The observation of Neptune’s smallest and most recently discovered moon, Hippocamp, is reported in a paper published this week in Nature. The paper details Hippocamp’s size and orbit pattern, and suggests insights into its formation.

The Voyager 2 spacecraft spotted six small inner moons orbiting Neptune when it flew by the planet in 1989. These inner moons are all thought to be younger than Neptune, having formed relatively soon after the capture of the planet’s largest moon, Triton. Each inner moon has probably been fragmented by comet impacts.

Mark Showalter and colleagues studied Neptune’s inner moons and rings using the Hubble Space Telescope, and found a seventh inner moon that went unobserved during the Voyager 2 flyby. This means Neptune has 14 moons in total. The discovery came through special image processing techniques, which enabled the authors to focus on Neptune’s inner satellites despite their rapid orbits.

The new moon - named Hippocamp, after the sea creature of Greek mythology - is Neptune’s smallest, with an average diameter of around 34 kilometres. Hippocamp orbits close to Proteus, the largest and outermost of the inner moons, and the authors propose that Hippocamp may have formed from ejected fragments of the larger satellite after a large comet impact. These findings provide further support for the idea that the development of Neptune’s inner moons has been shaped by numerous impacts, the authors conclude.