On August 16th, a panel convened by the International Astronomical Union in an attempt to vote on the definition of what a planet is. In doing so, they raised the possibility of increasing the number of planets from nine to twelve. Imagine--adding three planets to the solar system with the stroke of a pen. What a thrilling opportunity for science teachers, and for so many reasons. The vote raised a lot of questions and has been followed by a week of controversy. Today, the definition of "planet" may finally be decided.

When you think about it, science teachers probably spend too much time filling children's heads with science facts. Our textbooks are overstuffed, our assessment systems emphasize memorization over critical thinking, and we struggle to find time for inquiry. For decades, we've been telling our students "there are nine planets in our solar system" rather than "nine objects in our solar system meet our current definition of a planet." We shouldn't be surprised that students struggle to understand scientific knowledge as a social, human construct. But the IAU panel's recommendation gives us an opportunity to engage our students in conversations about what science is and how it works.

As teachers, do we want our students to walk away from our classrooms thinking that scientists know all there is to know about a field of study? Or do we want them to view science as an organic, evolving pursuit of understanding? In order for students to understand science, they need opportunities to examine why scientific knowledge is subject to revision, and more importantly, the process through which scientists achieve consensus. The debate about our solar system gives teachers a great opportunity to showcase science as a social process and help students understand that scientific knowledge can be tentative.

And let's face it--teenagers love to argue. Now we can show them that scientists do this too--they even argue about how we define objects in our solar system! The debate about what constitutes a planet has lasted for years! Although the panel's decision pertains to a major issue that is international in scope, scientists debate and discuss ideas and results in a search for meaning just about every day. Is this data point anomalous? Is this a valid measurement? Are these results reasonable? What constitutes a fair test of our hypothesis? Can we generalize these results to situations that are in some respects dissimilar to our experiment?

The IAU's recommendation provides a model of the process of science. Yes, it's a debate about a definition but the simple fact is that unless we agree on how we will measure something, or what counts as data or evidence, we'll continue to have difficulty talking about and agreeing upon what we observe. To quote one of my favorite films, "what we'll have here is a failure to communicate."

Some may argue that the definition of a planet is beyond the reach of K-12 students. I don't think so. Middle school students are fascinated with the "weird and mysterious." It doesn't take long in science class for them to see that Pluto is a bit of an oddball--its very distant, its orbit is very tilted, and sometimes it isn't even the farthest planet from the Sun. I would bet that 99 out of a 100 of my middle school students would challenge Pluto's status under our previous vague definition of planet.

I see this as a great opportunity to take advantage of my students' natural curiosity. I am still thinking about how I'll do this, but I think I'll begin by doing some activities that help them to appreciate how gravity works (at a level that is appropriate for them) and the difference between orbit and rotation. Then, I'll present them with what we know about the planets and say 10 or 15 other solar system bodies (density, mass, volume, radius, average distance from the Sun, what the object orbits (the Sun or one of the nine current planets), and so on). I'll include moons of several planets and the objects that would be considered planets under the new definition. I might ask my students to work in small groups to use the information to categorize all of the objects as either planets or smaller solar system bodies, and to come up with definitions to support their decisions. I'm sure that there will be differences between how each group came up with categories and definitions, and that's the teachable moment. We could set up a panel from each group to debate the proposed definitions, and hold a closed ballot vote that calls for a unanimous decision. If a decision isn't reached, the panel members would have to return to their groups to discuss the merits of the leading contenders, and return to the panel with recommendations. I'll conclude the unit by presenting the IAU's proposed definition and recent discoveries about objects that led scientists to reconsider this question and convene the panel, and asking students to re-categorize objects using the scientist's current view.

Our science textbooks will have to be rewritten. What a thrilling opportunity for educators. No matter how you pursue this news in your science class, I hope you'll take advantage of this unique news event. Let's help students experience science as it really is, and show them how new technologies and discoveries not only give us new evidence and insights into how the world works, but challenge us to revisit our earlier definitions and ideas. Do our current ideas still hold up in light of new evidence? Do we really know everything there is to know?

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More information:

Read it yourself: the IAU Resolution: The actual text

Questions and Answers about the IAU resolution:

Media/Images/movie to accompany IAU resolution: http://www.iau2006.org/mirror/www.iau.org/iau0601/iau0601a_video.html

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