The decision by the International Astronomical Union (IAU) in 2006 to redefine the term planet has caught the attention of the public and students from grade school to graduate school. The IAU’s decision has not changed the makeup of the solar system; it has merely offered a different way of thinking about the bodies that make it up. This activity will enhance students’ understanding of these new definitions through the use of a research-based instructional strategy called direct vocabulary instruction. This middle school vocabulary activity was developed for NASA’s Dawn mission by educators at Mid-continent Research for Education and Learning (McREL) and their colleagues on NASA’s Dawn team.