Next week, by a simple show of hands at an astronomy meeting, Earth could go from being one of nine planets to one of twelve — with unknown numbers yet to be discovered.

A seven-member panel appointed by the International Astronomical Union (IAU) has recommended a new definition of a planet: any body in orbit around a star that is not a star itself nor in orbit around a much larger planet, and that is massive enough for gravity to have squished it into an approximately spherical shape.

The IAU planned to introduce the notion in a draft resolution on 16 August, after Nature went to press, and will discuss it on 22 August during an open session at its general assembly in Prague, Czech Republic. Two days later, if a simple majority of IAU members vote for it or a slightly revised version, that will become the official scientific word on the topic.

Under the definition, most objects with a mass greater than 5×1020 kilograms, and typically with a diameter greater than 800 kilometres, would qualify as planets. That would instantly elevate the asteroid Ceres and Pluto's moon Charon to planet status, as Ceres is roughly spherical and Pluto and Charon can be described as a binary planet system. The Solar System's twelfth planet would be object 2003 UB313, nicknamed Xena, one of many 'trans-neptunian objects' lurking on the Solar System's fringes.

Not all are happy with the 'roundness' criterion, however. “It's not what I think of when I think of a planet,” says Michael Brown, an astronomer at the California Institute of Technology in Pasadena and co-discoverer of UB313. “I don't think of things that are a thousand times smaller than Earth.”

The discovery of UB313 beyond Pluto's orbit last year brought matters to a head. Astronomers have estimated that it is at least 2,400 kilometres in diameter — larger than Pluto, but smaller than any of the other eight planets.

If there is nothing accepted, we have to sit in the silly place where we are now.

All this demands a new definition of 'planet', some say. “If there is nothing accepted, we have to sit in the silly place where we are now: Pluto is a planet and UB313 is not,” says Iwan Williams, an astronomer at Queen Mary, University of London.

Pluto (left) and Charon might both be planets — but not in our league. Credit: D. VAN RAVENSWAAY/SPL

Williams chaired an earlier IAU committee that was meant to define a planet, but it deadlocked last November. “People came in with well-defined ideas of what a planet should be, expressed them strongly, and hardly anybody changed their mind,” says committee member Alan Boss, an astronomer at the Carnegie Institute of Washington. Opinions were divided between three options to define a planet orbiting a star: any object with a diameter greater than 2,000 kilometres; any object massive enough for gravity to make it round; or any object that dominates its region of space.

Faced with that stalemate, the IAU appointed another planet definition committee, chaired by astronomer and historian Owen Gingerich of Harvard University. The group met on 30 June for two days at the Paris Observatory. “We converged relatively quickly after the first day,” says panel member Richard Binzel of the Massachusetts Institute of Technology.

A definition based on gravity, the group concluded, made the most scientific sense. “It involves the most physics,” Binzel says.

To acknowledge that trans-neptunian objects are different to the other planets, the committee proposed a subcategory of planets known as 'plutons'. These are planets that take more than 200 years to orbit the Sun and would include Pluto, Charon and UB313.

The number of planets is likely to rise as more large trans-neptunian objects are found. After Ceres, other asteroids may also be eligible. Such changes may upset generations who grew up with nine planets. “If I have any concern, it's that the public will accept this,” says astronomer Ron Ekers, president of the IAU.

Binzel thinks there are ways around that. “My expectation is that children will memorize the eight classical planets, they will know that Ceres is a planet in the asteroid belt, and that there's a whole collection of planets out beyond Neptune, of which Pluto is the first.”

Convincing fellow astronomers may not be easy either. “There will be a long line of people waiting for the microphone to denounce it,” Boss predicted before he had heard details of the proposal. “I think there's a good chance nothing will be decided formally.”

That prospect dismays those who have been working on the new definition. “This is a very good compromise,” says Binzel, “and it's time to move forward.”