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Nereid (inset) may be the only moon of Neptune (background) that was around when the planet first formed, a new JWST study hints. (This composite image is not to scale).(Image credit: NASA/JPL)Share this article 0Join the conversationFollow usAdd us as a preferred source on GoogleSubscribe to our newsletter
One of Neptune’s most significant moons appears to have persisted in its orbit much longer than scientists had previously assumed — in fact, it might be the sole remnant from the planet’s earliest epochs.
Nereid, which ranks as Neptune’s third-largest satellite, is likely the only moon remaining from the time the ice giant coalesced early in the solar system’s 4.5 billion-year timeline, according to a recent examination of data from the James Webb Space Telescope (JWST).
These findings, unveiled on Wednesday, May 20, in the journal Science Advances, challenge the established narrative regarding Nereid’s past, which has been recounted since its discovery in 1949. The prior understanding was that Neptune “intercepted” this 220-mile-wide (350 kilometers) moon from a reservoir of icy bodies in the outer solar system known as the Kuiper Belt.
The observations from JWST were instrumental in this revised understanding, stated lead author Matthew Belyakov, a graduate student in planetary science at Caltech. “What JWST achieved for Nereid was to confirm its substantial water ice content and provide the overall profile of its light spectrum,” Belyakov informed Live Science during a phone conversation.
The telescope’s examination of the moon uncovered a composition distinctly different from that of known Kuiper Belt objects, he elaborated. Significantly, JWST has been operational long enough to enable direct comparisons with Kuiper Belt celestial bodies that the telescope itself has surveyed. “We are capable of comparing like with like,” Belyakov added.
The moon-stealing planet
Previously, the scientific community presumed that Nereid was “captured” by Neptune’s gravitational pull, similar to how the planet’s other known moons are believed to have been. This assumption was based on Nereid’s orbit, which is highly eccentric — suggesting the moon was acquired from a prior, more stable trajectory within the Kuiper Belt.
However, substantiating this hypothesis presents difficulties, considering Neptune possesses 16 known moons. The majority of these moons are diminutive, with the exception of Triton, which is slightly smaller than Earth’s moon. The system predominantly consists of small moons occupying irregular orbits.
“The challenge with Neptune is that we essentially have no regular satellites,” Belyakov stated.
Triton, Neptune’s most massive moon, constitutes 99% of the total mass within Neptune’s moon system and follows a “retrograde,” or backward, orbital path, Belyakov further explained. Triton is unequivocally an object that was captured — not solely due to its unusual orbit but also because observational data indicates its composition is more akin to Pluto’s than Neptune’s, he noted.
This JWST image of Neptune shows the planet’s rings and several of its 16 moons. The third-largest moon Nereid (not pictured) may be the only one leftover from the planet’s formation.
(Image credit: NASA, ESA, CSA, STScI; Image Processing: Joseph DePasquale (STScI), Naomi Rowe-Gurney (NASA-GSFC))
In the latest research, the authors conducted simulations modeling Triton’s capture from the Kuiper Belt and its potential impact on Neptune’s pre-existing moons. The simulations suggest that Triton’s arrival imparted a significant disturbance to the original Neptune moons. “Triton is captured and reorganizes the initial system, giving rise to an object similar to Nereid,” Belyakov clarified.
Moon mysteries
To further validate this hypothesis, Belyakov aims to obtain higher-resolution imagery of Nereid. JWST is in high demand among astronomers, and Belyakov’s team, which includes the renowned moon dynamicist and Caltech professor Konstantin Batygin, secured a brief observation window using the lowest resolution setting of JWST’s Near-Infrared Spectrograph (NIRSpec). The team intends to apply for access to a higher-resolution mode of NIRSpec in a future observation cycle, he indicated.
These observations carry implications not only for Neptune but also for the study of exoplanets and gas giants, Belyakov stressed.
“On a larger scale, the truth is we still lack a complete understanding of how moons form around Uranus and Neptune,” Belyakov remarked. “What we are generally discovering in the field of exoplanets is that planets comparable in size to Uranus and Neptune are exceedingly common. Therefore, if we do not grasp the formation process of moons around these bodies, it represents a substantial challenge.”
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The orbital histories of Uranus and Neptune add complexity to this investigation, he observed. Uranus is oddly canted on its side, possibly due to a significant collision with another celestial body in antiquity, meaning its initial generation of moons likely no longer exists.
“Similarly, at Neptune, we are also missing that primordial generation of moons,” Belyakov stated.The planet’s innermost moons today were reconstituted from the original moon population that fragmented following Triton’s arrival, the researchers noted.
“Thus, it’s conceivable,” he continued, “that Nereid represents the sole original, largely intact, relic of satellites that initially formed around these planets. That would constitute a highly significant discovery, as it would offer us a singular opportunity to examine and comprehend these satellites.”
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