SOLAR SYSTEM CLASSIFICATION: HOW MANY “PLANETS” ARE THERE?

The current debate regarding the definition of a “planet” provides an excellent opportunity for students to experience the process of science. This is a classic scientific problem – given a number of similar objects, how can we classify them into categories that are descriptive, accurate, and useful in terms of understanding the commonalities, histories, and origins of these objects? Classification is the first step in examining and understanding the natural world and allows scientists to communicate using a common vocabulary across continents, time, and languages.

The question of solar system classification has also captured the public’s attention in ways that illustrate many of the challenges faced as scientists attempt to communicate their process and findings to the general public. The definition of the word “planet” has been debated for thousands of years and we are currently learning about the objects in our own solar system and in other extra-solar systems at an amazing pace. Geologists and astronomers use many different words to describe the same object; Pluto is a planet, Kuiper Belt Object, small body, dwarf planet, trans-Neptunian Object, etc. The general public, educators, the media, and children have reacted emotionally to this question and want it resolved quickly. Unfortunately, the scientific process is slow and measured; this debate will not be decided by popular vote or a letter-writing campaign to “save Pluto.”

I have designed a hands-on, inquiry-based activity wherein students explore how they would classify a group of actual solar system objects based solely on physical properties such as density, inclination, diameter, distance from the object they orbit, etc. Students do not know the names of these objects, although it may help them to infer some of the more easily recognizable objects. Students decide how to group the objects into 8 or fewer categories that can be clearly distinguished from each other. This activity serves as (1) an introduction to the component bodies of our solar system, (2) a chance to classify objects based on data rather than simply “what it looks like” or personal preference, and (3) an opportunity to talk about the reality of the scientific process and how emotions, personal opinions, and politics can challenge an ideally impartial and scientific pursuit.