ours—through a violent collision. The team believes that the moon is a chunk of Proteus that broke off after a comet struck.

So where does Hippocamp fit among the diverse assortment of moons in the solar system? That’s up to the imagination. The moon is too far from Earth to tell; scientists have said that Hippocamp is roughly 100 million times fainter than the faintest star that is visible to the naked eye. Even to Hubble, an extremely powerful instrument, the moon appears as a fuzzy dot in photographs.

The moon christening is another milestone in the centuries-long study of Neptune’s moons, which bear the names of mythological figures. The first and largest, Triton, was found less than three weeks after Neptune itself was discovered, in 1846. More than a century would pass until the next moon, Nereid, was detected, in 1949. The rest were discovered in the past 40 years or so, via powerful ground-based telescopes and the Voyager mission, the vaunted NASA program that launched two probes on a tour of the solar system’s farthest planes. Voyager 2 spotted six small moons orbiting close to Neptune in 1989. Hippocamp orbits there, too, but the spacecraft missed it.

Mark R. Showalter / SETI Institute

Other moons are likely lurking out there, waiting to be found. Astronomers certainly have a more intimate understanding of our cosmic home than they did centuries ago. But they are still finding new neighbors. Last summer, astronomers announced the addition of 10 new moons around Jupiter, bringing its total to 79, the most in the solar system. Like Hippocamp, the objects appear as tiny glints of light in telescope observations. The smallest was about half a mile wide. Their names are still being considered, and will be approved by the International Astronomical Union’s Minor Planet Center, the organization in charge of confirming such discoveries.

The search for new moons extends beyond the solar system, to other stars and planets. Moons are not exclusive to our own. The universe may be swimming with them. Astronomers think they may have already found their first exomoon, as they’re called, orbiting a planet about 8,000 light-years away from Earth.

At that distance, the moon doesn’t show up in telescope images at all; it is seen instead in the slight wobble of the planet it orbits, tugging at it as it goes. Its discoverers believe that it isn’t a rocky or icy world like the moons we’re used to, but a massive, swirling ball of gas about the size of Neptune. The menagerie of moons in our solar system is rich and wondrous, but it’s only a taste of what else is out there.