When NASA’s Voyager 2 visited Neptune in 1989, it discovered six moons closer to the planet than its largest moon, Triton. Now, astronomers can add a new one to the roster, pushing the total number of moons around the ice giant to 14.
The latest addition is called Hippocamp, after the mythological seahorse, and it is the smallest moon of the system at only 34 kilometers (21 miles) across. It was first seen in 2013 using archival images from the Hubble Space Telescope that were snapped from 2004 onwards. It was officially designated as the 14th moon of Neptune on September 2018.
The long delay between the first images and confirmation had to do with the sophisticated imaging technique necessary to study such a diminutive moon. After all, the astronomers were studying an object not much larger than a city at a distance of 4.6 billion kilometers (2.9 billion miles). The team that made the discovery, led by Mark Showalter from the SETI Institute, have devoted three Hubble observation rounds to study Hippocamp and the other internal moons of Neptune.
As reported in their Nature paper, the team believes that Hippocamp provides further evidence that the internal moons have been shaped and possibly formed by a series of impacts. They believe that Hippocamp might have been a fragment of Proteus – the second-largest moon in the whole system – and was released after a cometary impact.
The surface of Proteus is heavily cratered and irregular with a 150-kilometer (93-mile) crater, called Pharos. Hippocamp would account for only 2 percent of the mass ejected by the impact that formed the Pharos crater, so more material could be hidden in a ring or a yet-to-be-discovered moon. Hippocamp’s orbit lies 105,000 kilometers (65,200 miles) from Neptune but only 12,000 kilometers (7,500 miles) from Proteus, a fact that strengthens the relationship between the two objects.
The seven internal moons would be, under this scenario, younger than Neptune. It is believed that Neptune had a system of moons when it formed, but then it captured Triton, which was at the time a dwarf planet-like body. The entry of such a massive object in the system caused havoc, destroying the original moons and forming a disk of rubble from which the other moons emerged.
Hippocamp is the first moon to be discovered around Neptune since Psamathe in 2003.