Room F, at Manchester Street School in Feilding, asks :-

What knocked over Uranus so that it is on its side?

Patrick Bowman, a physicist who teaches the astronomy course at Massey University's Albany campus, responded.

The answer to your question lies somewhere in the five billion year history of the planet, of which we know only a little. What follows is, at best, an educated guess.

All the planets rotate on an axis through their centre and for most of them that rotation is in the same direction as their revolution about the Sun (and the same direction that the Sun itself rotates). The Earth, for example, spins on its axis once every 24 hours, an axis that is tilted from upright (inclined) by about 23 degrees. Uranus is an exception. Its axis of rotation is inclined by more than 97 degrees, so its rotation is "retrograde". One possible explanation is that Uranus was hit by something big. Only something that was itself planet-sized (a "planetoid") could have hit Uranus hard enough to knock it over; but it might not be the only example of such a dramatic collision. Venus also has retrograde rotation: there the Sun rises in the West every 243 Earth days. A planetoid collision at the right place and angle could have overcome Venus' original rotation and set it spinning slowly in the opposite direction.

The biggest feature on Mercury, the Caloris Basin, has corresponding "Weird Terrain" on the opposite side; it seems the collision that formed the Basin disturbed the surface on the far side of the planet! Similarly, the leading theory for the origin of the Earth's moon is that the Earth was hit by a planetoid the size of Mars, spraying debris up into space which eventually coalesced to form the Moon.

There is evidence for wide-spread planet-sized impacts when the solar system was young, and this is currently the most popular explanation for Uranus' axial tilt.