If Pluto is a planet, so should be Xena, Dr. Brown has argued.

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. Dr. 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, Dr. Gingerich said. That knocks out Pluto, which crosses the orbit of Neptune, and Xena, which orbits among the icy wrecks of the Kuiper Belt, and Ceres, which is in the asteroid belt.

“Vociferous objectors have said they could accept this,” Dr. Gingerich said.

Reached in his office at Caltech, Dr. 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.

Dr. Gingerich cautioned that there were many things still to be sorted out. For example, the International Astronomical Union might consider creating a special name for Pluto and other dwarf-planets, like Xena and others yet to be discovered, that dwell out beyond Neptune. If it did, he said that “plutonians” seemed like 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.

But with two more days before the scheduled vote, there was no guarantee Pluto would not make a comeback and the definition of planethood 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,’’ Dr. Pasachoff said. “But someone pointed out that this definition will hold for all time and that it is more important to get it right.”