Say Hi, The newest known moon of Neptune

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According to a study published in the journal Nature on Wednesday, for the first time in years, a new small moon was found in orbit around the gas giant Neptune.


Hippocamp is a tiny moon whose discovery required some innovation on the part of its observers.
Mark R. Showalter, SETI Institute

Astronomers, led by Mark Showalter of the SETI Institute, discovered the use of the Hubble Space Telescope in an orbital way combined with an innovative way to track dim and little objects.


Hippocamp, known as Neptune’s smallest moon, is named for the sea beast from Helenic mythology, said to have a fish’s tail attached to a horse’s body. It’s only about 21 miles in diameter; by compared to Neptune is 30,599 miles in diameter.

This latest discovery was made by Mark Showalter, a prior research scientist at the SETI Institute, and his team, utilize observations from the Hubble Space Telescope.

One by one, he mottled 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 far worlds, was an interloper—a faint point of light behaving just as a moon would, except that no one seen it before.

After tracking the spot’s motion, Showalter and his colleagues were pleased that they’d mistake 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.


Hippocamp is tiny compared to Neptune’s other moons.
Mark R. Showalter, SETI Institute

“Astronomers have suspected since the 1980s that the small, inner moons of the giant planets get broken apart sometimes by impacts from comets,” Showalter said in an email. “I think Hippocamp makes a dramatic illustration that this idea is correct, because it is hard for us to understand how Hippocamp can be where we find it today unless it is a piece of the larger moon Proteus that was broken off in an impact perhaps 4 billion years ago.”

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