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

This image released by NASA shows the Voyager view of Nereid, a satellite of Neptune, obtained on Aug. 24, 1989.

CAPE CANAVERAL, Fla. — Neptune’s far-flung moon Nereid may be the last of the planet’s original companions that managed to survive a cosmic crash, scientists reported Wednesday.

Sixteen known moons circle Neptune, our solar system’s eighth and most distant planet. Neptune’s biggest moon, Triton, barged in from the solar system’s frigid outskirts billions of years ago, scattering the planet’s original moons and putting 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 no party crasher like Triton and likely survived by escaping into its extreme, elliptical orbit around Neptune.

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

Neptune has only been visited by one 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 in Greek mythology.

Roughly 220 miles (350 kilometres) across, Nereid has an extremely eccentric orbit for a moon. It takes practically an entire Earth year for Nereid to orbit Neptune, with the moon passing less than 1 million miles (1.4 million kilometres) from the giant icy planet at one end of its egg-shaped loop and as far as 6 million miles (9.6 million kilometres) at the other end.

Like so many other moons in the outer solar system, Nereid was long suspected of migrating to Neptune’s neighborhood from the frigid outlying expanse known as the Kuiper Belt. But using the Webb telescope, scientists determined that Nereid’s composition was inconsistent with Kuiper Belt objects — it had too much ice. That suggests it was part of Neptune’s system all along.

“We don’t have all that much evidence left around Neptune — the system doesn’t have very many moons left,” Belyakov said in an email. But the latest observations “strongly rule out” that Nereid wandered by like so many others and got ensnared by planetary gravity.

The findings appear in the journal Science Advances.

This is “an exciting result,” said Carnegie Science planetary astronomer Scott Sheppard, who was not part of the study.

The observations show for the first time that Nereid’s peculiar orbit matches “the history we might expect from a moon that originally formed close to Neptune and was later pushed outward from the capture of Triton,” Sheppard said in an email.

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

All three of the solar system’s other giant planets have more moons, with Saturn topping the charts at 292.

A visiting spacecraft could clinch the Neptunian system’s origin story, according to scientists, although none are currently planned.

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Marcia Dunn, The Associated Press