The moon’s size and orbit point to it being the remnant of a collision with Neptune’s moon Proteus.

Astronomers discovered a new moon orbiting Neptune: a 34-kilometer-wide chunk of rock they are calling Hippocamp. Hippocamp is now Neptune’s smallest moon and brings the planet’s total number of known satellites to 14.

“The discovery of Hippocamp provides a particularly dramatic illustration of the Neptune system’s battered history.” “Scientists have believed for quite a long time that the inner moons of Neptune have been broken apart multiple times by cometary impacts,” Mark Showalter, a senior research scientist at the SETI Institute in Mountain View, Calif., told Eos. “The discovery of Hippocamp, orbiting so close to the much larger moon Proteus, provides a particularly dramatic illustration of the Neptune system’s battered history.” Showalter is the lead author on the research paper announcing this discovery that published today in Nature.

Neptune’s moon Triton holds the vast majority of the mass that orbits the ice giant planet. Proteus is the next largest Neptunian satellite and the largest of the moons that orbit closer to Neptune than does Triton. Hippocamp is about 8% Proteus’s size, which itself is only 15% the size of Triton.

Past research suggests that Neptune’s inner moons and a nearby set of rings are younger than the planet. They likely formed in the chaotic time just after Neptune captured Triton and have fragmented multiple times during their history.

According to the researchers, Hippocamp’s size and orbital placement suggest that it may be an ancient fragment of Proteus. The team noted that Hippocamp’s volume is a small fraction of that of the debris estimated to have come from Proteus’s largest impact crater, Pharos, but they could not definitively say that Hippocamp formed from a Proteus impact.

Voyager 2 discovered six inner moons during its flyby of the Neptunian system in 1989 but missed Hippocamp because of poor image quality. Showalter and his team used the Hubble Space Telescope to tease out the satellite’s faint reflection over a 12-year period. On the basis of their observations, the researchers ruled out other undiscovered inner satellites larger than 24 kilometers in diameter.

“I would not be at all surprised if Neptune has smaller moons,” Showalter said, “but we will probably have to wait until NASA or [the European Space Agency] sends an orbiter to Neptune before we will know for sure.”

—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer

Correction, 20 February 2019: The article was updated to reflect Hippocamp’s volume.