CAPE CANAVERAL, Fla. (AP) 鈥� Neptune鈥檚 far-flung moon Nereid may be the last of the that managed to survive a cosmic crash, scientists reported Wednesday.
Sixteen known moons circle Neptune, our solar system鈥檚 eighth and most distant planet. Neptune鈥檚 biggest moon, Triton, barged in from the solar system鈥檚 frigid outskirts billions of years ago, scattering the planet鈥檚 original moons and putting them on destructive collision courses.
A team led by the California Institute of Technology used 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.
鈥淲hat we know about Nereid is very limited. For its size, Nereid is extremely understudied,鈥� said study author Matthew Belyakov, of Caltech.
Neptune has only been visited by one spacecraft, NASA鈥檚 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 kilometers) 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 kilometers) from the giant icy planet at one end of its egg-shaped loop and as far as 6 million miles (9.6 million kilometers) 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鈥檚 composition was inconsistent with Kuiper Belt objects 鈥� it had too much ice. That suggests it was part of Neptune’s system all along.
鈥淲e don鈥檛 have all that much evidence left around Neptune 鈥� the system doesn鈥檛 have very many moons left,鈥� Belyakov said in an email. But the latest observations 鈥渟trongly rule out鈥� that Nereid wandered by like so many others and got ensnared by planetary gravity.
The findings appear in the journal Science Advances.
This is 鈥渁n 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鈥檚 peculiar orbit matches 鈥渢he history we might expect from a moon that originally formed close to Neptune and was later pushed outward from the capture of Triton,鈥� 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鈥檚 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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