This boomerang moon has the most eccentric orbit of any moon in our solar system (Image: NASA)
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 – 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’s gravity, and that may be Nereid’s 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’s rogue path a mystery.
The answer could come from Nereid’s step-brother, Triton. This giant moon orbits Neptune in the opposite direction to Neptune’s own rotation, raising the possibility that it came from elsewhere and was captured by Neptune’s gravity (see “Living Snowballs”). That event could have thrown most of Neptune’s original clutch of moons completely out of the system, and sent Nereid on its wild ride.
Read more: Weird worlds: The solar system’s 10 strangest moons
