The beginning of a typical solar system "unit" in a traditional introductory astronomy course often consists of an overview of the different object types--planets, moons, and debris (asteroids, comets, and meteors), and now also Pluto's home, the Kuiper belt--prior to coverage of formation and more detail about the various object types. An important distinction that needs to be made prior to coverage of solar system formation or discussions of individual planets is between the two planet types: the Earth-like or "terrestrial" planets, and the Jupiter-like or "Jovian" planets. The following is a description of an activity designed to establish these categories through conclusions drawn by students after a guided analysis of planetary data.