In 1989, the NASA spacecraft Voyager 2 detected six moons of Neptune that are interior to the orbit of the planet’s largest moon, Triton1. In a paper in Nature, Showalter et al.2 report the discovery of a seventh inner moon, Hippocamp. Originally designated as S/2004 N 1 and Neptune XIV, this moon was found in images taken by NASA’s Hubble Space Telescope in 2004–05 and 2009, and then confirmed in further images captured in 2016. Hippocamp is only 34 kilometres wide, which makes it diminutive compared with its larger siblings, and it orbits Neptune just inside the orbit of Proteus — the planet’s second largest moon.
Discoveries in the past few years of moons and other objects that are associated with the outer planets of the Solar System have been serendipitous. For example, in 2012, astronomers found one of Neptune’s trojans3 — bodies that share the orbit of a planet or other celestial body — while looking for a target for the NASA spacecraft New Horizons after its flyby mission to Pluto. And, in 2018, planetary scientists reported the discovery of 12 moons of Jupiter (see go.nature.com/2ge8ekk), which were identified during a search for the elusive Planet X that is proposed to lurk in the outermost region of the Solar System4.
By contrast, Showalter and colleagues were actively hunting for moons to add to Neptune’s retinue. Showalter leads teams of researchers that specialize in finding small moons, using images from both space probes and satellites in orbit around Earth, including Hubble. In 1991, Showalter reported the discovery of Pan5, a small moon of Saturn that orbits in a gap in one of the planet’s rings, by meticulously searching images taken nine years earlier by Voyager 2. Showalter’s sleuthing has also netted small moons of Uranus (Mab and Cupid6) and Pluto (Kerberos and Styx7–9), using images from Hubble.
Many observation campaigns have focused Hubble’s most powerful instruments on Neptune. Nevertheless, Hippocamp eluded detection until Showalter et al. implemented a specialized 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.
Any moon with a circular orbit that is roughly aligned with a planet’s equator and that moves in the same direction as the planet’s rotation will follow a predictable path around the planet. As a result, sequential images can be transformed to match each other’s appearance by relocating pixels in an original image to locations in subsequent images that coincide with the moon’s orbital motion. Stacking the transformed images extends exposure times from a few hundred seconds to tens of minutes, up to about 37 minutes. The application of a similar technique led to the discovery10 of 2014 MU69 — an object in the outer Solar System that was visited by New Horizons in January this year (see Nature http://doi.org/gfskgt; 2019).
Using the same procedure, Showalter and colleagues also observed Naiad — Neptune’s innermost moon, which had not been seen since it was spotted by Voyager 2 in 1989. Following a comprehensive search using their image transformation and stacking technique, the authors concluded that there are no further moons larger than 24 km in diameter that are interior to the orbit of Proteus. Beyond Proteus, 200,000 km away from Neptune, there are no moons wider than 20 km.
The discovery of Hippocamp is intriguing because of the moon’s relationship to Proteus and the role that both objects might have had in the history of Neptune’s inner system. Hippocamp, the smallest known inner moon of Neptune, orbits just 12,000 km inside the orbit of Proteus, the planet’s largest inner moon (Fig. 1). Both moons migrate outwards because of gravitational interactions with Neptune, but smaller Hippocamp moves much more slowly than Proteus. Therefore, Hippocamp resides nearer to the location at which it formed than does Proteus, which suggests that the two bodies were much closer together in the past.
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. Although Showalter et al. propose that some of this debris coalesced to form Hippocamp, they do not explore the possibility that some of the dust remains today in the form of a ring, such as the gossamer rings of Jupiter11 or the Phoebe ring of Saturn12.
Phoebe, an outer moon of Saturn, resides in — and is presumed to be the source of — the enormous dust ring that bears its name. Phoebe’s own large crater, Jason, offers evidence of the moon’s violent past, as does Pharos for Proteus. Intriguingly, although Showalter and colleagues note that the volume of Hippocamp is only 2% of that missing from the Pharos impact basin, the volume of particles in the Phoebe ring would fill a crater that is only 1 km wide12 (by comparison, Jason is about 100 km in diameter13). Clearly, the impacts that produced these revealing scars generated much more debris than remains today in the form of a dust ring or a tiny moon.
Whether Hippocamp formed in place from material that did not originate from Proteus or was born of Proteus remains to be determined. Nevertheless, applying the techniques that were used to find it might result in the detection of other small moons around giant planets, or even planets that orbit distant stars.
Nature 566, 328-329 (2019)