Neptune's current inner moons eventually formed from some of the resultant debris. Triton is the solar system's only large moon that orbits in the opposite direction of its planet's spin. Advertisement Advertisement"Neptune is the only giant planet in the solar system without an ordered system of large regular satellites," Davis said. Nereid, one of Neptune's outermost moons, displays among the most elliptical orbits of any moon in the solar system. The story of Neptune's moons, Davis said, is a reminder that the solar system has a violent history.