The debate about Pluto's status returns to Tucson
Courtesy of NASA, This artist's concept, courtesy of NASA, shows the Pluto system from the surface of one of its candidate moons.

Many people grumbled when the International Astronomical Union kicked Pluto off the planet list in 2006, and a simple web search shows the controversy is far from over.

“The great planet debate” returns pages of links on Google, illustrating how contentious the issue remains for both the public and the scientific community.

The debate returns to Tucson this Friday, Nov. 21. Mike Brown, the CalTech astronomer whose discoveries helped lead to Pluto’s ouster as the ninth planet, speaks on “How I Killed Pluto and Why it Had it Coming” at 7:30 p.m. Friday at the University of Arizona’s Integrated Learning Center, Room 120. The talk is free to the public.

In August, the question of Pluto’s status fueled a three-day conference at the Johns Hopkins University Applied Physics Lab entitled “The Great Planet Debate: Science as Process.” During the conference, planetary scientist Mark Sykes and astronomer Neil deGrasse Tyson squared off for a one-on-one debate over the value of various planet definitions and the place for Pluto and other objects in our solar system. Sykes is director of the Tucson-based Planetary Science Institute, and Tyson is director of New York City’s Hayden Planetarium and host of the NOVA Science Now TV series.

The topic resurfaced again in late August, during the Planetary Science Institute’s annual retreat at Tucson’s Westward Look Resort. Larry Lebofsky, a PSI senior education specialist, presented “The Great Planet Debate: A Great Teaching Moment.”

Although some find it distressing that the scientific community can’t agree on what a planet is and what it isn’t, Lebofsky, who also is co-editor of Meteorite magazine, sees it as an opportunity to explain how science works.

“Science is not black and white,” Lebofsky said. “The answers aren’t always ‘yes’ or ‘no.’ There can be a range of answers that suit different scientific needs.”

To illustrate this point when teaching, Lebofsky asks groups of K-12 students to categorize balls of different sizes, colors and textures. Some students organize the balls by size, while others divide them by sport, color or some other system.

Then, Lebofsky shows the students that the same can be done with solar system objects. Some are gas giants, some have atmospheres, some are rocky, some are large, some are small.

“I let the students decide whether they think an object is a planet, and they disagree and come up with different definitions,” he said. “Then I tell them, ‘Guess what? Scientists look at it the same way. They can’t agree because they look at objects in different ways, too.’”

Sykes, for instance, says that if a non-stellar object is massive enough to be round and orbits a star, it ought to be a planet. The key here is that once an object gets that large, important geophysical processes begin. He lists 13 planets for our solar system, with room for more as new discoveries are made beyond Pluto.

On the other hand, Tyson wants to throw out the word “planet” and come up with new words and ways for organizing planet-like objects. He said Pluto is more like objects in the Kuiper Belt, a region of icy bodies beyond Neptune’s orbit.

Competing viewpoints and debate are what science is all about, and this high-profile argument provides educators with an opportunity to teach students that science is a process, not just a collection of facts, Lebofsky said.

Students learn to think critically when they’re asked to study and compare the properties of planets and moons, he added.

 “You look at Mars, and see canals,” he said. “And you notice they look a lot like dry riverbeds in Arizona. So you think there might have been water flowing. But then you go to an object like Titan (a moon of Saturn that is larger than the planet Mercury) and you see similar canals. But you discover it’s 100 degrees below zero on Titan, and they couldn’t have been cut by water. So then you think it might be another fluid, something that’s still liquid at those temperatures.”

“Facts change,” Lebofsky added. “What we know now may not be what we know two or three years from now. But learning to think critically and understanding how scientists organize facts to develop theories are lessons that will serve students for a lifetime.”

For more on the Great Planet Debate go to and search for “Great Planet Debate Ends in Stalemate.”

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