This Moon of Neptune May Have Survived a Wrecking-Ball Occasion

Neptune’s moon system is weird. Nearly half of its 16 recognized moons orbit backward relative to the planet’s spin, and one in every of these stands out as significantly odd: Triton, a big, icy world that’s about 1,680 miles throughout, barely smaller than Earth’s moon.

Elsewhere within the photo voltaic system, different massive moons orbit ahead, reflecting a probable origin story by which they coalesced from swirling mud and fuel when their planets had been nonetheless younger. Because of Triton’s distinctive conduct, astronomers have lengthy theorized that it didn’t type round Neptune in any respect. Instead, 4 billion years in the past, it flew in like a wrecking ball, kicking out any pre-existing moons and sending them off into spiraling orbits across the solar, by no means to be seen once more.

But astronomers have lengthy wondered whether or not a minimum of one of many planet’s present moons, the 210-mile-wide Nereid, could be the lone survivor of that occasion. This may clarify Nereid’s unusually huge and stretched-out orbit, which has it barely clinging onto the planet’s gravitational pull.

A research printed on Wednesday within the journal Science Advances offers new proof for this speculation. A staff led by Matthew Belyakov, a planetary scientist on the California Institute of Technology, used the James Webb Space Telescope to have a look at Nereid for simply 10 minutes; it seems solely as a dot of sunshine to telescopes as a result of Neptune is billions of miles from Earth. The staff discovered Nereid’s mirrored gentle regarded fairly completely different from objects within the Kuiper belt, the place it might need come from if it was not born round Neptune: Nereid has a barely bluer tinge than the pink objects within the belt. “It just doesn’t quite look like everything else,” Dr. Belyakov stated.

Next, the staff used pc fashions to simulate the arrival of Triton in Neptune’s orbit, to see how Nereid may have survived. Initially, Triton would have been on a large, looping course across the planet; if it stayed like that for too lengthy, 100 million to a billion years, no moons would have escaped its affect. “You need Triton to get out of the way fast enough,” stated Matija Ćuk, an astronomer on the SETI Institute in California who was not concerned within the research.

This may have occurred, the staff’s fashions confirmed, if Triton was hit by one or two massive objects within the first million years it was orbiting Neptune. “You just have to get lucky once or twice,” Dr. Belyakov stated. That would have been sufficient to squash Triton’s orbit right into a circle, leaving Nereid in its very eccentric — however crucially steady — orbit round Neptune that takes it thousands and thousands of miles from the planet.

Benjamin Sharkey, an astronomer on the University of Maryland, stated the findings may inform us extra in regards to the chaotic historical past of the early photo voltaic system — not simply round Neptune however round Uranus, too, which is assumed to have skilled its personal catastrophic occasion early on when an enormous impression knocked it on its facet. “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 stated.

“Nereid is absolutely vital in understanding how satellites form,” Dr. Belyakov added.

While most of Neptune’s different moons would have been captured after Triton’s grand entrance, there could be some others that survived its arrival. In specific, a bunch of moons near the planet might be fragments of the unique moons that had been damaged aside in collisions. “They might also be the leftovers,” Dr. Ćuk stated, “but they got reworked when Triton made a mess.”

To know the origin of Nereid for sure, we would want to ship a spacecraft to Neptune to review it. With no mission to Neptune at present in improvement, that’s unlikely to occur anytime quickly, however maybe in the future we would resolve to go and take a better look looking for solutions.

“It’s a criminally understudied moon,” Dr. Belyakov stated.