Ross McKenzie, of Invercargill, asks :-

Why are planets round?

Duncan Steel, a space researcher and author based in Nelson, responded.

In December 1991 the top fell off Mount Cook, reducing its height by ten metres. The Earth became more round, one might say. On the other hand, the Southern Alps seem to be rising by five to seven millimetres per year. A variety of natural agencies including earthquakes, volcanoes, and erosion by wind and rain, are all causing Earth's shape to alter: mostly slowly, but sometimes suddenly and perhaps calamitously.

Overall, large bodies such as the planets, the Sun and the stars are held together by gravity and tend to adopt the lowest-energy state possible. Water runs downhill, and stones and pebbles tumble or slide down!

In the case of a rocky planet like Earth (or Mercury, Venus and Mars) the physical strength of the materials of which it is made give it a crinkly edge in the form of mountains and canyons. If the outermost layers of a large body are made of a fluid, however, it will be more rounded. That's obviously the case with the Sun, and the giant planets Jupiter, Saturn, Uranus and Neptune.

It is Earth's dynamic geology, powered largely by heat released through radioactive decay of atoms such as uranium far below the surface, that raise up the mountains, continents and volcanoes, and making life like us possible.

If Earth did not spin on its axis then it would be closer to spherical. A point on the equator rotates around the centre at almost half a kilometre per second (almost 1,700 kph). This causes a bulge: the equatorial radius is 21 km greater than the polar radius.

In consequence, the point on Earth's surface farthest from the centre is not the top of Mount Everest (8.848 km above sea level), but the peak of Chimborazo, an inactive volcano in Ecuador (i.e. nearer to the equator).

There are millions of minor planets (synonym asteroids) in the solar system. Dependent on their size and composition (mainly rock and metal), these may be near-spherical in shape, or very irregular.

Asteroids larger than about 500 km in size tend to be well-rounded, because their self-gravity is stronger than the mechanical strength of the materials of which they are made: their gravity rounds them off.

Some smaller asteroids are shaped like gnarled bananas, wonky peanuts, or pimpled potatoes. One of the most peculiar is named Kleopatra: it looks like a dog's bone 200km long. It is too small to make it spherical.