A new moon has been discovered orbiting Neptune.
At only 21 miles wide, it’s among the smallest in the solar system – explaining how it’s eluded astronomers.
Named Hippocamp after a sea creature in Greek mythology, it brings the total number of satellites around the gas giant to 14.
It’s believed to have been formed about 4 billion years ago during a cosmic collision with another Neptunian moon called “Proteus”.
Hippocamp is one of the seven ‘inner moons’ – six of which were discovered 30 years ago during Voyager 2’s fly-by.
They are probably younger than the planet itself – forming soon after the largest known as Triton.
Alien hunter Dr Mark Showalter, of the SETI (Search for Extra-Terrestrial Intelligence) Institute in Mountain View, California, said: “Hippocamp is smaller than the other six.
“It orbits close to Proteus – the outermost and largest of these moons. Proteus has migrated outwards because of tidal interactions with Neptune.
“Our results suggest Hippocamp is probably an ancient fragment of Proteus – providing further support for the hypothesis that the inner Neptune system has been shaped by numerous impacts.”
Dr Showalter, who has now discovered six moons and three planetary rings, thinks Hippocamp was born when fragments came off Proteus when it was hit by a big comet.
The moon had been hiding in plain sight. It had already been captured in previous images of its nearby planet taken by Hubble.
But astronomers’ technical abilities were not enough to actually spot it – and only now has the tiny world actually been noticed and catalogued.
Dr Showalter and collleagues used a new specialised image-processing technique that enhanced the sensitivity of Hubble’s cameras.
This method effectively increases exposure times beyond the limit imposed by image
smearing that is caused by a moon’s orbital motion.
It enabled the researchers to see the inner moons despite the rapid speed at which they hurtle around Neptune.
It could enable the discovery of other moons and ‘exoplanets’ outside the solar system – some of which could harbour life.
The study published in Nature also found Hippocamp has a very strange orbit – which could also shed light on how the huge planet formed.
Dr Showalter said: “The discovery of tiny Hippocamp contributes to our understanding of the history of Neptune’s inner system.
“It orbits just 12,000 km (7,500 miles) interior to Proteus – a body with 4,000 times its volume.”
This mean’s Neptune’s smallest moon is close to its second largest. Proteus is the outermost and biggest of the inner moons with a diameter of 261 miles (420km).
Dr Showalter said: “Proteus and Hippocamp were even closer in the past because Proteus is migrating outwards owing to tidal interactions with Neptune.
“Hippocamp – with its much lower mass – migrates very slowly and remains close to its point of origin. It is therefore worth exploring the possible connection between these moons.”
He added: “It may have formed from ejected fragments of the larger satellite after a large comet impact.”
The first picture that included Hippocamp was actually taken in 2004 but scientists couldn’t see it as it was moving so quickly.
Astronomer Anne Verbiscer, of Virginia University who was not involved in the study, said: “Hippocamp, a previously undetected moon of Neptune, has a peculiar location and a tiny size relative to the planet’s other inner moons, which suggests a violent history for the region within 100,000 kilometres of the planet.
“Proteus sports an unusually large crater called Pharos- a telltale sign that the moon might have barely escaped destruction by impact.
“Whenever this impact occurred, it no doubt launched debris into orbit around Neptune.”
One of those splintered pieces of debris could have become the new moon.
In Greek mythology Hippocamp was a sea-horse – typically depicted as having the upper body of a horse with the lower body of a fish.