Neptune’s far-flung moon Nereid may be the last surviving member of the planet’s original companion system, scientists reported Wednesday, based on new observations from NASA’s James Webb Space Telescope.
The research, led by the California Institute of Technology and published in Science Advances, focused on what the team says is a crucial mismatch between earlier expectations and what Webb measured. Scientists had long suspected that Nereid came from farther out in the solar system and was later pulled into Neptune’s orbit, but the Caltech-led team reported that Nereid’s characteristics instead point to it having been part of Neptune’s system all along.
The study explained that the Neptune system has only a limited number of moons that can be studied in detail. Neptune has 16 known moons, and its largest, Triton, arrived after Neptune formed, scattering the planet’s earlier moons and putting many of them on collision paths, according to the account described by the study team.
In the team’s interpretation, Nereid did not behave like a “party crasher” that was captured from elsewhere. Instead, the observations suggest that Nereid likely survived by moving onto Neptune’s extreme, elliptical orbit, allowing it to endure after Triton’s arrival reshaped the system.
Study author Matthew Belyakov, of Caltech, said in an email that “What we know about Nereid is very limited,” adding that “For its size, Nereid is extremely understudied.” He also said the team’s latest observations “strongly rule out” the idea that Nereid wandered in and was ensnared by Neptune’s gravity like many other outer-solar-system objects.
The team placed the renewed focus on Nereid’s orbital and physical properties. Nereid’s diameter is described as roughly 220 miles (350 kilometers), and the moon’s orbit is described as extremely eccentric—taking practically an entire Earth year to complete a trip around Neptune. Over that elongated loop, the moon passes less than 1 million miles (1.4 million kilometers) from Neptune at its closest point and as far as 6 million miles (9.6 million kilometers) at the farthest.
The study also described how the moon’s composition measured with Webb does not match the expected makeup of objects from the Kuiper Belt. The researchers said Nereid’s material appears to contain too much ice to be consistent with Kuiper Belt bodies, and they said that result supports the idea that Nereid belonged to Neptune’s system from the start.
Belyakov and colleagues said the disruption that Triton caused likely also explains why Neptune’s innermost moons formed from shattered remnants of the originals that Triton’s arrival disrupted. The account described Triton’s long-ago arrival as one of the major turning points for Neptune’s moon inventory.
Outside scientists who were not part of the study said the results fit with broader modeling of Neptune’s history. Scott Sheppard, of Carnegie Science, said the observations show, for the first time, 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,” in an email.
The study’s team also noted what remains beyond reach without direct exploration. Neptune has only been visited by one spacecraft, NASA’s Voyager 2 in 1989, and scientists said a visiting spacecraft could ultimately help clinch the origin story, though none are currently planned.
All three of the solar system’s other giant planets have more moons than Neptune, with Saturn topping the count at 292, according to the reporting. Scientists say the limited number of Neptune moons makes each new measurement more consequential for reconstructing the planet’s past.