Pluto’s status has been contested for many years

Astronomers meeting in the Czech capital have voted to strip Pluto of its status as a planet.

About 2,500 experts were in Prague for the International Astronomical Union’s (IAU) general assembly.

The scientists rejected a proposal that would have retained Pluto as a planet and brought three other objects into the cosmic club.

Pluto has been considered a planet since its discovery in 1930 by the American Clyde Tombaugh.

The ninth planet will now effectively be airbrushed out of school and university textbooks.

“The eight planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune,” said the IAU resolution, which was passed following a week of stormy debate.

Professor Iwan Williams chaired the IAU working group that has been working over recent months to define the term “planet”.

“I have a slight tear in my eye today, yes; but at the end of the day we have to describe the Solar System as it really is, not as we would like it to be,” the Queen Mary University of London, UK, scientist told the BBC.

PLUTO – A ‘DEMOTED PLANET’   - Named after underworld god - Average of 5.9bn km to Sun - Orbits Sun every 248 years - Diameter of 2,360km - Has at least three moons - Rotates every 6.8 days - Gravity about 6f Earth’s -Surface temperature -233C - Nasa probe visits in 2015

The initial proposal put before the IAU to raise the number of planets in the Solar System to 12 – adding the asteroid Ceres, Pluto’s “moon” Charon and the distant object known as 2003 UB313 – met with opposition.

Robin Catchpole, of the Institute of Astronomy in Cambridge, UK, told the BBC News website: “My own personal opinion was to leave things as they were; I met Clyde Tombaugh and thought how nice it was to shake hands with someone who had discovered a planet.

“But since the IAU brought out the proposal for new planets I had been against it – it was going to be very confusing. The best of the alternatives was to leave the major planets as they are and then demote Pluto. So I think this is a far superior situation.”

Louis Friedman, executive director of the Planetary Society in California, US, commented: “The classification doesn’t matter. Pluto – and all Solar System objects – are mysterious and exciting new worlds that need to be explored and better understood.”

Dwarf planet

Amid dramatic scenes which saw astronomers waving yellow ballot papers in the air, the IAU meeting voted through new definition criteria.

They agreed that to qualify as a planet, a celestial body must be in orbit around a star while not itself being a star. It also must be large enough in 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.”

Pluto was automatically disqualified because its oblong orbit overlaps with Neptune’s.

It will now join a new category of “dwarf planets”.

Pluto’s status has been contested for many years as it is further away and considerably smaller than the eight other “traditional” planets in our Solar System.

Its orbit around the Sun is also highly inclined to the plane of those big planets.

In addition, since the early 1990s, astronomers have found several objects of comparable size to Pluto in an outer region of the Solar System called the Kuiper Belt.

Some astronomers have long argued that Pluto belongs with this population of small, icy worlds.

Allowances were once made for Pluto on account of its size. At just 2,360km (1,467 miles) across, Pluto is smaller even than some moons in the Solar System. But until recently, it was still the biggest known object in the Kuiper Belt.

That changed with the discovery of 2003 UB313 by Professor Mike Brown and colleagues at the California Institute of Technology (Caltech). After being measured with the Hubble Space Telescope, it was shown to be some 3,000km (1,864 miles) in diameter, making it larger than Pluto.

Named after the god of the underworld in Roman mythology, Pluto orbits the Sun at an average distance of 5.9 billion kilometres (3.7 billion miles) taking 247.9 Earth years to complete a single circuit of the Sun.

An unmanned US spacecraft, New Horizons, is due to fly by Pluto and the Kuiper Belt in 2015.