Pluto may lose status as solar system's ninth planet

January 30, 1999|By BRUCE HAMILTON

Some say Pluto isn't planetary, but local star-gazers disagree.

For 69 years, the icy ball of rock and gas has been called a planet. But several scientists, including a University of Maryland astronomer, are thinking of revoking that title.

"It has been clear for decades that Pluto did not fit in with the pattern of the other planets," wrote Mike A'Hearn, a University of Maryland professor, in an Internet statement.

A'Hearn serves as president of the International Astronomical Union's Planetary Systems Sciences Division. The union recently held a conference, renewing the debate over Pluto's status.


A'Hearn wants to classify it as a "Trans-Neptunian Object." Pluto would be the first in that category, which would include asteroids in the Kuyper Belt.

Pluto is an oddball for several reasons. With a diameter of 1,440 miles, it is the smallest planet. Earth's moon is bigger.

All the other outer solar system planets are gaseous but Pluto is solid. It also has the most eccentric orbit, an ellipse with a period of 248 years that occasionally brings it closer to the sun than Neptune.

There is no strict definition of "planet," a Greek word meaning "wanderer." Before Clyde Tombaugh discovered Pluto in 1930, there were only eight known planets in the solar system.

Whether Pluto should be subtracted is debatable.

"I think it's still a planet," said Rodney Martin, a teacher who runs the Washington County Planetarium. "It has a lot of the characteristics that we associate with planets," he said, such as an atmosphere, seasons and weather.

"I give Pluto credit as being a planet," said Frank Moon, president of Tri State Astronomers. "I don't know what level of expertise you would have to achieve to say it's not." Asteroids tend to be potato-shaped, he said.

Pluto is a sphere because it is massive enough to have the gravity needed for that form. Moon also points to its atmosphere as evidence of planet-ness. "A non-planetary object is unlikely to have any atmosphere at all," he said.

Elizabeth Wasiluk, director of the Berkeley County Planetarium, said she has mixed feelings. "Personally, I don't know. I'd hate to say it isn't a planet. For so many years I believed it was. Why don't we just call it a possible planet since we're not sure?"

Jeanne Mozier, a Berkeley Springs, W.Va., astrologer, said her star charts won't change either way. "It's not going to change anything I do," she said. "Pluto has its place in the symbology."

The decision to change Pluto's classification is "a practical matter," according to A'Hearn, "rather than a matter of scientific definition." Planet or not, Pluto won't change. A decision will be made "if it provides useful distinctions," according to A'Hearn.

Neptune is now the planet furthest from the sun. On Feb. 9, Pluto once again moves past it, according to Wasiluk. Will Pluto become known as a former planet?

"The jury is still out," Wasiluk said.

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