New research suggests that Nereid, an unusual moon of Neptune, could be the sole survivor of the satellites that formed with the planet. Triton, shown here beneath a crescent Neptune in Voyager 2's parting view, may have disrupted Neptune's original moon system. Or another Triton-like object might have done so even earlier on. NASA / JPL The giant outer planets of the solar system host a ragtag bunch of moons, some large and round, some small and misshapen. Generally, round moons are the native satellites that formed alongside their planets, orbiting in the same direction the planet rotates. Others, called irregular moons , travel on wider, more elliptical paths, often in the opposite direction from their planet’s spin. Astronomers think these are captured objects formed elsewhere and were ensnared by the gravity of the gas and ice giants early in the solar system’s history. Neptune, however, doesn’t quite fit the pattern. Nearly half of its 16 known moons travel backwards around the planet.…