NASA - National Aeronautics and Space Administration

The information in this document was accurate as of the original publication date.

Audience: Educators
Grades: 9-12


During the annual meeting of the International Astronomical Union in the summer of 2006, members in attendance approved a newer definition for "planet" and other bodies. In this lesson, students learn about the characteristics of planets, comets, asteroids and trans-Neptunian objects through a classification activity. The students can then apply what they have learned by participating in a formal debate about a hypothetical solar system object discovered by the New Horizons spacecraft and by defining the term "planet."

What Is a Planet? [203KB PDF file]