In 2013, astronomer Mark Showalter of the SETI Institute was going over Neptune photos snapped by the Hubble when he noticed a white blob about 65,400 miles from Neptune in one of them. This same blob appeared in multiple images taken by Hubble between 2004 to 2009. Showalter had just discovered a new moon orbiting Neptune.
But the tiny moon, just 20 miles across, shouldn’t exist. Named Hippocamp, it sits extremely close to another one of Neptune’s moons called Proteus. The latter measures 260 miles across and has enough gravitational pull that it should have pushed Hippocamp aside or swallowed it up.
Six years later and a team of planetary scientists believe they know what’s going on. Hippocamp is most likely a chunk of Proteus that broke away after a comet slammed into Proteus billions of years ago.
“The first thing we realized was that you wouldn’t expect to find such a tiny moon right next to Neptune’s biggest inner moon,” said Showalter. “In the distant past, given the slow migration outward of the larger moon, Proteus was once where Hippocamp is now.”
Images of Proteus from Voyager 2 give planetary scientists “smoking-gun evidence” for this scenario. When the Voyager 2 was making its trek through the outer solar system, it snapped a picture of a large crater that was almost large enough to break the moon apart.
“In 1989, we thought the crater was the end of the story,” says Showalter. “With Hubble, now we know that a little piece of Proteus got left behind and we see it today as Hippocamp.”
Hippocamp could be considered a third-generation Neptune moon. Triton, the largest moon, was captured by Neptune billions of years ago according to NASA – and is believed to have wreaked havoc on Neptune’s original moon system.
Triton settled into a circular orbit and the debris from shattered Neptunian moons re-coalesced into a second generation of natural satellites.
Then comets came in and caused more headaches for Neptune’s moon system leading to the formation of Hippocamp.
As for the tiny moon’s name? The International Astronomical Union has rules for naming features throughout the solar system from craters to moons. For small satellites of Neptune:
Gods and goddesses associated with Neptune/Poseidon mythology or generic mythological aquatic beings
Hippocamp is from Greek mythology and is a half-horse half-fish.