Nereid Could be Neptune's Only Original Moon

New research suggests that Nereid, an unusual moon of Neptune, could be the sole survivor of the satellites that formed with the planet... Nearly half of its 16 known moons travel backwards around the planet. One of them is not like the others: Like a swan chick among ducklings, Triton is six times larger than the system's next-largest moon. Nearly as big as Earth's Moon, Triton has long been considered a Pluto-like world, captured in Neptune's orbit when the planet was still young. Now, research suggests that Triton's arrival -- or perhaps even the arrival of some earlier, Triton-like object...