Neptune’s distant moon, Nereid, may be the last remaining moon from Neptune’s original system, surviving after a major cosmic collision. Scientists made this announcement on Wednesday. As the eighth and farthest planet from the sun, Neptune has 16 known moons. Triton, its largest moon, is believed to have arrived from the cold outer regions of the solar system billions of years ago, disrupting the planet’s initial moons and leading them into destructive paths.
Researchers from the California Institute of Technology utilized NASA’s Webb Space Telescope to study Nereid. Their findings indicate that, unlike Triton, Nereid avoided collision by settling into a highly elliptical orbit around Neptune. “Our knowledge about Nereid is quite limited. For its size, Nereid is significantly under-researched,” stated Matthew Belyakov, a study author from Caltech. NASA’s Voyager 2, which visited Neptune in 1989, is the only spacecraft to have approached the planet. Nereid was discovered much earlier, in 1949, by Dutch astronomer Gerard Kuiper, who named it after the sea nymphs from Greek mythology.
Measuring about 220 miles (350 kilometers) in diameter, Nereid has an extraordinarily eccentric orbit for a moon. It takes nearly a full Earth year to complete an orbit around Neptune, ranging from less than 1 million miles (1.4 million kilometers) to as far as 6 million miles (9.6 million kilometers) in its elongated path.
Nereid was long thought to have migrated from the Kuiper Belt, a distant area of icy bodies. However, using the Webb telescope, scientists found that Nereid’s composition, containing excess ice, conflicted with typical Kuiper Belt objects. This indicates that it likely originated within Neptune’s system. “There’s not much evidence left around Neptune; its system has very few remaining moons,” Belyakov mentioned in an email. Recent observations strongly suggest Nereid didn’t simply wander near the planet and become trapped by its gravitational pull. These findings were published in the journal Science Advances.
Carnegie Science planetary astronomer Scott Sheppard, not involved in the study, called the results significant. He noted that for the first time, observations show Nereid’s unusual orbit aligns with a moon formed close to Neptune and pushed outward by Triton’s capture. According to Belyakov and his team, Neptune’s innermost moons likely formed from remnants of these original moons, destroyed by Triton’s intrusion. The other three gas giants in our solar system have more moons, with Saturn leading at 292.
A spacecraft mission could confirm the origins of Neptune’s moon system, though there are no current plans. The findings emphasize the need for further exploration to understand Neptune’s complex system of moons.
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