For billions of years, a small moon orbiting the ice giant Neptune hid amid the dusky starlight. Now, the minuscule world has a name—and scientists are beginning to piece together its violent history.
“It was just incredibly difficult to detect,” says the SETI Institute’s Mark Showalter, who first spotted the moon in 2013 and describes it today in the journal Nature. The newly described satellite brings Neptune’s clutch of known orbital companions up to 14. A diver, Showalter named the tiny moon Hippocamp after the mythological beast that gave rise to the genus name of one of his favorite aquatic creatures: seahorses.
“When it came time to pick a name out of Greek and Roman mythologies from the seas, it was like, Oh, that’s not a hard one,” he says.
The hardest part was finding the small moon in the first place. Nestled relatively close to Neptune and just 21 miles wide, the rocky object is small enough to fit within the narrowest portion of the English Channel. Finding it took several years of careful scouring through pictures from the Hubble Space Telescope.
“You can only find these [moons] because folks like Mark Showalter have figured out how to wring every last photon out of those images,” says Heidi Hammelof the American Universities Research Association.
Not necessarily intending to go moon-hunting, Showalter happened upon the small world after developing a new way to search for faint structures around Neptune. He and his colleagues had aimed Hubble at the planet in 2004, 2005, and 2009 hoping to get a good look at the bizarre ring fragments arcing around the big blue planet. Because those arcs are so feeble, the team stacked long-exposure Hubble images atop one another, which amplifies the telescope’s sensitivity. (See how an Earth-bound telescope found a strange giant storm on Neptune.)
Then, Showalter decided to use the technique to look for moons.
This iconic Hubble image of the spiral galaxy NGC 1300 is suffused with detail—bright blue young stars, the dust lanes spiraling around the bright nucleus, distant galaxies shining through.
One by one, he spotted Neptune’s known inner moons Proteus, Thalassa, Galatea, Despina, Larissa, and Naiad, a moon that hadn’t been seen since the Voyager 2 spacecraft cruised by in 1989. Yet there, among those faraway worlds, was an interloper—a faint point of light behaving just as a moon would, except that no one had seen it before.
After tracking the spot’s motion, Showalter and his colleagues were satisfied that they’d stumbled upon an actual moon and not just a camera artifact; based on its brightness and on data from another observing campaign in 2016, they estimated its size. As for what it’s like?
“The truth is, it’s a dot,” Showalter says.
A history of cometary violence
More puzzling, perhaps, is Hippocamp’s orbit, which is jammed up against that of Proteus, the largest of Neptune’s inner moons. The two are so close—roughly 7,500 miles apart—that it’s unlikely Hippocamp randomly ended up there.
Instead, Showalter suspects that it formed after a comet slammed into Proteus and launched shrapnel into orbit around Neptune. Eventually, that debris clumped together into a new moon. If this scenario is true, Hippocamp’s birth is probably recorded in the form of a giant Protean crater called Pharos.
But that wasn’t its only experience with violent impacts. Since then, it’s possible that comets have broken Hippocamp apart multiple times, essentially putting it through a blender that jumbles the moon’s materials and turns it into a transient ring. (Find out why astronomers think Saturn’s famous rings may be surprisingly young.)
“It’s in a place around Neptune where you’re far enough away that when you break something apart, it’ll eventually form back into the moon,” Showalter says. “A lot of the inner moons of Jupiter, Saturn, Uranus, and Neptune have probably broken apart multiple times over their history.”
But telling the detailed stories of Hippocamp and other objects at the fringe of the easily observable solar system requires an instrument much sharper than Hubble, or a spacecraft sent into the deep.
“You can find them with Hubble,” Hammel says. “But you cannot take it to the step of trying to understand the detailed geology, the detailed chemistry of the objects out there at the edge of the solar system.”