Pluto demoted to dwarf planet

Pluto is no longer a planet, after the world's astronomers voted to change the definition of what counts as one.

The International Astronomical Union voted on 24 August at its meeting in Prague to reclassify Pluto as a "dwarf planet", leaving the solar system with eight planets.

The new definition

Under the new rules, a planet must orbit the Sun, be massive enough for its own gravity to pull it into a round shape, and have cleared the neighbourhood around its orbit of other objects.

Pluto fails the third test, because it shares its region of space with many other icy objects in an area beyond Neptune known as the Kuiper Belt.

Objects that meet the first two tests but not the third will be called dwarf planets. They include Pluto, the large asteroid Ceres and a distant object discovered last year, currently known as 2003 UB313.

Why the change?

Pluto was discovered in 1930 by Clyde Tombaugh, an American astronomer, and was named after a suggestion by Venetia Burney, an 11-year-old girl from Oxford.

It has always been an oddity. It is smaller than our Moon, and its orbit is tilted and stretched compared with those of the other planets.

In recent years, astronomers have discovered many similar objects in the outer solar system. The discovery of 2003 UB313, which is larger than Pluto, forced the issue. Astronomers had to decide whether to add it and possibly dozens of others as planets, or to change the definition.

A controversial vote

The decision has been controversial. Some astronomers say the definition is poorly worded and that only a few hundred of the thousands of members of the union voted. Others say the change is long overdue.

Alan Stern, who leads NASA's New Horizons mission, which was launched in January and is on its way to Pluto, has criticised the decision.

Textbooks rewritten

Generations of schoolchildren have learned the names of nine planets, and many people are sad to see Pluto demoted. Teachers and publishers will now have to update their lessons and books.

Astronomers say that Pluto has not changed, only the way we describe it, and that it remains a fascinating world worth studying.

New Horizons is due to reach Pluto in 2015.