One of the longest-running controversies in astronomy — Pluto’s right to be called a planet — will be resolved by extending the accolade to three more celestial bodies, an international panel of astronomers will announce today.

However, the 12 planets will be divided into two broad categories — “classical” planets such as Earth and Jupiter, and “dwarfs” such as Pluto and the three new members. Pluto thus survives as a planet, but outside the solar system’s Premiership.

There will also be a new group of “plutons” — dwarf planets beyond Neptune that take more than 200 years to orbit the Sun — if the International Astronomical Union (IAU) general assembly approves the plan in Prague next Thursday. The other winners will be Ceres, hitherto regarded as the largest asteroid; Charon, once considered to be Pluto’s moon; and a newly discovered object officially called 2003 UB313, but nicknamed Xena after the television warrior princess played by Lucy Lawless.

All conform to the new definition of a planet agreed unanimously by a seven-strong IAU committee that has spent two years deciding the criteria that command planetary status. This semantic issue has caused bitter disagreements between astronomers for years, as there has been no official ruling on what constitutes a planet. Broadly, large round objects that are not stars and which orbit the Sun rather than another planet have been considered planets, but no upper or lower limit has been set for size.

While the credentials of the first eight planets are not disputed, Pluto’s status has long been controversial as it is so much smaller. Its diameter of about 2,306km (1,433 miles) is closer to that of Ceres, previously seen as an asteroid, than to Mercury, the next smallest official planet.

Many astronomers have argued that were Pluto to have been discovered today, it would never have been described as a planet, and at least one major museum — the American Museum of Natural History in New York — has removed it from its planetarium.

Under the new system, a celestial object will be called a planet if it has sufficient mass, and hence gravity, to be roughly spherical in shape, and if it orbits the Sun rather than another planet.

Owen Gingerich, of Harvard University, who chaired the IAU planet definition committee, said that several members were so worried that they would not reach a consensus after vigorous discussions that they had a sleepless night:

“But by the end of a long day, the miracle had happened: we had reached a unanimous agreement.”

Richard Binzel, a panellist from the Massachusetts Institute of Technology, said: “Our goal was to find a scientific basis for a new definition of planet and we chose gravity as the determining factor. Nature decides whether or not an object is a planet.”

Ceres qualifies as it is round, though at 952km across it is even smaller than Pluto. Xena, which was discovered last year from images taken in 2003, also makes the cut on grounds of shape and orbit. Its name has yet to be confirmed by the IAU, and this is unlikely to be resolved at the Prague meeting.

Charon is more complicated: though it has long been seen as Pluto’s satellite, the two bodies are broadly similar in size, both are roughly spherical, and their common centre of gravity lies outside Pluto itself. This means the two count as a “double planet” under the new system.

At least 12 other bodies in the solar system might also qualify as planets under the new system, but observations have yet to confirm that they are sufficiently large and round. These include “plutons” such as Sedna and Quaoar, and large asteroids such as Vesta and Pallas.

Leading astronomers welcomed the new definition, which is widely expected to be approved next week.

Lord Rees of Ludlow, the Astronomer-Royal and president of the Royal Society, said: “It is a semantic issue, but it has arisen because the constituents of the solar system turn out to be much more numerous and variegated than was foreseen when Pluto was discovered.

“If we were starting from present knowledge we probably would not have classified Pluto as a planet. But this is a very sensible compromise.”

DON’T FORGET THE PLUTONS

Why do we need a new definition?

Modern telescopes have revealed many new objects orbiting the Sun, some of which are bigger than Pluto, hitherto the smallest

What is the definition?

A planet is a planet if its gravity has made it roughly spherical, and if it orbits the Sun but not another planet

So is Pluto still a planet?

Yes. But so are three other small objects, Ceres, Charon and 2003 UB313 (Xena)

How many planets are there in the solar system now?

Twelve — the traditional nine plus Ceres, Charon and Xena

Are there any subdivisions?

The main eight are “classical” planets. The new three, plus Pluto, are “dwarf” planets. Small planets that take more than 200 years to orbit the Sun — Pluto, Charon and Xena — are “plutons”

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