What is a Planet?
Overview: Students learn about the characteristics of planets, comets, asteroids, and
trans-Neptunian objects through a classification activity. Students can then apply what
they have learned by participating in a formal debate about a solar system object
discovered by the New Horizons spacecraft and by defining the term ‘planet.’
Target Grade Level: 9-12
Estimated Duration: 3 class periods or about 135 minutes
Learning Goals: Students will be able to…
• Compare and contrast the characteristics of planets, comets, asteroids, and trans-
• Create a definition for the term planet.
• Formulate an argument for or against the planet status of a hypothetical solar
system object discovered via telescope and then observed in a fly-by of the New
Benchmarks (AAAS, 1993)
The Nature of Science, 1A: The Scientific World View
National Science Education Standards (NRC, 1996)
History and Nature of Science, Standard G: Nature of science
Table of Contents:
Materials and Procedure
Blank Characteristic Card
Classifying Solar System Objects
IAU Member Analysis sheet
Debate Role and Stance: Opening/Closing
Debate Role and Stance: Topic Presenter
Debate Role and Stance: Rebuttal Presenter
Debate Format sheet
“Gravity Rules” article by Alan Stern
Extensions, Adaptations, and References
Standards Addressed, detailed
Why classify? Classification arises from the human desire to catalog objects,
compare and contrast them, look for patterns among them, and communicate about them.
We create classification schemes based on characteristics that are observable or
measurable, and we organize the objects being classified according to this scheme.
Classification can help clarify