Neptune's distant and mysterious moon Nereid may be the last surviving original companion of the planet, according to a new study published in the journal Science Advances. Researchers from the California Institute of Technology used NASA's Webb Space Telescope to analyze the moon's composition, finding evidence that it formed within Neptune's system rather than being captured from elsewhere.
Surviving a Cosmic Crash
Neptune has 16 known moons, but its largest, Triton, is believed to have been an interloper that barged in from the Kuiper Belt billions of years ago. This cosmic intrusion scattered Neptune's original moons, putting many on destructive collision courses. Nereid, however, appears to have survived by escaping into its extreme, elliptical orbit.
"What we know about Nereid is very limited. For its size, Nereid is extremely understudied," said study author Matthew Belyakov of Caltech. The moon was discovered in 1949 by Dutch astronomer Gerard Kuiper, who named it after the sea nymphs of Greek mythology.
Unusual Orbit and Composition
Nereid is roughly 220 miles (350 kilometers) across and has one of the most eccentric orbits of any moon. It takes almost an entire Earth year to orbit Neptune, coming as close as 1 million miles (1.4 million kilometers) and as far as 6 million miles (9.6 million kilometers) from the planet.
Scientists had long suspected Nereid might be a captured Kuiper Belt object, but the Webb telescope's observations revealed its composition contains too much ice to be consistent with such an origin. This strongly suggests Nereid formed near Neptune and was pushed outward when Triton was captured.
Implications for Neptune's System
"We don't have all that much evidence left around Neptune — the system doesn't have very many moons left," Belyakov noted. The new findings "strongly rule out" that Nereid wandered into Neptune's gravity like many other moons in the outer solar system.
Carnegie Science planetary astronomer Scott Sheppard, who was not involved in the study, called it "an exciting result." He explained that Nereid's peculiar orbit matches "the history we might expect from a moon that originally formed close to Neptune and was later pushed outward from the capture of Triton."
Neptune's innermost moons are likely fragments of the original moons that were destroyed by Triton's arrival. All three other giant planets in the solar system have more moons, with Saturn leading at 292.
Scientists say a dedicated spacecraft mission to Neptune could confirm this origin story, though none are currently planned.
