Defining 'planet': Newfound world forces action

The members have said they "want to get something done, pronto," Stern told SPACE.com. He said it's possible a proposal could be finalized in a week or two and made public. Still, group members have clearly different ideas in mind.

A synopsis of Stern's thinking:

A planet is a body that directly orbits a star, is large enough to be round because of self gravity, and is not so large that it triggers nuclear fusion in its interior.

"I think there's a consensus moving in this direction," Stern said.

The actual definition will, at least, be more complex than that. Stern favors calling the smaller objects dwarf planets, for example. Other astronomers prefer the term minor planet. Another term bandied about is Kuiper Belt planets. Some don't like the idea of applying the planet label at all.

Let there be 8
Brian Marsden, who is also on the IAU Working Group and who runs the Minor Planet Center where data on objects like these end up, says a simple definition like Stern's makes sense from a theoretical point of view.

If adopted, the wording would bring our solar system's tally of known planets to about two dozen, Marsden said.

But practically speaking, Marsden, who expects it will take "somewhat more than a week or two to come up with a policy," prefers another approach.

"The only sensible solution is to accept that the solar system contains the eight planets known a century or so ago," Marsden said via email, "and to add new members only if they are larger than, say, Mars — or maybe even the Earth."

(Stern and others contend that such large worlds indeed await discovery.)

The discovery of 2003 UB313 presents "the best chance to resolve the problem," Marsden said. "I doubt that all astronomers will be happy with the outcome, but I would hope that what is decided is enough of a compromise that most of them are."

Forget science
Mike Brown yesterday attempted to shift the whole debate away from science.

In Brown's mind — and he admits to changing it recently — Pluto is too enshrined in our culture, from place mats to postage stamps, to strip it of planethood.

"Some astronomers have rather desperately attempted to concoct solutions which keep Pluto a planet, but none of these are at all satisfactory, as they also require calling dozens of other objects planets," Brown wrote on his web site this week. "While people are perhaps prepared to go from nine to 10 planets when something previously unknown is discovered, it seems unlikely that many people would be happy if astronomers suddenly said, 'we just decided, in fact, that there are 23 planets, and we decided to let you know right now.'"

Brown's team is taking a stand.

"We declare that the new object, with a size larger than Pluto, is indeed a planet," Brown wrote. "A cultural planet, a historical planet. I will not argue that it is a scientific planet, because there is no good scientific definition which fits our solar system and our culture, and I have decided to let culture win this one."

He advises the public to "ignore the distracting debates" of the scientists.

It seems clear the IAU Working Group plans to ignore Brown, at least insofar as they expect to forge a scientific definition.

Yet no matter what the group comes up with, you can bank on at least one more year of debate. For a definition to be made official, it must be voted on at an IAU General Assembly meeting. The next one is in Prague in August, 2006.

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