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Is Rekindling the Pluto Planet Debate a Good Idea?

Critics accuse Pluto boosters of beating a dead planet



NASA

Pluto lovers, don't despair: Researchers have not given up the fight for the former ninth planet. Many of them put up a fuss two years ago when the International Astronomical Union (IAU) downgraded Pluto to the status of mere dwarf planet. Now they plan to revive the debate, this time under the banner of public understanding of science.

Researchers on both sides of the issue are set to gather in August at Johns Hopkins University Applied Physics Laboratory in Laurel, Md., for what's being called "The Great Planet Debate: Science as Process." The goal, says the conference's co-organizer Mark Sykes, director of the Planetary Science Institute in Tucson, Ariz., is to teach the public that science is a process of constant revision and refinement. "People should be exposed to that process," he says. "The IAU process gave the impression that science is done by a bunch of scientists voting behind closed doors."

The schedule for the upcoming conference includes back-to-back talks on the current IAU definition of a planet (a round body that has cleared its orbit of competitors) as well as an earlier version, preferred by researchers such as Sykes. Also listed is a talk on "challenges and opportunities" for teachers, who are invited to attend. "One of the problems over the last couple of years," Sykes says, "has been [that] teachers have been confused what to teach"

The lingering resistance to the IAU's decision irks some researchers. "I think fighting it is doing more damage to our reputation than anything," says Harold Levison, a planetary scientist at the Southwest Research Institute in Boulder, Colo. He agrees with the IAU's conclusion, he says, but adds that he would have rather seen planets divided into two groups: major and minor. The IAU could still decide to revisit the subject in a meeting scheduled for next year.

Despite the rift, textbook publishers seem to be taking Pluto's demotion in stride. Some contacted by ScientificAmerican.com say they anticipated it. An early draft of Perspectives on Astronomy (first edition, 2007), written before the August 2006 IAU meeting, excluded Pluto from a map of the planets in the solar system, says text co-author Dana Backman, a planetary scientist at NASA Ames Research Center, Moffett Field, Calif.

"The IAU decision was expressing what we already believed, and had put words to paper about," Backman says. He provided an excerpt of the published first chapter by e-mail, which briefly recounted Pluto's demotion but did not describe it as controversial.

Planetary Sciences (first edition, 2001) counted Pluto among the planets but will exclude it in the second edition, due out in 2009, according to Backman's fellow Ames researcher Jack Lissauer, co-author of the text, who shared the 2007 Chambliss Astronomical Writing Award from the American Astronomical Society with co-author Imke De Pater.

Pluto has always stood out from the other planets. At roughly a fifth the mass of the moon, it is the largest of the icy bodies that make up the Kuiper belt beyond Neptune's orbit. Unlike the four inner (terrestrial) planets, it has a tenuous atmosphere at best.

The outer four Jovian planets are massive and gaseous. None resemble the former ninth planet, which also has a distinctly eccentric, or elliptical, orbit that crosses that of Neptune. "When you look at a plot like that [of eccentricity], there's eight planets," Levison says. "It just jumps out and bites you in the butt."

Then, in 2003 astronomers discovered a tenth "planet," Eris, which displaced Pluto as the largest object in the Kuiper belt. The finding raised the specter of discovering dozens more objects that might also be considered planets, Levison notes. (Pity the poor schoolchildren, forced to learn ever more complicated mnemonic devices.) So two summers ago, the IAU convened a meeting in Prague to decide on the first formal definition of a planet.

An IAU-appointed working group initially proposed to define a planet based primarily on its roundness. That would have included not only Pluto and Eris, but also Pluto's moon Charon and Ceres, the largest asteroid in the belt between Mars and Jupiter.

After a highly publicized debate, the working group modified the definition to require that the round body also had cleared its orbital zone of competitors, which none of those four objects had done. The IAU voters gave their nod and reclassified the aforementioned group (except for Charon) as "dwarf planets."

Not everyone was on board. Researchers working on NASA's New Horizons mission to Pluto and the Kuiper belt, launched in 2006, for example, suddenly found themselves examining a dwarf instead of a true planet. Some 300 planetary scientists signed a petition protesting the IAU definition and calling for a new one.

Those rejecting the dwarf planet label point out that the IAU definition still requires scientific expertise to interpret. Neptune, for example, does not prevent Pluto from crossing its orbit, and Jupiter's orbit is not entirely cleared of rubble—it contains hundreds of rocks, known as the Trojan asteroids, albeit locked in two clumps by the gas giant's gravity.

Lissauer says he and his textbook co-author limited their use of the term dwarf planet because of such caveats. "We don't state the IAU definition of round and clearing its zone," he says, "because we suspect that if a Mercury-mass object were to be discovered at 100 AU [astronomical units] it would be accepted as a planet even if it doesn't clear its zone." (One hundred AU is around nine billion miles, or 15 billion kilometers.)

In a recent issue of the journal Science, Sykes argues the case for the round-body definition. A sufficiently massive object settles into the shape of a ball under its own gravity. When that happens, he says, denser material heads to the core and geologic processes kick in, including volcanism, plate tectonics and erosion.

The round-body definition provides planetary scientists with a useful guide to selecting objects in the solar system that could shed light on processes here on Earth, says Sykes, who is part of the science team for NASA's Dawn mission to dwarf planet Ceres and Vesta, the two largest members of the asteroid belt between Mars and Jupiter. "When you're studying different properties of different objects," he says, "you're saying, how are these things related? The IAU way of organizing things provides absolutely no insight."

Such a claim doesn't match up with evidence in the classroom, says Ed Prather, associate staff scientist and senior lecturer with the Steward Observatory at the University of Arizona in Tucson, who teaches Astronomy 101 to nonscience majors.

Prather says that even before the Prague meeting, he taught his classes to distinguish asteroids and Kuiper belt objects from the eight major planets based on how they formed: Denser material in the protoplanetary disk around the early sun would have congregated closer in, resulting in terrestrial planets, whereas gases floated farther out, forming the Jovian (gas giant) planets.

He says he routinely polls his classes on the controversy and, typically, about 70 percent of students come away thinking: "Yep, there's no reason it should be a planet. It's just a rock sitting out farther away." Defining a planet as a round body creates problems of its own, he notes, because it doesn't automatically distinguish planets from moons.

Whether they embrace or reject Pluto's downgrade, researchers say they are concerned about the public's understanding of the scientific process. "Science is not about organizations providing truth to the masses," Sykes says. "Science is a lot more fun. It's a lot messier," he says.

It's the messiness that worries Prather: "I think this should be a critical discussion among scientists, not among educators or the public." Otherwise, he says, the public might come to perceive scientists as making decisions based on personal feelings and historical convention, not a rigorous process. As soon as that happens, he says, "we've dropped the ball."

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