A tiny moon has been found orbiting the gas giant Neptune.

A new study published in the journal Nature has introduced the world to ‘Hippocamp’ – an object just 33km in diameter orbiting Neptune.

The discovery was made using observations from the Hubble Space Telescope by scientists at the at SETI Institute.

Hippocamp is unusually close to a much larger Neptunian moon called Proteus.

Normally, a moon like Proteus should have gravitationally swept aside or swallowed the smaller moon while clearing out its orbital path.

The experts say Hippocamp is likely to have been chipped-off from the larger moon after a collision with a comet billions of years ago.

“The first thing we realised was that you wouldn't expect to find such a tiny moon right next to Neptune's biggest inner moon,” said Mark Showalter of the SETI Institute in Mountain View, California.

“In the distant past, given the slow migration outward of the larger moon, Proteus was once where Hippocamp is now.”

This scenario is supported by Voyager 2 images from 1989 that show a large impact crater on Proteus, almost large enough to have shattered the moon.

“In 1989, we thought the crater was the end of the story,” Showalter said.

“With Hubble, now we know that a little piece of Proteus got left behind and we see it today as Hippocamp.”

Neptune's satellite system has a violent and tortured history.

Many billions of years ago, Neptune captured the large moon Triton from the Kuiper Belt, a large region of icy and rocky objects beyond the orbit of Neptune.

Triton's gravity would have torn up Neptune's original satellite system, leaving it in a circular orbit before the debris from shattered Neptunian moons re-coalesced into a second generation of natural satellites.

However, comet bombardment has continued to tear things up, leading to the birth of Hippocamp, which might be considered a third-generation satellite.

Hippocamp is a half-horse half-fish from Greek mythology. The scientific name for the seahorse is Hippocampus, also the name of an important part of the human brain.

The rules of the International Astronomical Union require that the moons of Neptune are named after Greek and Roman mythology of the undersea world.