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News Service

News Service:

Annette Hacker, director,
(515) 294-3720

Office: (515) 294-4777

8-24-06

Contacts:

Lee Anne Willson, Physics and Astronomy, lwillson@iastate.edu

Mike Krapfl, News Service, (515) 294-4917, mkrapfl@iastate.edu

Iowa State professor votes yes for new definition of a planet

AMES, Iowa -- Iowa State University's Lee Anne Willson joined astronomers from around the world as they voted today to approve a new three-part definition of a planet.

The new definition says planets must orbit the sun. They must have enough mass for their gravity to pull the planet together into a nearly round shape. And they must be big enough to have cleared the part of the solar system around their orbits.

The new definition demotes Pluto to dwarf-planet status because it crosses the orbit of Neptune and therefore has not cleared the neighborhood around its orbit.

The new definition also creates a third tier of objects orbiting the sun to be known as "small solar system bodies."

Willson, a University Professor of physics and astronomy at Iowa State, said she thinks the astronomers debating the issue at a meeting of the International Astronomical Union in Prague ultimately came up with a good definition.

"This definition is good because the conditions are both reasonably easy to verify, allowing one to recognize, 'Yes, this is a planet' and, 'No, this is not one,'" she wrote in an e-mail from Prague.

The original proposal presented to the astronomers defined a planet as any round object that independently orbits the sun. That would have kept Pluto as a planet and added Pluto's moon Charon, the asteroid Ceres and the newly discovered Xena to the list of planets.

"I think there is enough contrast between the major planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn Uranus, Neptune) and the smaller bodies (Pluto, Ceres, etc.) that it is useful to make a distinction between them," Willson wrote. "The original proposal did not make that distinction, and I think that is what bothered a lot of astronomers."

Ultimately, Willson said the astronomers' vote signified the distinctions between bodies in the solar system that they want to stress.

"We like the idea of distinctions based on something that can be measured, such as roundness," she wrote. "The measurement that distinguishes the eight big ones from the rest is that they dominate their neighborhoods. They are also the only bodies capable of perturbing each other; the smaller ones, including Pluto, have no measurable effect on the motions of the major planets."

And so changing Pluto's status makes a lot of sense to Willson.

"When Pluto was first discovered, it was thought to be perturbing Neptune," she wrote. "That has since been demonstrated not to be the case, so in that sense, we are finally correcting a mistake made seventy-some years ago."

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Quick look

Lee Anne Willson, a University Professor of physics and astronomy at Iowa State University, joined the world's astronomers in voting today for a new definition of a planet that demotes Pluto to a dwarf planet.

Quote

"We like the idea of distinctions based on something that can be measured, such as roundness. The measurement that distinguishes the eight big ones from the rest is that they dominate their neighborhoods. They are also the only bodies capable of perturbing each other; the smaller ones, including Pluto, have no measurable effect on the motions of the major planets."

Lee Anne Willson, University Professor of physics and astronomy at Iowa State