ONE of the longest-running controversies in astronomy — the question of what constitutes a planet — could soon be resolved by a radical proposal to abolish the term as a catch-all category.

The organisation responsible for naming celestial objects is to revise its qualifications for planetary status after discoveries that have challenged the traditional model of nine planets orbiting the Sun. In the past five years powerful telescopes have revealed several large objects that orbit farther out than Pluto, and at least one of these, tentatively named Xena, is bigger than the smallest member of the Sun’s official planetary family.

This has led increasing numbers of scientists to question whether Pluto should be in the same club as Earth, Mars and Jupiter, while others believe that the category should be extended to include small icy worlds in the outer reaches of the solar system. Along with Xena, the nickname for an object known officially as 2003 UB313, bodies called Sedna and Quaoar (pronounced kwa-whar) have been identified at the edge of the solar system. Although both are smaller than the smallest official planet, many astronomers believe that these all belong to a similar category of overgrown, icy asteroids in the Kuiper Belt beyond Neptune.

To address the problem, the International Astronomical Union (IAU), which oversees such classifications, is considering abandoning the use of the term “planet” unless it is accompanied by a qualifier. Under a proposal from a 19-member nomenclature panel, leaked to the journal Nature, the rocky inner planets, such as Earth and Mars, would be known as terrestrial planets. Jupiter, Saturn, Uranus and Neptune would be gas-giant planets and everything more distant than that would be trans-Neptunian planets.

The panel considers “planet” to be too broad a category to be precise. “If we’re going to use the word ‘planet’, we should put an adjective in front of it,” Brian Marsden, a panel member from the Harvard-Smithsonian Institute for Astrophysics in Cambridge, Massachusetts, said.

Professor Iwan Williams, of Queen Mary, University of London, the chairman of the group, plans to send a final version to the IAU within two weeks.

The idea of dividing the planets into varieties has been broadly welcomed by astronomers, although some have taken issue with the language proposed by the panel. Alan Stern, of the Southwest Research Institute, in Boulder, Colorado, said that it would be better to call Pluto, Xena and Sedna “ice dwarfs” than “trans-Neptunian planets” because it is more descriptive. He said: “I don’t believe we should classify planetary types by location. We should use properties of the objects as a guide.”

The status of Pluto has long been disputed, with many scientists considering it to be too small and too different from the other planets to deserve its popular designation. The American Museum of Natural History, in New York, recently provoked controversy by excluding Pluto from its display on the planets.

Pluto’s diameter is slightly more than 2,253km (1,400 miles), a figure that is closer in size to the largest official asteroid, Ceres, than to Mercury, the next smallest planet. Ceres, which orbits in the asteroid belt between Mars and Jupiter, is 941km (585 miles) in diameter, whereas Mercury has a diameter of more than 4,828km (3,000 miles).

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