Orbital objects

A new definition of planet may enlarge exclusive club

August 16, 2006|By MICHAEL STROH | MICHAEL STROH,SUN REPORTER

Brace yourself: The solar system is about to get a whole lot bigger.

Throughout history only nine orbs in the sun's entourage have earned the title "planet." But in a change bound to ripple through classrooms and publishing houses across the country, astronomers meeting in Prague, Czech Republic, today are proposing to add three new objects to the solar system's most exclusive club.

And that's just for starters. Astronomers said dozens - even hundreds - of new "planets" could be added to the official rolls in coming years.

"Our solar system is changing in fundamental ways," said Richard Binzel, a planetary scientist at the Massachusetts Institute of Technology and a member of the International Astronomical Union committee proposing the new planet candidates.

"I'm sure the schoolchildren of America are up to the challenge," he added.

The recommendation must still be approved by the IAU, astronomy's governing body and official arbiter of celestial nomenclature. If it passes, it would bring the number of official planets to 12 and mark the first expansion of the rolls since 1930, when Pluto was discovered.

Today's announcement is the culmination of a rancorous two-year debate over what qualifies as a planet, a definition that scientists until now have managed to live without.

In 2004 the IAU formed a committee to address the question, mostly to quell long-standing bickering over Pluto's planethood credentials and figure out what to call the slew of new Pluto-scale iceballs that astronomers were discovering beyond Neptune.

The new definition is also intended to help classify the odd, planet-like objects increasingly found orbiting other stars.

According to the proposed definition, a planet is any object that is spherical as a result of gravity, that orbits a star and that is itself neither a star nor a satellite of another planet.

Nature decides

"In the end, it's nature that tells whether an object is going to be a planet, not an arbitrary line in the sand," said MIT's Binzel.

Based on the new criteria, the revised planetary lineup includes: Ceres, the solar system's largest asteroid; 2003 UB313, an icy orb that holds the record as the most far-flung object in the solar system; and Charon, Pluto's largest moon.

Charon is only 70 miles larger than Pluto, which itself is just two-thirds the size of Earth's moon. But scientists said that it passes the planet test because of not size but gravity.

Most moons in the solar system, including Earth's, have a center of gravity that lies deep below the surface of the host planet. Charon's center of gravity lies in space between the two planets, essentially making it and Pluto a double planet.

IAU officials said that at least a dozen other objects are on its potential planet "watch list."

Lots of `plutons'

The IAU committee also proposed a new category of planets called "plutons" - Pluto-size objects that reside in a little-explored zone beyond Neptune dubbed the Kuiper Belt. Because the icy orbs found there are smaller and trace more unusual orbits than the classical planets, astronomers felt they merited a special category.

Pluto and two potential inductees - Charon and 2003 UB313 - would be plutons. Many more could be on the way: Since 1992, more than 1,000 objects have been discovered in the Kuiper Belt.

Astronomers emphasized yesterday that the definition's specifics could change in the days ahead as 2,500 astronomers from 75 countries debate the proposal at the International Astronomical Union's general meeting in Prague.

A final vote is scheduled Aug. 24.

Science goes on

But even astronomers concede that whatever the IAU decides, it will have little impact on workaday science.

"In scientific discussions, the word planet is hardly ever used," noted Michael Brown, the California Institute of Technology astronomer who led the team that discovered 2003 UB313 and several other planet candidates.

The planet debate has struck a nerve nonetheless.

"To our generation, the word planet meant something special," said Brown. "I had a poster on my wall as a kid of the nine planets and would trace out the orbits with my finger. With 53, it's not going to happen."

But Alan Stern, a planetary scientist at the Southwest Research Institute in Boulder, Colo., says that science must change to reflect the reality of new discoveries.

"Whoever said that the number of planets has to be easily memorizable? Life's complicated. Is anybody bothered by the fact that there are more than 50 stars or 50 rivers or 50 mountains?" said Stern. "I don't know where this idea that nine is a special number came from."

Stern, principal investigator of NASA's New Horizons mission to Pluto, says that advances in computer software and telescope technology are likely to turn up even more objects in the Kuiper Belt in the years ahead, including potentially orbs the size of Mars or even Earth. "It's bizarro land," he said.

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