A massive moon that orbited Uranus in the past may explain the planet’s extreme tilt (Image: NASA/ESA/M. Showalter/SETI Institute)

Please try to resist the childish jokes, but the fact is that the odd tilt of Uranus may be the result of a particularly large moon.

Uranus spins on an axis almost parallel with the plane of the solar system, rather than perpendicular to it – though why it does this nobody knows. One theory is that the tilt is the result of a collision with an Earth-sized object, but this “hasn’t succeeded in explaining much of anything”, says Ignacio Mosqueira of the SETI Institute in Mountain View, California. Why, for example, are the orbits of Uranus’s 27 known moons not also tilted?

Now Gwenaël Boué and Jacques Laskar at the Paris Observatory in France have come up with another explanation: Uranus may once have had an unusually massive extra moon. If the moon had 1 per cent of the mass of Uranus – and orbited at a certain distance – it would slightly unbalance the planet and increase its wobble about its axis. After about 2 million years, the wobbling could have become exaggerated enough to tip the planet on its side, their model has shown (arxiv.org/abs/0912.0181).


The moon may since have been ejected by the tug of another planet passing nearby. Its fate is unclear, but it may have crashed into another gas giant if it is not still roaming the solar system.

William Ward at the Southwest Research Institute in Colorado finds the theory plausible but points out there is no evidence for the extra moon other than the effect Boué and Laskar suggest it has had on Uranus’s orientation.