It just won’t end. Two years after the International Astronomical Union demoted Pluto from a planet to a dwarf, the bickering goes on.
I’m here at the “Great Planet Debate”, an event put on by the John Hopkins University’s Applied Physics Laboratory in Laurel, Maryland, where several hundred scientists, teachers and members of the public have shown up to watch two scientists duke it out over definitions. The organizers say a Internet video stream of the debate will be posted here soon.
On one side, they’ve got Mark Sykes, director of the Planetary Science Institute in Tucson, Arizona, a Pluto proponent full of good-natured bluster. And on the other is Neil deGrasse Tyson, the hyperarticulate director of the Hayden Planetarium in New York City, who famously pushed Pluto out of its customary box as the ninth planet in his museum’s exhibits. It was supposed to be a debate about planetary definitions — but it quickly became all about that attention-seeking Pluto. The two clearly were having fun as they milked it for all it was worth.
“The word ‘planet’ has lost all scientific value,” says Tyson. “We’re in desperate need of a new lexicon to accommodate our new knowledge.”
Tysons point is that subcategories and new terms should be developed as scientists discover new things — and that strict categories only limit thinking. William Herschel, the discoverer of Uranus, thought he had found a comet. The first asteroids were called planets until it was realized that a huge population of objects sat between Mars and Jupiter. Now, Tyson argues, Kuiper belt object is a good new term for things like Pluto — or dwarf planet.
Sykes is in favor of a more inclusive definition — based on “roundness” — that would include as planets big asteroids such as Ceres. His main problem of the IAU decision was the vagueness of a secondary part of the definition — that a planet must be able to “clear” its neighborhood of other bodies. Sykes says that part of the reason for the IAU decision was a fear that, under a more inclusive rule, the solar system would eventually contain hundreds of planets. “My God, we can’t have more than 10 planets,” he said earlier in the day, holding up his hands in mock horror. “We’re out of fingers!”
But really, this whole event — three days of presentations, posters and panel discussions — has less to do with science and more to do with a debate about how science is taught. (The event also probably had a little to do with maintaining status for NASA’s New Horizons mission to Pluto. And maybe a little to do with making money — think of all the books, T-shirts and bumper stickers that this ongoing debate has spawned and sustained.)
Several generations of people grew up with nine planets, memorizing them in order. Tyson said he got emails that, in the wake of the Pluto demotion, would say: “My Very Energetic Mother Just Served Us Nine Pizzas. ‘I just learned this mnemonic. Now what am I going to do?’”
Sara Seager, an exoplanet astronomer at the Massachusetts Institute of Technology, saw firsthand how the issue still resonates with the general public. On the shuttle ride from her hotel to APL this morning, her driver asked if she was going to the Pluto debate. She said yes. “And he looked at us very sadly and said, ‘Please, please bring Pluto back.’”