Meet the newest known moon of Neptune

The wee world, named after a mythical sea creature, has a surprisingly brutal past.

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In 1989, Voyager 2 flew by Neptune, right, and spotted six small inner moons, composite at left, but the tiny Hippocamp went undetected—until now. Mark Showalter discovered the tiny moon using the Hubble telescope.Each moon is probably younger than Neptune fragmented by comet impacts. Composite (not to scale)

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 journalNature. 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...

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