How it works

The word planet comes from the Greek for wanderer, and for most of history it needed no definition. A planet was one of the lights that moved against the stars. The trouble began when telescopes started finding more and more things going round the Sun.

The three tests. The International Astronomical Union voted on them in Prague on 24 August 2006. To be a planet, a body in our Solar System has to do all three.

The test	The Earth	Pluto	Ceres	A small asteroid
It goes round the Sun	Yes	Yes	Yes	Yes
Its own gravity has pulled it into a ball	Yes	Yes	Yes	No
It has cleared its path of other bodies	Yes	No	No	No
So it is	A planet	A dwarf planet	A dwarf planet	A small body

A moon fails the first test, because it goes round a planet. That is why our Moon and the big moons of Jupiter and Saturn are not on the list, although several are bigger than Pluto.

What “cleared its path” means. Each of the eight planets is by far the biggest thing anywhere near its own orbit. Over billions of years it has swept up the smaller bodies there, flung them away, or caught them as moons. Pluto has not. Its orbit runs from 30 to 49 times the Earth’s distance from the Sun, through a belt full of other icy bodies that it is much too small to shift. Ceres is in the same position in the asteroid belt, where it makes up about a quarter of all the mass.

The five dwarf planets. These are the ones the Union has named so far. It expects there to be many more, perhaps more than a hundred, still to be found or confirmed.

Name	Where	Width	Found
Ceres	The asteroid belt, between Mars and Jupiter	About 950 km	1801
Pluto	Beyond Neptune	2,377 km	1930
Haumea	Beyond Neptune	About 2,300 km at its longest, but shaped like an egg and only about 1,000 km through its shortest way	2003 or 2004 (two teams claim it)
Makemake	Beyond Neptune	About 1,430 km	2005
Eris	Beyond Neptune, further out than Pluto	2,326 km	2005

For comparison, the Moon is 3,476 km wide and the Earth 12,742 km. The width of Ceres is twice the radius NASA gives, worked out here. The size of Eris was measured in 2010 by timing how long it hid a star, and Haumea’s in 2017 in the same way.

Eris. Eris was found on 5 January 2005 by Mike Brown, Chad Trujillo and David Rabinowitz, on pictures taken at Palomar Observatory in 2003. It is very nearly the size of Pluto, about 50 km narrower, and lies much further from the Sun. Its discovery is what forced the question.

The argument. The third test is the one people quarrel with. Some scientists who study planets would rather call anything a planet that is big enough to be round, which would bring in Pluto, Ceres and the large moons as well. The definition also speaks only of bodies that go round the Sun, so it says nothing about the thousands of planets now known round other stars. For now, the 2006 wording is the official one.