Neptune's moon Nereid might be the only satellite surviving from the planet's original system, researchers report in a new study.
This goes against the previous theory, which has long suggested that Nereid is actually an object captured from the Kuiper Belt, the ring of icy bodies beyond Neptune's orbit. In the early history of the outer solar system, it was chaos. Neptune's gravity is thought to have captured its largest moon, Triton, from the vast disk of the Kuiper Belt, which is home to countless small bodies, including a number of dwarf planets. This capture wreaked havoc on the Neptune system and, until now, it's been thought that the planet's third-largest moon, Nereid, was also an object captured from the Kuiper Belt. But a new paper begs to differ.
Uncovering a mystery
The Nereid-came-from-the-Kuiper-Belt theory prevailed for so many years because the moon has an irregular and eccentric orbit but is still an intact object, which is rare in moons that are original to the systems in which they orbit.
Original moons like this are so rare because, if an object like Triton is pulled into the system by gravity, the other objects orbiting that planet are typically destroyed or scattered by the event. So it was thought that Nereid couldn't be original, as it couldn't have survived such an encounter.
But the skepticism around the real origins of this moon began all the way back in 1949, when the moon was first discovered by astronomer Gerard Kuiper, the namesake of the Kuiper Belt.
In his discovery paper, Kuiper wrote, "There is some reason to hope that this object may become a clue to the unusual cosmogonic problem presented by the Neptune system, and as such is of more than routine interest."
To find the true nature of this moon, the team took a two-part approach: making observations with JWST and performing simulations of the dynamics of Neptune and its moons in the system's early years. With just over 10 minutes of observations with JWST, the team was able to collect enough data to show that the moon is very much unlike known Kuiper Belt objects (KBOs).
In comparing the moon to KBOs, they found that Nereid is much richer in water ice, is much brighter and reflective, and is also bluer than those objects. Also, volatile organics are common on KBOs, but the team did not find them on Nereid.
These observations with JWST highlighted just how different Nereid is from KBOs, making a fairly strong case that the object didn't come from the Kuiper Belt and is instead an original satellite of Neptune. But to add evidence to this theory, the team took things a step further, creating simulations of the Neptune system in its early years when Triton was first captured.
In these simulations, the team was able to show how Triton being captured into the Neptune system actually knocked Nereid into its current, highly eccentric orbit.
These new findings paint the history of this planetary system in a whole new light. Instead of a system of objects accumulated after Triton's chaotic entry, we have this incredible tale of Nereid's survival.
With this mystery beginning in 1949, today in 2026 we may finally have some answers.
"It takes a long time to do science," Belyakov said. JWST "has a finite lifetime," he added. "If we're not funding scientists to research during James Webb's lifetime, then maybe in decades to come we won't have the data that we would have had otherwise. So this study would not have been possible to any other point other than now."
This work was described in a study published May 20 in the journal Science Advances.
