But roundness, he said, is not a very interesting attribute to use in classifying astronomical bodies. “A Plutophile is well served by this definition,” he said. “It is one of the few that allow you to utter Pluto and Jupiter in the same breath.”

But Alan Stern of the Southwest Research Institute in Boulder, Colo., called the definition “a nice solution that works both inside and outside the solar system.”

Everybody agrees that a little clarity is needed when it comes to categorizing the members of the solar system. The proposed definition would come as a relief to schoolchildren and others who have rallied to the cause of Pluto.

The planet (if that is what it is) has been an oddball ever since Clyde Tombaugh spied it wandering in the outer reaches of the solar system beyond Neptune in 1930. Not only is it much smaller than the other eight planets, only a fiftieth the mass of Earth, but its orbit is unusually elliptical and inclined to the plane that marks the orbits of the other planets. In recent decades, however, other objects with orbits like Pluto’s have been discovered in the Kuiper Belt, a junkyard of icy debris beyond Neptune.

Many astronomers began to argue that it made more sense to think of Pluto as a Kuiper Belt object, a minor planet instead of a planet. When it was reported that the Hayden Planetarium had done just that in its new Rose Center, which opened in 2000, a firestorm erupted. Schoolchildren rushed to the defense of lonely little Pluto.

Two years ago, the International Astronomical Union appointed a group to come up with a definition that would resolve this tension. The group, led by Iwan Williams of Queen Mary University in London, deadlocked. This year a new group with broader roots took up the problem. After a sleepless night in Paris this spring, what Dr. Gingerich calls a miracle took place: “We had reached unanimous agreement.”

In a nod to the idea of classifying Pluto with the Kuiper Belt, the group proposed calling planets with elongated orbits beyond Neptune “Plutons,” while emphasizing that they would still be planets.