Teaching What a Planet Is

In the latest issue of Astronomy Education Review (AER), there is an interesting article titled, "Teaching What a Planet Is: A Roundtable on the Educational Implications of the New Definition of a Planet." The article looks at the International Astronomical Union's controversial definition of a planet from the points of view of 14 astronomers and educators. From AER:

We now have a reasonable definition of a planet, including the new category of dwarf planet. Why, then, does the public find this decision so controversial, especially in America? I suggest the following three reasons:

1. Bad connotations of the word dwarf. Although astronomers have a long tradition of using this word (e.g., dwarf star, dwarf galaxy), this is not a common word in general usage. To many people, apparently, dwarf has a negative connotation, which was not intended by the IAU. (There may also be some problems with translating this word into other languages.)

2. Affection for Clyde Tombaugh and the "poor boy makes good" story of his discovery of Pluto. This seems to be primarily an American reaction. Some of my non-U.S. colleagues suggest that much of the support for Pluto could be a form of U.S. nationalism.

3. Concern about teaching the new definitions or about asking students to "unlearn" the nine planets they memorized in grade school. This seems to me to be a minor concern. This is only nomenclature, and classifying it as a dwarf planet in no way diminishes the scientific importance of Pluto. I see this as a teaching opportunity. At the simplest level, it is a story of new discoveries: (1) that Pluto is a lot smaller (less massive) than it was thought to be when it was discovered and classified as a planet, and (2) that we have found other large trans-Neptunian objects (of which Pluto is the prototype) that reveal a fascinating part of the Solar System that was undreamed of when Pluto was discovered. An even more important lesson about the nature of science is that (3) scientists change their ideas when new data become available.