Debate Lingers Over Definition for a Planet

By KENNETH CHANG

Published: September 1, 2006

More than 300 scientists have signed a petition protesting the definition of “planet” decided by the International Astronomical Union last week. That definition demoted Pluto, leaving the solar system with eight planets.

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Petition Protesting the I.A.U. Planet Definition (ipetitions.com)The petition states: “We, as planetary scientists and astronomers, do not agree with the I.A.U.’s definition of a planet, nor will we use it. A better definition is needed.”

Mark V. Sykes, director of the Planetary Science Institute in Tucson, Ariz., and S. Alan Stern, of the Space Science and Engineering Division of the Southwest Research Institute in Boulder, Colo., organized the petition. Many planetary scientists, they said, were dissatisfied with the new definition, some because of how it was decided, some because they did not find it sound.

The signers of the petition included NASA scientists, astronomers at major observatories, university professors and graduate students.

The astronomical union allowed only scientists attending a conference last week in Prague to vote.

The group’s definition for a planet specifies three conditions: the object orbits the Sun, it is large enough for its gravity to pull it into a round shape, and it “has cleared the neighborhood around its orbit.”

The last condition excludes Pluto because it is located among many other icy bodies in a ring of debris known as the Kuiper Belt. But it strikes some scientists as imprecise.

For example, Earth has not cleared out all of the asteroids that cross its path. Even Jupiter, the largest planet, has not entirely cleared out its neighborhood; asteroids known as the Trojans share its orbit.

“It’s not functional,” Dr. Sykes said of the definition, “and there are a lot of people that feel that way.”

Dr. Stern and Dr. Sykes are organizing an international conference to revisit the question next year.

Dr. Sykes said: “I think the community at this point and time can organize itself to have this discussion and come up with its own consensus, and the I.A.U. can catch up later.”