I recently heard a science teacher say that the astronmers’ current debate and formulation of a new definition for a planet was great because it could be used to teach students about the nature of science. Right on!

In a recent New York Times article, Dennis Overbye wrote that “a committee of astronomers and historians has proposed a new definition of the word “planet” that would expand at a stroke the family of planets from 9 to 12 and leave textbooks and charts in thousands of classrooms out of date.” The definition would result in the addition of of three new planets (go to a USA Today article to see a wonderful graphic on the “new solar system.”

The new definition (very abbreviated form here) which is that an object (to be called a planet) be massive enough that gravity has formed it into a sphere and that it circles a star and not some other planet.

Some astronomers classify the new definition as “terrible.” Other astronomers want the definition approved.

Some astronomers claim that there are really only eight planets (you can name them, starting with Mercury and ending with Neptune. That would leave out Pluto, as many astronomers already have. But the new definition would put Pluto back into the family, and add Charon, Ceres, and newly discovered “body” 2003 UB313, nicknamed Xena.

This situation is a perfect setting for classroom debate. Students could make use of articles that have been published online, and could reference the International Astronomical Union conference taking place now in Prague. If you go to this site, you will find this statement: The IAU is the arbiter of planetary and satellite nomenclature since its inception in 1919. The boundary between planets and other solar system objects has never been defined and the recent discoveries of new objects in the solar system has made it necessary for the IAU to address this issue.

The decision process and a draft Resolution for the Definition of a Planet will be published in the conference newspaper during the first week of the General Assembly.

Using a strategy known as “structured controversy” students, organized into small teams, take a side, and research the planetary definition issue to prepare for the debate. You might refer to Chapter 9 of the Art of Teaching Science for more details on this strategy. When you reach this page, scroll down, and you will see a graphic outlining the strategy.

What do you think of the new definition?