Neptune's tiniest moon was likely once a part of its second biggest.
In 2013, scientists discovered a tiny moon orbiting Neptune. They gave it the forgettable name S/2004 N 1, but it soon earned the nickname of “the moon that shouldn’t be there,” because its size and placement didn’t make sense to astronomers.
Now, a team of planetary scientists has not only given Neptune’s 14th moon a more memorable moniker — Hippocamp — but also unraveled some of the mystery surrounding its origin.
Tiny, but Fast
In a paper published on Wednesday in the journal Nature, the researchers shared the results of several years of analysis about Neptune’s tiniest moon.
According to their research, Hippocamp is just 20 miles in diameter and a thousandth of the mass of Proteus, the much-larger moon next to it. It also zips around Neptune at a speed of about 20,000 miles an hour, which is 10 times faster than our Moon circles Earth.
The new name, meanwhile, refers to a mythological sea creature that’s half-horse and half-fish — fitting for a moon orbiting the planet named after the Roman god of the sea.
So that’s what the researchers learned about Hippocamp, but it doesn’t answer the question as to why “the moon that shouldn’t be there” is there.
The answer, according to the researchers, is that it probably wasn’t always.
If Hippocamp was around 4 billion years ago, Proteus would have likely destroyed the smaller moon while clearing its orbit around Neptune. This led the researchers to conclude that Hippocamp likely formed when a chunk of Proteus broke off following a collision with a comet or asteroid.
“This is the first really great example of a moon that got created as a result of an impact,” researcher Mark Showalter told Space.com.
READ MORE: Tiny Neptune Moon Spotted by Hubble May Have Broken From Larger Moon [SETI Institute]
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