Download the Mach newsletter.
Vid David Freeman
Say hello to Hippocamp, a heavenly pipsqueak that makes great waves at the edge of our solar system. Using NASA's Hubble Space Telescope, astronomers have confirmed the existence of this little moon in orbit around Neptune.
With a diameter of just 20 miles, the newly-defined moon ̵
Hippocamp was first seen in images taken by Hubble in 2013. But its presence – located near a much larger moon called Proteus a mystery: Why hadn't Hippocamp been merged with or deposited by Proteus, an irregularly shaped 260-mile-long is about 1000 times more massive?
Some are called S / 2004 N1, which Hippocamp was originally known, "the moon that should not be there."
But after several years of slitting – including a new look at images of the Neptune system that Hubble and NASA's Voyager 2 spacecraft took, astronomers led by Mark Showalter from the SETI Institute in Mountain View, California, decided that Hippocamp is a piece of Proteus who had been cut off from the larger moon when a comet struck it billions of years ago.
There is no smoke gun for a collision so old. But as Showalter and his colleagues say in a paper about his research, published Thursday in the journal Nature, a large battleground conforming to a comet strike can be seen on pictures of Proteus taken in 1989 by NASA's spacecraft Voyager 2.
"The Origin Scenario We discussed in the magazine is consistent with everything we know about the history of the Neptune system, "Showalter told NBC News MACH in an email. "It's the only working scenario we've identified. If someone else in astronomical society comes up with an alternative explanation, we'd love to hear about it."
It is a great backstory for such a punk moon, but other astronomers agree that it is probably the right one.
Sara Seager, a planetary scientist at MIT in Cambridge, Massachusetts, said in an email that the explanation was sensible. She called it "a fascinating hypothesis of the little moon's origin – with solid support from the Hubble data gathered in recent years."
Hundreds of moons have been found in our solar system. But the Proteus-Hippocamp couple "gives a dramatic illustration that men are sometimes shattered by comets," said Jack Lissauer, a researcher at NASA's research center Ames in Mountain View, California, and a co-author
The discovery of Hippocamp gives 14 numbers of moons discovered in orbit around Neptune. There are seven inner moons, including Hippocamp, along with six outer moons and Neptune's largest moon, Triton.
Triton is a bit oddball because it is the only moon in the solar system with a so-called retrograde orbit, which means that it revolves in the opposite direction of its planet's rotation.
Showalter said we have learned almost everything we can about Hippocamp, at least for the moment. But he added that if NASA or the European Space Agency were to circulate spacecraft around Neptune or its planetary neighbors, Uranus, "we could learn more about a few days about the sum of everything we have ever learned about any planet so far!"
] Want more stories about space?
FOLLOW NBC NEWS MACH ON TWITTER FACEBOOK AND INSTAGRAM.