Most objects in the solar system rotate in the same direction, which is counterclockwise with respect to an observer looking at the solar system from its "north." But some objects, most notably the planet Venus and the dwarf planet Pluto, rotate clockwise, or retrograde. This presents a severe problem for the nebular hypothesis of the formation of the solar system. Astronomers have commonly "solved" this problem by postulating giant impacts on Venus and Pluto, and generally hold the latter impact responsible for Pluto's system of moons. But Venus has no moons, and thus any giant-impact theory begs the question of why Pluto should have three moons while Venus has none.