Neptune’s moon system is bizarre. Nearly half of its 16 known moons orbit backward relative to the planet’s spin, and one of these stands out as particularly odd: Triton, a large, icy world that is about 1,680 miles across, slightly smaller than Earth’s moon.
Elsewhere in the solar system, other large moons orbit forward, reflecting a likely origin story in which they coalesced from swirling dust and gas when their planets were still young. Because of Triton’s exceptional behavior, astronomers have long theorized that it didn’t form around Neptune at all. Instead, four billion years ago, it flew in like a wrecking ball, kicking out any pre-existing moons and sending them off into spiraling orbits around the sun, never to be seen again.
But astronomers have long wondered whether at least one of the planet’s current moons, the 210-mile-wide Nereid, might be the lone survivor of that event. This could explain Nereid’s unusually wide and stretched-out orbit, which has it barely clinging onto the planet’s gravitational pull.
A study published on Wednesday in the journal Science Advances provides new evidence for this hypothesis. A team led by Matthew Belyakov, a planetary scientist at the California Institute of Technology, used the James Webb Space Telescope to look at Nereid for just 10 minutes; it appears only as a dot of light to telescopes because Neptune is billions of miles from Earth. The team found Nereid’s reflected light looked quite different from objects in the Kuiper belt, where it might have come from if it was not born around Neptune: Nereid has a slightly bluer tinge than the red objects in the belt. “It just doesn’t quite look like everything else,” Dr. Belyakov said.
Next, the team used computer models to simulate the arrival of Triton in Neptune’s orbit, to see how Nereid could have survived. Initially, Triton would have been on a wide, looping course around the planet; if it stayed like that for too long, 100 million to a billion years, no moons would have escaped its influence. “You need Triton to get out of the way fast enough,” said Matija Ćuk, an astronomer at the SETI Institute in California who was not involved in the study.
This could have happened, the team’s models showed, if Triton was hit by one or two large objects in the first million years it was orbiting Neptune. “You just have to get lucky once or twice,” Dr. Belyakov said. That would have been enough to squash Triton’s orbit into a circle, leaving Nereid in its very eccentric — but crucially stable — orbit around Neptune that takes it millions of miles from the planet.
Benjamin Sharkey, an astronomer at the University of Maryland, said the findings could tell us more about the chaotic history of the early solar system — not just around Neptune but around Uranus, too, which is thought to have experienced its own catastrophic event early on when a giant impact knocked it on its side. “We don’t know what either Neptune or Uranus’s original systems looked like, and Nereid is a great example of this provocative mystery,” he said.
“Nereid is absolutely vital in understanding how satellites form,” Dr. Belyakov added.
While most of Neptune’s other moons would have been captured after Triton’s grand entrance, there might be some others that survived its arrival. In particular, a group of moons close to the planet could be fragments of the original moons that were broken apart in collisions. “They might also be the leftovers,” Dr. Ćuk said, “but they got reworked when Triton made a mess.”
To know the origin of Nereid for certain, we would need to send a spacecraft to Neptune to study it. With no mission to Neptune currently in development, that’s unlikely to happen anytime soon, but perhaps one day we might decide to go and take a closer look in search of answers.
“It’s a criminally understudied moon,” Dr. Belyakov said.
