For Pluto, a Smaller World After All

"Pluto is a dwarf planet, but we are now faced with the absurdity that a dwarf planet is not a planet," Gingerich retorted. "Is a human dwarf not a human?"

The controversy over how to define Pluto began when scientists realized it is much smaller than it was thought to be when it was discovered in 1930. Early data indicating that it was large enough to disturb the orbits of Neptune and Uranus turned out to be observational errors.

As a result, astronomers felt there was something distinctive about Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. But like the late Supreme Court justice Potter Stewart, who said he could not define pornography but knew it when he saw it, the scientists found that coming up with a definition that would include only eight planets was surprisingly difficult.

One option would be to call any round object orbiting the sun and larger than a certain size a planet. If that minimum size were arbitrarily set small enough to include Pluto, it would be a planet. But so would an even more distant object found in 2003 that is larger than Pluto, dubbed 2003 UB313, or, unofficially, Xena.

Another possibility, advocated by Gingerich's committee, would have been to call any orbiting object sufficiently large for gravity to pull it into a round shape a planet. That would have included Pluto, Xena and Ceres -- a object long considered an asteroid that orbits between Mars and Jupiter -- and perhaps others.

This definition would have preserved the status of the eight "classical" planets by calling the smaller objects "plutons." But geologists objected, saying the word "pluton" hadalready been claimed -- in geology, it is the term for magma that works its way into rocks.

Yesterday, astronomers who focus on dynamical properties -- how planets influence their surroundings -- won the day: Because the eight classical planets are relatively large, they dominate their orbits, sweeping smaller objects before them.

"Either of these definitions is technically good," said Ekers, referring to the Gingerich definition and the one finally adopted. "One would have been easier for describing to children and the public, and the resolution which is passed is fine for scientists but is a little more difficult for the public."

Other astronomers remained divided. Andrew Cheng, Harold F. Weaver and Karl Glazebrook of Johns Hopkins University said the new definition about planets being able to clear their neighborhoods of other objects is muddled and confusing. But colleagues William P. Blair and Richard Conn Henry said demoting Pluto makes sense.

Astronomer Mike Brown, who helped discover Xena and would have stood to gain fame from Gingerich's original definition, said the astronomy group had come up with the right decision. "A mistake was made, and science has corrected the mistake," said Brown, of the California Institute of Technology.

But Gingerich, who is also a historian, said astronomers had blown it by ignoring public sentiment and the historical significance of Pluto: "We are an expensive science, and if we don't have public support, we are not going to be able to do our work."