Planet definition debate revives, and Pluto’s in trouble

Fans of Pluto, beware. The diminutive world on the outer edges of the solar system may yet have its membership in the planetary club revoked.

It looked as if Pluto had been saved when the International Astronomical Union put forth a new planetary definition last week that set the bar so low even Pluto could clear it: The requirements said a planet must be round and orbit the sun.

When it was unveiled, IAU leaders predicted swift approval this week by the 3,000 astronomers gathered for a conference in the Czech Republic. Instead, the proposal has aroused heated discussions in the bars and restaurants of old Prague.

One group of dissenters is floating a proposal that would demote Pluto to “dwarf planet” status, leaving just eight planets in the solar system.

“It’s interesting and very exciting,” Lars Lindberg Christensen, a spokesman for the IAU, said Saturday of the debate.

Political warfare is usually not something astronomers engage in. But when it comes to tampering with the solar system, even scientists can apparently get their blood up, he said.

The astronomers, whose conference ends Friday, are scheduled to vote Thursday on the issue.

Given the passions aroused, Christensen said it was impossible to predict the outcome, other than what emerges will likely be “somewhat different” from last week’s proposal.

After months of debate, an executive committee of the IAU proposed the definition that not only keeps Pluto, but adds three new planets. They would be UB313, Pluto’s moon Charon and the largest member of the asteroid belt, Ceres.

Opponents complain the definition just makes things more confusing. Several of the new planets would be smaller than Earth’s moon. Worse, under the new definition, there could be dozens more planets added as other objects are found at the outer edges of the solar system.

The dissenting group, including members from Uruguay, France, Brazil, Italy and elsewhere, has what it calls a simpler idea. To be a planet, they suggest, the object must not only be round, but be “by far” the largest body in its local population.

Under this definition, Pluto would become a “dwarf planet” because it would be just another member of the distant Kuiper Belt of small, icy worlds beyond Neptune. Charon and UB313 would fall victim to the same classification.

As for Ceres, it would presumably go back to being another asteroid, though some might argue that as the largest asteroid, it might still qualify.

In any event, Christensen says, the release of the competing proposal is “completely premature. It’s just one of many ideas circulating.”