The future of the Solar System — or at least that of some of its nomenclature — may be thrown into turmoil by scientists who are calling for a boycott of a new definition of a planet.

The ayes have it: astronomers have criticized the vote to define a planet. Credit: M. CIZEK/AFP/GETTY

On 24 August, delegates at the general assembly of the International Astronomical Union (IAU) in Prague voted to define the planets of the Solar System by three criteria. To deserve planet status, the assembly agreed, a chunk of rock or ball of gas must be big enough for its gravity to have made it round, must orbit the Sun but not be a satellite of another planet, and must have cleared other bits of debris from its orbit. Round objects, including Pluto, that failed on the final count became not planets but 'dwarf planets'.

The definition originally proposed on 16 August by the IAU would have had just two criteria — roundness and not being a moon. This was rejected by members at the meeting where the three-part definition was voted on as the final word on the subject. But many IAU members were not in Prague for the vote, and some are furious at the outcome.

We do not classify objects in astronomy by what they are near, we classify them by their properties. Alan Stern

“I am just disgusted by the way the IAU, which is meant to represent the best in science, handled this matter,” says Alan Stern, a planetary scientist at the Southwest Research Institute in Boulder, Colorado. As principal investigator for NASA's New Horizons mission to Pluto, he has a particular interest in its status. But he says the issue is not really Pluto's status so much as the idea of putting objects in orbital contexts. “We do not classify objects in astronomy by what they are near,” he says. “We classify them by their properties.”

The day after the new definition was unveiled, Stern was among a dozen scientists who launched a petition to contest it. By e-mail, they sought the support of their colleagues for the following statement: “We, as planetary scientists and astronomers, do not agree with the IAU's definition of a planet, nor will we use it. A better definition is needed.” More than 200 people had added their names to the petition as Nature went to press on Tuesday.

Stern thinks that requiring a planet to have 'cleared its orbit' rules out some of the Solar System's other eight planets. These include Neptune, the orbit of which is crossed by Pluto, and Jupiter, which shares its orbit round the Sun with the Trojan asteroids.

The 'clearing' criterion was introduced when astronomers who study the dynamics of the Solar System insisted that the definition should recognize their idea of what constitutes a planet — an object with a mass that dominates its orbital zone. Owen Gingerich, chair of the committee that proposed the 16 August resolution, thinks the IAU had no choice but to bend to the dynamicists' demands. “They may not have had a majority for anything positive, but they could rouse a strong negative majority simply because there are so many little fiefdoms,” he explains.

“The dynamics part of the definition is a rather complex one,” says Ron Ekers, past IAU president. Couching the idea in terms of a planet 'clearing its orbit' was intended to make the issue easier for the public to understand. But it may well end up confusing matters.

Some organizations have already said they will accept the IAU's new definition. Encyclopaedia Britannica, for example, issued a statement saying that some of its articles on Pluto and the Solar System were updated online the same day the IAU's pronouncement was made. According to a spokesperson, later revisions may reflect any uncertainty, but “the vote by the IAU is considered binding — until the next vote, whether it's next year or next century”. NASA, too, promised to abide by the definition, adding that it will “continue pursuing exploration of the most scientifically interesting objects in the solar system, regardless of how they are categorized”.

Others are waiting to see how strong the counter-movement becomes. A black ribbon was tied around the Pluto panel at the Smithsonian Institution's National Air and Space Museum in Washington DC when the IAU's verdict was announced, says curator Andrew Johnston. But it has since been removed. “We're going to let things calm down for at least a few weeks before we decide to make any changes,” he says.

One thing that particularly irks critics is the way the decision was made. The IAU has nearly 9,000 members, but only 2,500 people attended the Prague meeting and only a few hundred were present for the vote. The IAU should have used the Internet to gauge wider opinion, and then allowed electronic voting, according to those who oppose the definition.

“The IAU seems to be rooted in the pre-Internet age,” says Mark Sykes, director of the Planetary Science Institute in Tucson, Arizona, who instigated the petition. “The rules of the IAU say that resolutions are passed by those present and voting,” says Catherine Cesarsky, director of the European Southern Observatory and newly elected president of the IAU.

NASA's New Horizons probe should reach Pluto in 2015, regardless of whether it is still a planet. Credit: JHUAPL/SWRI

Sykes allows that agreement on a "better definition" might be hard to come by, but is still pressing for the current one to be scrapped. He thinks the IAU would be better off without any definition at all rather than the one they have chosen. “If they can determine that this process was flawed and nullify it, then I think that would be in their best interests,” he says.

“If enough people are completely unhappy, we could go through the process again,” says Ekers. But a new resolution would have to wait for the next general assembly in 2009 in Rio de Janeiro. The IAU may issue a clarifying statement in the next week or two, but is hesitating to do so now. “Perhaps we need to make our next statement when things are a little less emotional,” says Ekers.