A miniature moon that whizzes around Neptune at breakneck speed has been tracked by astronomers working from the US.

The speck of a moon, no more than 21 miles across, hurtles around the distant gas giant at about 20,000 miles an hour, 10 times faster than our own moon circles Earth, scientists said.

“This is the smallest known moon around the farthest known planet in the solar system,” said Mark Showalter, a senior research scientist at the Seti Institute, in Mountain View, California.

The scientists first saw the moon in enhanced images from the Hubble space telescope, which showed the tiny body 65,000 miles away from Neptune. It orbits the planet once every 22 hours.

The astronomers began the work that led to the discovery in 2004 but required further images, which were taken in 2009 and 2016, to confirm its presence and the nature of its orbit.

The moon is too small and too distant to be seen in individual images taken by the Hubble telescope. But it showed up when Showalter and his colleagues stacked a sequence of images on top of each other. For the process to work the scientists had to calculate how fast any moon would be moving and shift the pixels in each image to take account of the motion.

“We could take eight images with five-minute exposures and turn them into one image with a 40-minute exposure,” Showalter said. “You can’t normally do that because the moons would smear out, but we essentially took the orbital motion out of the images. And when we did that this extra dot showed up. We realised we were looking at a tiny little moon.”

Under guidelines set down by the International Astronomical Union, Neptunian moons must be named after sea creatures and gods from Greek and Roman mythology. Since Showalter is a keen scuba diver and a fan of seahorses, he named the moon Hippocamp, a nod to the seahorse genus, and the name of a horse-fish sea monster often depicted pulling Neptune’s chariot. “It’s always gratifying when you can find a piece of real estate in the solar system and get to name it,” he said.

In 1989 Nasa’s Voyager 2 probe shot past Neptune and photographed six inner moons orbiting the planet in the same direction as it spins. All are thought to have formed around the same time as the planet itself. Seven more moons, which may have been captured by Neptune’s gravity after it formed, lie further out.

Hippocamp circles just inside the orbit of Proteus, the largest of Neptune’s six inner moons. Proteus’s 250-mile-wide body bears a huge crater from a violent collision deep in its past.

Writing in the journal Nature, Showalter and his colleagues speculate that Hippocamp formed from the material that was blasted into space when a comet or asteroid clattered into Proteus.

Showalter said Neptune was a planet shrouded in mystery. Beyond its moons– 14 have been found – the planet has a number of complete and partial rings, some of which appear to be fading away.

“It’s a good reason why Nasa and the European Space Agency need to get together and send a mission to Neptune,” said Showalter. “All we know from close-up studies of Neptune are from Voyager. There’s a great big hole in our knowledge of the outer solar system.”