Pluto is a planet no more after the world's leading astronomers redrew the solar system today, ruling that only eight celestial bodies deserved the prestigious title.

Discovered in 1930 by Clyde Tombaugh, a 24-year-old American astronomer, Pluto's status as the ninth, oddly small planet had been in jeopardy since the 1990s, when powerful telescopes revealed objects just as large also making distant orbits in the darkness beyond Neptune.

For the last week, an assembly of 2,500 delegates at the International Astronomical Union (IAU) has been debating Pluto's future, as well as the status of three other bodies: Ceres, until now regarded as the solar system's largest asteroid; Charon, one of Pluto’s moons; and a newly discovered object called 2003 UB313.

One proposal was for the solar system to be expanded to contain 12 planets in two tiers, but today, by a show of hands in its concluding session, the IAU decided that there could be only eight.

"The eight planets are Mercury, Earth, Venus, Mars, Jupiter, Saturn, Uranus and Neptune," said the IAU resolution that confirmed Pluto's relegation to "dwarf planet" status, where it will not even be the largest object.

Just 1,433 miles (2,306km) across, Pluto is smaller than 2003 UB313, an icy being unofficially known as Xena whose diameter of 1,900 miles (3,000km) was only confirmed earlier this year.

Ceres and Charon, the largest of Pluto’s three moons, will also be added to the group of dwarf planets that astronomers believe could grow to include dozens of members in the years to come.

Today's meeting also created a new catch-all category for other significant asteroids and comets: they will be known as "small solar system bodies".

Today's vote was the culmination of two years of debate and semantic adjustment by a seven-strong IAU committee that has worked to define just what makes a planet in an increasingly complex and visible solar system.

Eventually, that definition — "A celestial body that is in orbit around the Sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a... nearly round shape, and has cleared the neighbourhood around its orbit" — made Pluto's case hard to argue.

In 1999, the IAU was forced to issue a statement saying it had no plans for Pluto's demotion. Soon afterwards, it was excluded from an exhibition about the solar system that the American Museum of Natural History in New York.

The next piece of good news for Pluto may not now come until 2015, when Nasa's New Horizons spacecraft arrives at the edge of the solar system to take the first close-up images of its surface. In the end, it was a planet for just over a quarter of a Pluto year, a Pluto spring.

Explore World News

• World Agenda

• Iraq News

• US & Americas News

• Europe News

• Middle East News

• Asia News

• Africa News

• Ireland News

Times Recommends

• Rio Tinto four held as China prepares case
• Diplomats must keep heads down and trousers up
• When in Rome should one make like Berlusconi?