Skip to main content

Three New Planets with the Stroke of a Pen? Great News for Science Teachers

On August16th, a panel convened by the International Astronomical Union in an attempt to voteon the definition of what a planet is. In doing so, they raised the possibility of increasing the number of planets from nineto twelve. Imagine--adding three planets to the solar system with the strokeof a pen. What a thrilling opportunity for science teachers, and for so manyreasons. 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 youthink about it, science teachers probably spend too much time fillingchildren's heads with science facts. Our textbooks are overstuffed, ourassessment systems emphasize memorization over critical thinking, and westruggle 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 oursolar system meet our current definition of a planet." We shouldn't besurprised that students struggle to understand scientific knowledge as asocial, human construct. But the IAU panel's recommendation gives us anopportunity to engage our students in conversations about what science is andhow it works.

Asteachers, do we want our students to walk away from our classrooms thinkingthat scientists know all there is to know about a field of study? Or do we wantthem to view science as an organic, evolving pursuit of understanding? In orderfor students to understand science, they need opportunities to examine whyscientific knowledge is subject to revision, and more importantly, the processthrough which scientists achieve consensus. The debate about our solar systemgives teachers a great opportunity to showcase science as a social process andhelp students understand that scientific knowledge can be tentative.

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

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

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

I see thisas a great opportunity to take advantage of my students' natural curiosity. Iam still thinking about how I'll do this, but I think I'll begin by doing someactivities that help them to appreciate how gravity works (at a level that isappropriate for them) and the difference between orbit and rotation. Then, I'llpresent them with what we know about the planets and say 10 or 15 other solarsystem bodies (density, mass, volume, radius, average distance from the Sun,what the object orbits (the Sun or one of the nine current planets), and soon). I'll include moons of several planets and the objects that would beconsidered planets under the new definition. I might ask my students to work insmall groups to use the information to categorize all of the objects as eitherplanets or smaller solar system bodies, and to come up with definitions tosupport their decisions. I'm sure that there will be differences between howeach group came up with categories and definitions, and that's the teachablemoment. We could set up a panel from each group to debate the proposeddefinitions, 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 theirgroups to discuss the merits of the leading contenders, and return to the panelwith recommendations. I'll conclude the unit by presenting the IAU's proposeddefinition and recent discoveries about objects that led scientists toreconsider this question and convene the panel, and asking students tore-categorize objects using the scientist's current view.

Our sciencetextbooks will have to be rewritten. What a thrilling opportunity foreducators. No matter how you pursue this news in your science class, I hopeyou'll take advantage of this unique news event. Let's help students experiencescience as it really is, and show them how new technologies and discoveries notonly give us new evidence and insights into how the world works, but challengeus to revisit our earlier definitions and ideas. Do our current ideas stillhold up in light of new evidence? Do we really know everything there is to know?

Aboutthe author:


Read ityourself: the IAU Resolution: The actual text

Questionsand Answers about the IAU resolution:

Media/Images/movieto accompany IAU resolution:

Defining Moments: The Saga's History

Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: