"I think that today can go down as the 'day we lost Pluto,' " said Jay Pasachoff of Williams College, in Williamstown, Mass., in an e-mail message from Prague.

Under fire from other astronomers and the public, a committee appointed by the International Astronomical Union revised and then revised again a definition proposed last week that would have expanded the number of official planets to 12, locking in Pluto and the newly discovered Xena in the outer solar system, as well as the asteroid Ceres and Pluto's moon Charon.

The new definition offered on Tuesday would set up a three-tiered classification scheme with eight "planets"; a group of "dwarf planets" that would include Pluto, Ceres, Xena and many other icy balls in the outer solar system; and thousands of "smaller solar system bodies," such as comets and asteroids.

The bottom line, said the Harvard astronomer Owen Gingerich, chairman of the Planet Definition Committee of the IAU, is that in the new definition, "Pluto is not a planet."

"There's not happiness all around, believe me," he added.

The new proposal was hashed out in a couple of open meetings, the first of which was described by participants as tumultuous, and the second as more congenial. Astronomers are supposed to vote on Thursday on this or some other definition, but whether a consensus is emerging depends on whom you ask.

Some astronomers expressed anger that the original definition of a planet had been developed in isolation and then dropped on them only a week before the big vote. Others continued to question whether it was so important to decide the question now.

Among its defects, some astronomers say, the newer definition abandons any pretense of being applicable to planetary systems beyond our own solar system.

Pluto's shortcomings

To many astronomers, Pluto's tiny size and unusually tilted orbit make it a better match to the icy balls floating in the outskirts of the solar system in what is known as the Kuiper Belt than to the traditional planets like Jupiter and Mars. The issue has been forced on astronomers by the discovery of such a ball even larger than Pluto, nicknamed Xena by its discoverer, Mike Brown of the California Institute of Technology.

Neither Pluto nor Xena should be considered planets, Brown has argued. But if Pluto is a planet, then Xena should be, too, he said.

What makes a planet?

The committee's original prime criterion was roundness, meaning that a planet had to be big enough so that gravity would overcome internal forces and squash it into a roughly spherical shape. But a large contingent of astronomers, led by Julio Fernandez of the University of the Republic, in Montevideo, Uruguay, has argued that a planet must also be massive enough to clear other objects out of its orbital zone. Gingerich admitted, "They are in control of things."

So the newest resolution includes the requirement for orbital dominance as a condition for full-fledged planethood, Gingerich said. That knocks out Pluto, which crosses the orbit of Neptune; Xena, which orbits among the icy wrecks of the Kuiper Belt; and Ceres, which is in the asteroid belt.

Reached in his office at Caltech, Brown, who, as the discoverer of Xena, has the most to lose by its and Pluto's demotion, said he thought he could live with the new proposal. "It essentially demotes Pluto to something other than a real planet, which is reasonable," he wrote in an e-mail message.

Gingerich cautioned that there were many things still to be sorted out. For example, the IAU might or might not create a special name for Pluto and other dwarf planets, such as Xena and others yet to be discovered, that dwell beyond Neptune. If so, he said that "plutonians" seemed a likelier choice than the previous suggestion "plutons." That term was protested by geologists, who pointed out that it was already used in earth science for nuggets of molten rock that have solidified and reached the surface.

With two days before the scheduled vote, there was no guarantee that Pluto would not make a comeback and that the definition of planethood might be rewritten again.

"Some people think that the astronomers will look stupid if we can't agree on a definition or if we don't even know what a planet is," Pasachoff said. "But someone pointed out that this definition will hold for all time and that it is more important to get it right."