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.)
S. Alan Stern, a Pluto scientist at the Southwest Research Institute in Boulder, Colo., says it's too early to reclassify anything. As astronomers continue to search the Kuiper Belt, "we may even find objects as big as Earth. We're going to be teaching schoolchildren in 25 years that there are not nine planets, but 90."
Marsden's critics accuse him of "nefarious" efforts outside scientific circles to undermine Pluto's status. He has questioned Pluto's status at least since 1980, but says, "I was not alone about that." Scientists have debated the issue for years.
Marsden is no stranger to controversy. Last March he issued an IAU statement reporting the discovery of a comet calculated to pass within 30,000 miles of -- and maybe collide with -- Earth in 2028. The announcement made news around the world but proved embarrassingly premature.
Improved calculations the next day showed the comet would actually miss Earth by 600,000 miles or more. "I've been been in quite a lot of trouble in the last year," Marsden acknowledged.
University of Maryland astronomer Michael A'Hearn has suggested dual status for Pluto -- listing it as both Planet No. 9 and Minor Planet No. 10,000. Marsden loves the idea.
Stern hates it. He wants a separate list for TNOs, with Pluto ranked as TN-1. But Marsden is opposed.
"Maybe I've been too democratic about it," Marsden said. "Maybe I should have made the decision, and that's that." For now, he's still seeking scientific consensus, but it remains elusive.
Just in case the deadlock persists, the IAU has yet another committee at work. It's searching for an alternative object to be named and honored as Minor Planet No. 10,000.
Pluto, meanwhile, goes on about its business. On Feb. 10, it will cross back into "trans-Neptunian" space after 20 years inside Neptune's realm. It will then reclaim its traditional place as the ninth planet from the sun.
The Planet Pluto Name: Roman god of the underworld. Chosen for initials PL, for astronomer Percival Lowell, who theorized its existence in 1904.
Discovery: Clyde Tombaugh, 1930
Diameter: 1,400 miles
Moon: Charon, 750 miles in diameter
Pluto's Day: 6.39 Earth days
Pluto's Year: 248 Earth years
Distance from sun: 2.8 billion to 4.6 billion miles
Composition: Frozen water, methane, carbon monoxide, nitrogen
Pub Date: 1/26/99