Experts' vote could mean demotion for Pluto

Astronomy group to offer definition in 'planet' debate

Jim Erickson, Rocky Mountain News

Published August 12, 2006 at midnight

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NASA's New Horizons spacecraft is streaking at 43,500 mph toward a 2015 rendezvous with Pluto. When it arrives, will it encounter the solar system's ninth planet or a demoted celestial has-been?

Nearly 3,000 astronomers and planetary scientists from around the globe will gather in Prague, Czech Republic, next week to answer that question by voting on an official definition of "planet."

For decades, schoolchildren have learned that nine planets circle the sun. But that number could drop to eight or soar to dozens, depending on the new International Astronomical Union definition.

The wording of the proposed definition has been a guarded secret since it was drafted in June.

The clandestine nature of the deliberations has heightened anticipation among scientists who have argued about Pluto's status for years. Many say it's time to resolve this thorny issue and move on.

"I have a well-developed opinion on this topic, but I'm willing to abide by whatever the IAU decides," University of Colorado planetary scientist Larry Esposito said.

"I'm willing to give up my position for the interest of a good, solid definition that professional astronomers can agree to," said Esposito, a team leader on the Cassini mission to Saturn.

News leaks about the planet definition began to spout late this week, as the authors prepared to present a draft resolution to the IAU's executive committee Sunday in Prague. The IAU is the official arbiter of all issues related to astronomical nomenclature.

In a story that aired Thursday, unnamed sources told National Public Radio the proposed definition would include Pluto in a new class of small planets. A source also told the Rocky Mountain News on Thursday that a member of the seven-person definition panel said Pluto will remain a planet.

IAU Vice President Bob Williams described the reports as "interesting speculation" but would neither confirm nor deny them.

'Just not big enough'

The need for a bulletproof definition gained urgency last year after the discovery of 2003 UB313 - nicknamed Xena - an icy object in the solar system's outer fringes.

Hubble Space Telescope photos show Xena has a diameter of 1,490 miles, give or take 60 miles. Pluto is 1,422 miles across.

If Xena rivals Pluto in size, shouldn't it be classified as a planet? The leader of the discovery team, Caltech's Michael Brown, thought so; he dubbed Xena "the 10th planet."

But others, including CU's Esposito, consider Xena, Pluto and their kin as little more than cosmic debris.

Xena and Pluto reside in a band of icy, sun-orbiting bodies beyond Neptune, called the Kuiper Belt. More than 1,000 so-called Kuiper Belt Objects have been discovered since 1992.

"These Kuiper Belt Objects are to my mind more leftovers than planets, more debris or detritus," Esposito said.

Esposito's solar system would contain eight planets. Pluto, the faint, far-off iceball discovered by U.S. astronomer Clyde Tombaugh in 1930, would not make the cut. Same for the rest of the Kuiper Belt and all the asteroids, rocky bodies that orbit the sun between Mars and Jupiter.

"They're just not big enough," Esposito said. "I would reserve the planet designation for something more like the planets Mercury through Neptune."

If pressed, Esposito said he would be willing to grant Pluto - but no other Kuiper Belt Objects or asteroids - planethood due to "historical error."

When it was discovered, astronomers mistakenly believed Pluto was as big as Earth, he said.

Long road

So what's a planet? To settle the issue, the International Astronomical Union formed a 19-member scientific committee. After working for about two years, the group disbanded early this year without settling on a definition, said member Alan Stern, a Boulder planetary scientist.

This spring, the IAU formed a new seven-person panel in a last-ditch effort to reach a consensus before the 12-day IAU general assembly, which starts Monday. Members are expected to vote on the resolution Aug. 24.

Harvard University astronomer and historian Owen Gingerich chaired the new panel. The group reached a unanimous recommendation after meeting in Paris for two days in June.

"If we didn't get it settled in time for Prague, there isn't another congress for three years," Gingerich said. "And this situation is awkward to live with."

Gingerich said he can't divulge the resolution's wording before it is presented in Prague. But the definition consists of "three or four short paragraphs" and avoids jargon that would confuse laymen, he said.

Gingerich said the starting point for his group's deliberations was the work of the disbanded 19-member committee.

Stern, lead scientist on the $700 million New Horizons mission to Pluto, said the earlier committee developed three "very slightly different" versions of a definition. All three included the idea that a planet must be smaller than a star, yet big enough that its own gravity pulls it into a roughly spherical shape.

One of the three versions also stated that a planet must be bigger than 620 miles (1,000 kilometers) in diameter. That's roughly the size at which celestial objects become round due to self-gravity, Stern said.

Another version said a planet must be bigger than 1,240 miles (2,000 kilometers) across.

"It's a shame the whole thing fell apart, because it was really done," Stern said. "It was sort of as if we'd agreed not only on what brand of car to buy but what model, and we were just arguing about the paint color."

The IAU's Williams, who finalized the draft resolution's wording this week, said it presents a definition "that is not likely to be too different than those three (from the 19-member committee), but there may be some new wrinkles in it."

As for Stern's personal definition of a planet, it's a simple one: An object large enough to be rounded by self-gravity but smaller than a star.

Chance for Pluto

Under that definition, large Kuiper Belt Objects, such as Pluto and Xena, qualify as planets. So do a few of the largest asteroids.

Stern and a few other planetary scientists call such objects dwarf planets.

The Alan Stern solar system contains anywhere from a few dozen dwarf planets to 1,000 - many of them still waiting to be discovered.

"Things like Pluto are dwarf planets in the same way that a Chihuahua is a dog," he said. "It's still a dog. It's just a small type of dog."

David Grinspoon, curator of astrobiology at the Denver Museum of Nature & Science, sees the debate over planetary semantics as "a little bit silly" because it diverts attention from more important issues.

Even so, he had a personal definition of planet on the tip of his tongue: A gravitationally rounded object in orbit around a star.

"And yes, that means there are many, many more than nine planets in our solar system - hundreds, possibly thousands," said Grinspoon, author of the book Lonely Planets.

"To me, the fact that we define it in such a way that it means we haven't discovered all the planets in our solar system is kind of exciting and enticing, and I don't find it bothersome at all," he said.

More important than the wording of the definition is the fact that a burst of discoveries over the past decade forced astronomers to grapple with the planet issue, Grinspoon said.

In addition to the Kuiper Belt Objects at the edge of our solar system, scientists have discovered more than 200 planets orbiting stars beyond our sun - the so-called extrasolar planets.

So what if you you're a planetary scientist or an interested layman and you disagree with the new IAU definition - assuming that it passes in Prague?

"You can ignore it if you wish. We have no authoritative power," Williams said.

"But there is some cachet associated with the international body of astronomers," he said. "And it is our intention to really do things that are sensible - that avoid the giggle factor - and yet reflect the increased understanding we have of our solar system."

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