What counts as a planet

For 76 years the Solar System had nine planets. The ninth was Pluto, found in 1930. Then in 2006 astronomers took a vote, and there were eight. Pluto had not changed at all. What changed was the meaning of a word.

Pluto had always been the odd one out. It is tiny. It is only two thirds as wide as our own Moon, and it weighs about a sixth as much. All eight of the others are far bigger.

It also has company. Pluto goes round the Sun far out beyond Neptune, in a wide belt of icy bodies, and it shares that part of space with a great many of them. In January 2005 astronomers found another one, now called Eris, which is very nearly Pluto’s size. If Pluto was a planet then so was Eris, and so, before long, would dozens of others be.

So astronomers had to decide what a planet is. In August 2006 they agreed on three tests. A planet goes round the Sun. It is heavy enough for its own gravity to have pulled it into a ball. And it has cleared its path of other bodies of any size. Pluto passes the first two and fails the third. A body like that is now called a dwarf planet.

This had happened once before. Ceres, found in 1801, was called a planet at first. Then so many other small bodies turned up between Mars and Jupiter that it was put with them as an asteroid. Since 2006 it has been a dwarf planet too.

None of this makes Pluto dull. In July 2015 the spacecraft New Horizons flew past and sent back the first close pictures. They show a world with five moons and a pale plain the shape of a heart. Not everyone agrees with the 2006 decision, and people still argue about it.