麻豆传媒

WHILE most moons gently circle their planets, Nereid swoops vertiginously. This otherwise undistinguished satellite of Neptune, moderately lumpy and middling in size, travels on the most eccentric orbit of any moon in the solar system 鈥� a roller-coaster ride that takes it soaring out more than 9 million kilometres from the planet, and then plunging back to within 1.4 million kilometres of it.

Most moons with irregular orbits are thought to be former comets or asteroids captured by their parent planet鈥檚 gravity, and that may be Nereid鈥檚 story too. But its composition does not resemble that of the other loose objects in the Kuiper belt, the area of the outer solar system that would most likely have been its original home. Instead, it probably formed from the disc of leftover material that once orbited Neptune. Such moons normally follow a circular orbit around their planet, however, leaving Nereid鈥檚 rogue path a mystery.

The answer could come from Nereid鈥檚 step-brother, Triton. This giant moon orbits Neptune in the opposite direction to Neptune鈥檚 own rotation, raising the possibility that it came from elsewhere and was captured by Neptune鈥檚 gravity (see 鈥淟iving Snowballs鈥�). That event could have thrown most of Neptune鈥檚 original clutch of moons completely out of the system, and sent Nereid on its wild ride.

Read more: Weird worlds: The solar system鈥檚 10 strangest moons