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What One Fewer Planet Means to Our Worldview

By Shankar Vedantam
Washington Post Staff Writer
Monday, August 28, 2006

Is Pluto a planet?

The world's astronomers met in Prague last week to vote on this question, and in a sort of cosmic game of "Survivor," they voted Pluto off the solar system.

Many people were anguished. One colleague asked, "Don't you think it's at least possible that somewhere we're being voted off the solar system?"

This space is designed to explore human behavior, and the topic today is why people care so intensely about whether Pluto is a planet. As Johns Hopkins astronomer William P. Blair put it, "Pluto hasn't changed just because of our nomenclature. It is the same today as it was yesterday and as it has been for thousands of years."

Blair is basically asking whether it is rational to care about definitions and categories, which always involve a degree of arbitrariness. (The astronomers, after all, had to take a vote . It could have gone another way, and we would now have 12 planets instead of eight.)

While Blair makes perfect sense, he also misses the point entirely: Whether or not it is rational, human beings do care intensely about definitions. Some of our most contentious public debates are about definitions. Is the conflict raging in Iraq a civil war? In the abortion debate over when life begins, what exactly do we mean by life?

Definitions and categories are the handles by which we grasp the world. If we change the handles, we change how we see the world.

Peter Lipton, a University of Cambridge philosopher of science, argues that science itself is a composite of external reality and human interpretation of that reality. This is why, after a paradigm shift such as the redefinition of a planet, reality itself can feel different. Whether we say the solar system has eight planets or nine or 12 makes no difference to the solar system, but it makes an enormous difference to us.

Much of the business of science, in fact, has to do with the construction and demolition of categories. No sooner had the astronomers devised their new definition of a planet -- the idea that planets need to be large enough to "clear out their neighborhoods" of smaller bodies -- than others began testing the solidity of the definition.

"You have the Trojan asteroids that arch 60 degrees in front of and behind Jupiter, so what in the world they mean by 'clearing out' I don't know," said Owen Gingerich, a Harvard astronomer and historian.

If Pluto cannot be a planet because its orbit intersects with Neptune's, how is it that Neptune -- which definitely is a planet -- can have an orbit that intersects with Pluto's? "Clearly Neptune has not cleared its orbit," declared Karl Glazebrook, an astronomer at Johns Hopkins.

Defenders of the new definition, including outgoing International Astronomical Union President Ronald D. Ekers and Caltech astronomer Mike Brown, dismissed such criticism as lawyerly nitpicking, a sure sign that a debate over definitions will rage for some time.

The reason people care so much about one definition rather than another is because definitions are markers for group identity, said Barbara King, a biological anthropologist at the College of William and Mary who studies social behavior in primates. Wanting to see the world a particular way is an extension of our innate tendency to form groups, coalitions and tribes.

For a Democrat who thinks the war in Iraq is a mistake, for example, it makes sense to define the ongoing carnage as a civil war. For a Republican who thinks the war is justified, it makes sense to define the internal conflict as a hurdle that can be overcome. Arguing about the definition of a civil war, therefore, is an effective (and ostensibly high-minded) shortcut to arguing about politics.

But what in the world does eight planets or nine have to do with group identity and social behavior? Knowledge, King said, is also wrapped up in social experience. King's 12-year-old daughter, for example, is upset that Pluto is no longer a planet, partly because one of her cherished memories is of a trip to Flagstaff, Ariz., where the family went to see the place where an astronomer discovered Pluto. Questioning the importance of Pluto implicitly undermines the importance of that family trip.

People often find it threatening when the categories with which they are familiar are challenged, agreed New York psychiatrist Jack Drescher, who has studied how boundaries of gender and sexuality are constructed. Drescher recently wrote a paper exploring the issue, and cited a 1948 comment in a book by sex researcher Alfred Kinsey:

"The world is not to be divided into sheep and goats. Not all things are black nor all things white. It is a fundamental of taxonomy that nature rarely deals with discrete categories. Only the human mind invents categories and tries to force facts into separated pigeon-holes. The living world is a continuum in each and every one of its aspects."

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