Is Pluto a planet? You decide

From the time of its discovery in 1930 until 1992, Pluto was a happy, full-fledged planet. A little odd maybe with its tipped, eccentric orbit and tiny size, but the ninth planet just the same. That all began to unravel when asteroid 1992QB1 was discovered by astronomers David Jewitt and Jane Luu on August 30 that year.

QB1 was the first object found in the Kuiper (KYE-per) belt , an orbiting reservoir of asteroids similar to but much larger than the asteroid belt between Mars and Jupiter but much larger. While the inner belt is populated by rocky asteroids, the Kuiper Belt objects, located farther from the sun in solar system's deep freeze zone, are composed of mixtures of water, methane and ammonia ices and rock.

QB1 is about 100 miles across and orbits a little farther from the sun than Pluto. Here was an asteroid out near Pluto orbiting in a similar way. Were there others? QB1 would turn out to be the proverbial tip of the iceberg. As searches expanded and bigger telescopes were put to the task, dozens then hundreds of new icy asteroids were found in the Kuiper Belt. As of today, over 1,000 are known with an estimated 70,000 larger than 100 km (62 miles) remaining to be found. Suddenly Pluto had lots of company. In 2005 of Mike Brown and his team at Palomar Observatory found Eris, an asteroid more massive than Pluto and nearly equal in size.

That's when astronomers began to wonder whether Pluto should still be considered a planet or just one of the many asteroids in the Kuiper Belt. The discovery of so many small worlds at the edge of the solar system made us realize we lived in a much less tidy but vastly more interesting place than we'd thought.

Dr. Hal Levison at the Department of Space Studies at Southwest Research Institute   in Boulder and Alan Stern, the principal investigator for the New Horizons mission to Pluto, went head to head on the topic at a workshop I attended in Boulder, Colorado earlier this month at the  Laboratory for Atmospheric and Space Physics .

While Levison plants his flag firmly in the Pluto-as-asteroid-camp, Stern would not only keep Pluto a planet but expand the definition to include many of the solar system's larger moons. They share common ground in their mutual dislike of the definition of a planet introduced and voted on in August 2006 by members of the International Astronomical Union. One may wonder whether science is best served by 420 astronomers taking a vote on a proposal.

This is what shook out. For a body to qualify as a planet it must:

1. Orbit the sun

2. Have sufficient mass (gravity) to crush itself into a nearly spherical shape

3. Be gravitationally dominant in its orbital zone. In other words, it has to clear its orbit of all other smaller bodies

By this definition, Pluto, unable to clear its orbital neighborhood, loses planetary status. Almost as a kindness, it and several other large, round asteroids, were given the special designation of dwarf planet.

While Levison takes exception on the roundness issue, citing Earth's moon and Jupiter's four largest moons as being round enough to at least in part qualify as planets, he agrees that Pluto and the thousands of other asteroids in the Kuiper Belt are related by location, size (small bodies compared to the 8 planets), inability to clear their orbits and dynamics. The last refers to how the whole lot of them were cast into the outer solar system by the planet Neptune as it migrated to its present orbit in its youth.

"There are 8 important bodies and lots of smaller ones," said Levison. He added that if we went back to the old way of doing business and kept Pluto a planet we'd have a 1000 planets today, since there are nearly 1000 roughly spherical asteroids 250 miles across and larger. "It's a lot easier to go from 9 to 8 instead of 9 to more than a 1000," remarked Levison.

Alan Stern respectfully disagreed. "Just give it the Star Trek test," he said. "Orbit it, look at it and make the most obvious call." If the current definition of a planet were carried forward into a fictional distant century, the conversation between Spock and Captain Kirk might go something like this: "I can't say whether it's a planet yet Jim. We'll first have to determine its shape and study its orbital mechanics before we can classify it. To say it's a planet based on current information would be not be logical Captain." Kirk is fuming at this point.

Stern offers his own "geophysical" definition of a planet that relies on an objects essential qualities rather than on how well it cleans up its orbital neighborhood. Drum roll please:

A planet is anything big enough to crush itself into a sphere but small enough that it's unable to fire up nuclear fusion in its core and burn like a star. He used stars to further illuminate his point. Big or small, hot or cool, double or solitary, stars are still stars. Looking at Hal with a a wry smile, Stern further added (and I paraphrase): "You're a large man and I'm a short one, but we're both men." By Stern's definition, Earth and moon become a double planet, Jupiter's four largest moons are planets as are some well-known asteroids like Ceres and Eris. Objects found orbiting other stars as well as rogue bodies wandering around with no star to call home - all planets.

While I've been one to see Pluto as just a big asteroid, Alan's sweeping definition felt like a fresh breeze of common sense. He pointed out that the "clearing the neighborhood" requirement is heavily biased against objects in the outer solar system whose orbits cover much vaster regions of space that those of planets in the inner solar system. In fact, if you put the Earth where Pluto is, the gravitational might of our planet wouldn't be enough to clear its orbit. Does that mean Earth's not really a planet unless it's close to the sun? Crazy talk.

So what are we to take home about the point-counterpoint about Pluto and planets? At least two things. First, it's going to take time and lots more discussion before astronomers arrive at a definition they can agree upon. Second, science rarely deals in black and white. That's both its frustration (for some) and beauty. Both Levison and Stern see the discovery of the Kuiper Belt asteroids and the hundreds of new planets around stars beyond the sun as one of the great revolutions of astronomy on par with Copernicus' theory of the sun-centered solar system. The fact that definitions are in upheaval means a profound change in our understanding is underway.

Nature whispers in our ears through sophisticated instruments and computer models. Our charge is to listen carefully, take time to draw conclusions and not be wedded to a particular idea too soon. Listening to two scientists with opposing views treat the Pluto -planet issue with humor and respect gave me a new respect for how science works.