Reclassify Planet Pluto? Defenders go into orbit

'War of the Worlds' erupts over labeling orb as a lesser light

January 26, 1999|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

Don't look now, but astronomers say that Pluto, the littlest member of the family of planets, has begun to look less like its siblings, and a whole lot more like the neighbors -- the ice dwarfs at the edges of our solar system.

The resemblance has sparked a proposal by the International Astronomical Union to catalog Pluto as merely the largest of the solar system's 10,000 so-called "minor planets" -- not planets at all, but a grab bag of asteroids, spent comets and other orbiting flotsam.

The idea has unleashed a kind of "War of the Worlds." Pluto's defenders accuse Brian Marsden, director of the IAU's Minor Planets Center in Cambridge, Mass., of ignoring tradition and attempting to demote Pluto from planethood.

Marsden calls Pluto's guardians emotional and their opposition "silly." He insists, "It's a practical convenience for astronomers. There's no demotion involved."

New discoveries in the past seven years have revealed a striking resemblance between Pluto and a growing list of icy objects spotted drifting in the Kuiper Belt, beyond the orbit of the eighth planet, Neptune. The smallest planet now seems more like the biggest of these "ice dwarfs."

But traditionalists and astronomers who specialize in Pluto studies say it's an unseemly, unnecessary and ill-timed assault on the planet's legitimacy.

"My entire career has been based on studying Pluto," said Marc Buie of the Lowell Observatory in Arizona. "At some level it almost feels like a personal slap."

The truth is, Pluto has never fit with its siblings. Almost from the day in 1930 when it was discovered by 24-year-old Lowell Observatory astronomer Clyde Tombaugh, astronomers questioned whether Pluto was really a planet.

It is smaller than Earth's moon, and nothing like the giant gas planets of the outer solar system. Its orbit is tilted 17 degrees from those of the other planets, and it spins backward on its axis.

Pluto wanders as far as 4.6 billion miles from the sun, and as near as 2.8 billion miles. For 20 years of each 248-year orbit, Pluto drifts closer to the sun than does Neptune.

And that's about all anyone knew about Pluto until 1978, when astronomers detected its single moon, Charon.

Charon's behavior revealed that Pluto was only 1,400 miles across. Unlike any of its sister planets, it's composed mostly of water, methane, nitrogen and carbon monoxide ices.

Pluto seemed like an oddball until 1992, when astronomers began finding the first of what may be tens of thousands of icy, comet-like objects orbiting the sun beyond Neptune's orbit.

So far, 89 such "trans-Neptunian objects" (TNOs) have been seen and tracked. They range in size from 50 to 400 miles across. Their realm -- the Kuiper, or Trans-Neptunian Belt -- extends outward from Neptune's orbit to nearly twice that distance from the sun.

From their size, composition and orbits, "it's become quite clear in the last few years that Pluto is one of these," Marsden said. Those that cross inside Neptune's orbit like Pluto have even been named "Plutinos."

Last year, Marsden began to give them names and numbers under the official IAU catalog of minor planets.

The catalog helps astronomers sort out new discoveries from known objects. It now lists 9,913 objects that are neither comets nor planets. Most are asteroids of various types and spent comets.

With about 100 new objects found each month, Marsden knew that No. 10,000 would likely be assigned by this March. Such round numbers are regarded as special honors. So, last year he proposed that the IAU add the TNOs to the minor planets catalog, and make Pluto No. 10,000.

The idea drew unexpected criticism and alternative proposals from astronomers with concerns beyond the obvious impact on textbooks and a science catechism memorized by millions of schoolchildren for 70 years.

Pluto is the only planet never visited by spacecraft from Earth. NASA officials have assured astronomers that the listing would not change their support for Pluto research. But Buie and others worry that it might devalue the planet in the eyes of Congress.

That could threaten funding for Pluto studies, especially the $500 million Pluto Express mission scheduled for early in the next decade.

"Unfortunately, planetary science seems to be dripping with more politics than other scientific fields," Buie said. "It has to do with the big bucks required to carry on these space missions."

In any case, Buie argues, the IAU can't demote Pluto until it can define what a true planet is. And no one ever has.

Buie would bestow planethood on any object massive enough that its own gravity has pulled it into a spherical shape. "Pluto is well above this threshold," he said. (So is Ceres, a 568-mile-long asteroid once ranked as a planet but later demoted to Minor Planet No. 1.)

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