The heated debate over what constitutes a planet has reignited following last week's confirmation that the most distant planet-like object object ever seen in the solar system is larger than Pluto. But astronomers tasked with settling the issue say the argument could drag on for years.

The International Astronomical Union (IAU), responsible for resolving such issues, assembled a special working group to decide on the definition two years ago, when a large new body called Sedna was found in the outer solar system.

But since then, several other large worlds have been discovered, including 2003 UB313, unofficially dubbed Xena. This body became widely known as the "tenth planet" as it appeared to be larger than Pluto, which is about 2300 kilometres across. Now, new results from an independent team appear to confirm this, finding Xena is about 30% wider than Pluto.

However, astronomers are bitterly divided over what constitutes a planet. And when the IAU's working group was forced to issue its verdict in October 2005, it failed to find a definition all 19 members could agree on. So it simply reported on the relative popularity of three different proposals – each group member was allowed to vote for more than one proposal.

Keeping Pluto

A narrow majority of 11 members favoured deeming anything larger than 2000 kilometres a planet. Under this scheme, Pluto would remain a planet and it would be joined by several newly discovered worlds, including Xena. But some group members argued such a size cut-off was arbitrary, set only so Pluto could retain the title of ninth planet.

Another option attempted to come up with a scientific justification for a size cut-off. In this plan, planets would have to be massive enough for their gravity to hold them in a stable shape – a requirement that could be met by objects as small as 600 kilometres across.

"It complicates matters because we get some dozens of new planets, but on the other hand, there's some scientific justification" for the size cut-off, says group member Brian Marsden of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, US, who had the idea.

In this scheme, which eight group members voted for, all planets would be sorted into four sub-categories based on their location and composition, those being:

• Terrestrial planets, such as Earth and Mars

• Jovian planets – gas giants such as Jupiter

• Cisjovian planets – large asteroids such as Ceres

• Trans-Neptunian planets, such as Pluto

Another proposal argued that a planet is the dominant body in its immediate neighbourhood. This would demote Pluto, as it is one of several bodies of similar size in the Kuiper Belt – a ring of icy objects beyond Neptune. Six group members voted for this option, which would leave the solar system with eight official planets.

Division and discord

Because the group was so divided it simply issued a report on its discord, and not a resolution for the IAU's executive committee to put up for a wider vote. That vote would most likely occur at an IAU general assembly meeting, which occur every three years. The next will take place in Prague, Czech Republic, in August 2006.

That meeting could see a vote if the executive committee, a representative from an IAU member nation, or one of the IAU's divisions puts forward a resolution on the definition of a planet. "But I don't think that is likely given the difficulty the experts had in coming up with a consensus," says Robert Williams, one of 10 members of the IAU's executive committee and an astronomer at the Space Telescope Science Institute in Baltimore, Maryland, US.

Williams adds that waiting may also have scientific merit because astronomers have been focusing on defining planets in our solar system, while little is known about the formation and evolution of the 170 or so known planets around other stars.

"We're trying to define things we don't fully understand," he says. "It may be a bit frustrating that we're not quite at the point where we can agree on what a planet is. But the more important thing is a fundamental understanding of what's going on – I would put that as a higher priority than the naming convention we adopt."

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