Neptune’s mysterious moon Nereid may be an original, study shows

CAPE CANAVERAL, Fla. (AP) — Neptune’s distant moon Nereid may be the last of the planet’s original companions to manage to survive a cosmic crash, scientists reported Wednesday.

Sixteen known moons surround Neptune, the eighth and most distant planet in our solar system. Neptune’s largest moon, Triton, erupted from the icy outskirts of the solar system billions of years ago, scattering the planet’s original moons and setting them on destructive collision courses.

A team led by the California Institute of Technology used NASA’s Webb Space Telescope to study Nereid. Their observations suggest that Nereid is not a party animal like Triton and likely survived by escaping into its extreme elliptical orbit around Neptune.

“What we know about Nereid is very limited. Given its size, Nereid is extremely understudied,” said study author Matthew Belyakov of Caltech.

Neptune has only been visited by a single spacecraft, NASA’s Voyager 2, in 1989. Nereid was discovered 40 years earlier by Dutch astronomer Gerard Kuiper, who named the moon after the sea nymphs of Greek mythology.

With a diameter of about 350 kilometers, Nereid has an extremely eccentric orbit for a moon. It takes almost an entire Earth year for Nereid to orbit Neptune, with the Moon passing within a million miles (1.4 million kilometers) of the icy giant planet at one end of its egg-shaped loop and up to 6 million miles (9.6 million kilometers) at the other end.

Like so many other moons in the outer solar system, Nereid has long been suspected of having migrated to Neptune’s neighborhood from the icy expanse known as the Kuiper Belt. But using the Webb telescope, scientists determined that Nereid’s composition was not consistent with that of Kuiper Belt objects: it contained too much ice. This suggests that it has always been part of the Neptune system.

“We don’t have much evidence around Neptune anymore — the system doesn’t have many moons anymore,” Belyakov said in an email. But the latest observations “strongly exclude” that Nereid wandered like so many others and was trapped by planetary gravity.

The results appear in the journal Science Advances.

It’s “an exciting result,” said Carnegie Science planetary astronomer Scott Sheppard, who was not involved in the study.

The observations show for the first time that Nereid’s particular orbit fits “the story we might expect from a moon that originally formed near Neptune and was then pushed outward after Triton was captured,” Sheppard said in an email.

Neptune’s innermost moons likely formed from the shattered remains of the originals that were Triton’s victims, according to Belyakov and his team.

The other three giant planets in the solar system have more moons, with Saturn leading the way with 292.

Scientists say a visiting spacecraft could confirm the Neptunian system’s origin story, although none is currently planned.

The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Department of Science Education and the Robert Wood Johnson Foundation. The AP is solely responsible for all content.