Charles, of St Martins School, asks :-

Why are planets and moons round?

Duncan Steel, the author of "Target Earth" (Reader's Digest) responded.

Whilst it is true that most planets and moons appear to be round, this is not precisely the case. Think about our own planet. The seas attain a spherical shape because they are made of a fluid - water! - that has no strength. Rock, on the other hand, is strong, and so it resists the attempt of gravity to pull it flat, and so the Earth is bumpy.

This finite strength of solid material leads to us having mountains eight kilometres or so high. Mars, which is smaller than the Earth and therefore has lesser gravity, has taller protrusions from its surface: the volcano called Olympus Mons is three times as high as Mount Everest, and Mars also has a canyon far bigger than the Grand Canyon.

The moons of Mars, Phobos and Deimos, are not round at all, but rather angular. They are only a handful of kilometres across. Similarly if one considers the natural satellite systems of the giant planets Jupiter and Saturn, one finds that the larger objects are spherical, but the smaller moons are irregular in shape.

As a general rule a body needs to be at least about three hundred kilometres in size if it is to have sufficient self-gravity to pull it into a roughly spherical shape. Looking at the asteroids (also called minor planets), the largest is about 1000 kilometres across, and it is round. Similarly the other large asteroids are close to being spherical, gravity pulling them into that profile.

But smaller asteroids have weird shapes. NASA obtained radar images of an asteroid called Kleopatra, which is 200 kilometres long. It is shaped somewhat like a dog's bone!