Uranus May Have Had an Extra Moon

Gwenaël Boué and Jacques Laskar of France's Paris Observatory believe that Uranus' unusual tilt may have been caused by a moon that no longer exists.

Unlike the other planets, whose axes are perpendicular to the plane of the Solar System, Uranus' axis is almost parallel to the plane of the Solar System.

Some astronomers think that Uranus once collided with an object about the size of the Earth, and that this collision caused Uranus to become tilted in this way. However, this hypothesis does not explain why Uranus' moons are not similarly tilted, as Ignacio Mosqueira of California's SETI Institute has explained.

Boué and Laskar argue that an extra moon about the size of the Earth could have caused Uranus to wobble and eventually, after two million years, cause Uranus to tip over on its side.

Later on, the gravitational pull of a passing planet could have pulled this moon away from its orbit around Uranus.

William Ward of Colorado's Southwest Research Institute points out that there is no other evidence for the existence of an extra moon around Uranus.