Assumption refuted: Neptune’s moon Nereid was previously considered a celestial body from the Kuiper Belt captured by the planet – just like most of its fellow moons. But now new data from the James Webb Telescope reveal: Nereid is probably the only original satellite of Neptune – the last representative of what were once several regular moons of this ice planet. They were ejected when Neptune captured the large moon Triton. Only Nereid survived, but it ended up in an extremely eccentric orbit, as astronomers report in Science Advances.
Neptune is the outermost and least explored planet in our solar system. Because of its great distance from Earth, many features of this ice planet are difficult to see with telescopes. Most of the data on Neptune comes from the flyby of the Voyager 2 space probe in 1989. They provided the first information on the magnetic field, the structure of the gas envelope and showed that Neptune also has extremely thin rings and moons.
Nereid: An exotic species even among the irregular moons
What’s unusual, however, is that “Neptune is the only giant planet that lacks a system of regular, intact satellites,” explain Matthew Belyakov from the California Institute of Technology and his colleagues. According to common assumptions, Neptune’s 16 moons did not form together with their planet, but were either captured later or formed from debris from early collisions. Even Triton, Neptune’s largest moon, is considered a captured Kuiper Belt object.
Astronomers previously assumed something similar for Nereid, the third largest satellite of the distant ice giant. The 340-kilometer-wide, ice-rich satellite orbits Neptune in a highly eccentric orbit that is unique in the entire solar system. “Even among irregular moons, Nereid is an outlier: it is the largest, most eccentric, and closest to its planet among them,” Belyakov and his team report. That’s why planetary researchers previously suspected that Nereid also originally came from the further out Kuiper Belt.
Spectral search for traces on the outer edge of our solar system
But new observation data from the James Webb Space Telescope now refutes this assumption. For their study, Belyakov and his team analyzed Nereid’s spectrum using Webb near-infrared data and compared it with the spectra of various Kuiper Belt objects and icy moons. “The data from the Webb telescope reveal three different spectral types of Kuiper Belt objects: water-rich, methanol-rich and CO2-rich celestial bodies,” report the researchers.
At first glance, the spectra of Neptune’s moon showed some similarities with those of water-rich Kuiper Belt objects: Nereid also shows the typical spectral signatures of water ice, while signatures of methanol and other organic compounds are largely missing. But Neptune’s moon differs significantly from the water-rich celestial bodies on the outer edge of the solar system in other features: its albedo is higher, the water ice spectral signatures are steeper and shifted further towards the bluish, as Belyakov and his team found.
Nereid did not come from the Kuiper Belt
This means that the moon of Neptune is most likely not a former representative of the Kuiper belt objects. Even taking into account the change in the lunar surface caused by later impacts, too many differences remain, the team explains. “This suggests that Nereid could not have formed together with the Kuiper Belt objects in the outer protoplanetary disk,” the astronomers said. Instead, Nereid’s spectrum suggests a more local origin. This contradicts the common capture scenario for this moon of Neptune.
But where does this satellite of Neptune come from? According to Belyakov and his team, two theoretical scenarios remain: On the one hand, Nereid could have come from the close environment of young Neptune and been captured by it. However, the parameters of Nereid’s orbit and the physical conditions that would be necessary for such a capture of a planetesimal speak against this, as the researchers explain.
The last original moon of Neptune
The second scenario, however, is more likely. According to this, Nereid emerged together with Neptune and is therefore a regular satellite of the ice planet. Initially, this moon orbited Neptune in a normal, nearly circular orbit. However, this changed when Neptune captured the 2,700 kilometer wide Triton and made it its satellite. The resulting gravitational turbulence steered Nereid out of its orbit and brought it into its current, highly eccentric orbit, as the team reconstructed using a model.
“The simulated evolution of this system over millions of years demonstrates that Triton can disrupt original, regular satellites of Neptune in such a way that they subsequently trace the orbital axis and periapsis of Nereid with an accuracy of ten percent,” write Belyakov and his colleagues. Other regular moons that were initially present were thrown completely out of the system by the Triton capture.
“According to our origin story, Nereid is the only regular moon of Neptune that is still intact,” the researchers state. All other satellites were either ejected or destroyed. “Today’s inner moons of Neptune, including Proteus, were formed from the debris of these destroyed satellites,” the team continued.
Source: Matthew Belyakov (California Institute of Technology) et al., Science Advances, 2026; doi: 10.1126/sciadv.aeb1429