Neptune’s Moon Nereid Might Be the Sole Intact Survivor of an Ancient Lunar Collision

A study suggests that the ice giant’s largest moon, Triton, made a dramatic entrance to the Neptunian system long ago, kicking Nereid outward and destroying the planet’s other original lunar companions

May 21, 2026 4:16 p.m.

Image of an entirely blue planet — Neptune, which appears blue because of methane in its atmosphere, has a strange set of moons. NASA / JPL

Neptune, the farthest planet from the sun, is surrounded by 16 known moons—and many of them are weirdos.

Now, researchers propose that one of those oddballs, a wide-orbiting moon called Nereid, is the sole intact survivor of an ancient lunar collision. The findings, described May 20 in the journal Science Advances, could provide insight into the giant planet’s history, and thus, that of the early solar system.

The solar system’s eighth planet has a strange collection of moons compared to its fellow outer planets, Jupiter, Saturn and Uranus. Those three have more typical lunar companions, with several large satellites that orbit in line with their planet’s equators and smaller, farther moons with tilted orbits.

Neptune, on the other hand, has one strange, giant moon: Triton makes up more than 99 percent of the mass of all 16 known satellites. What’s more, Triton orbits Neptune in the opposite direction that the host planet circles the sun. That suggests the 1,680-mile-wide moon didn’t form from the leftover gas and dust from Neptune’s birth. Instead, researchers suspect Triton is an object from the Kuiper Belt—a region of icy objects beyond Neptune—that flew into Neptune’s gravitational trap.

Scientists previously thought that Nereid, the ice giant’s third-largest moon at about 210 miles wide, was also captured later in Neptune’s life because of its wide elliptical orbit. However, it’s bigger, brighter and has a different path than the host planet’s other far-orbiting moons suspected of coming from the Kuiper Belt.

“Nereid always is an outlier,” says study co-author Matthew Belyakov, a planetary scientist at Caltech, to Science News’ Lisa Grossman.

Fun fact: Nereid’s discovery

This moon was identified in 1949 by astronomer Gerard P. Kuiper with a ground-based telescope. It was the last Neptunian moon to be discovered prior to Voyager 2’s flyby. The satellite was named after the Nereids, sea nymphs in Greek mythology.

So, he and his colleagues observed infrared light reflected by Nereid using the James Webb Space Telescope, which stared at the moon for about ten minutes in November 2024. The light signature looked distinct from that of Kuiper Belt objects, and it resembled patterns seen from Uranus’ moons, hinting that Nereid probably wasn’t from the icy, rocky region.

The team then ran computer simulations of the evolution of Neptune’s moons. The analysis revealed that if Triton flew into the Neptunian system—and stayed there—its activity could have flung pre-existing moons into eccentric orbits that matched Nereid’s path.

“In the cases where Triton survives, rather than get destroyed or kicked into Neptune, around 25 percent of the time one or more moons can survive the Triton encounter on distant orbits,” Belyakov tells CNN’s Jacopo Prisco. That’s more favorable than the calculated likelihood of Nereid being captured, he adds.

The findings hint that within the solar system’s first 200 million years, Triton’s dramatic entrance into Neptune’s orbit may have destroyed the planet’s original moons, save Nereid, which got kicked outward into its present-day path.

Triton’s capture “subsequently caused havoc, gravitationally scattering Neptune’s original moons hither and thither but mostly out of Neptune orbit,” Carolyn Porco, a planetary scientist who worked on NASA’s Cassini, Voyager and New Horizons missions and was not involved in the study, tells CNN. She describes the recent research as a lovely and simple study of the Neptunian moon system’s history.

Additionally, the planet’s current innermost moons might be broken “leftovers” of the original lunar companions, “but they got reworked when Triton made a mess,” says Matija Ćuk, an astronomer at the SETI Institute who was not involved in the study, to the New York Times’ Jonathan O’Callaghan.

Nereid is still a mysterious place, and Belyakov describes it as “criminally understudied” to the Times. Our best images of the faraway moon are just a few pixels across and were snapped by the Voyager 2 spacecraft when it was about 2.9 million miles from the celestial body in 1989.

a fuzzy white object on a black background — An image of Nereid captured by the Voyager 2 spacecraft NASA / JPL

“Understanding what transpired at Neptune is one of the ways that we can solve what happened in the early solar system, and Nereid is important for pinning down key events like Triton’s capture,” Belyakov says in a statement. “We’re hoping this work motivates people to do creative observations of Nereid, even though it is faint and distant.”

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