What Is a Planet, Anyway?

Name: _________________________________       Date: _____________

In 2006, the list of planets suddenly shrank from nine to eight after astronomers demoted Pluto to dwarf planet status. In this Planet Diary activity, you'll learn the new definition of a planet and why Pluto doesn't cut it. You'll also find out how new discoveries fueled debate over what a planet is.

1. Start with this Planet Diary report Pity Poor Pluto: It's Been Demoted. What are the IAU's new requirements for a planet?
2. Explain why Pluto no longer qualifies as a planet.
3. Read NASA's Pluto and the Dwarf Planets. How do dwarf planets differ from the eight planets?
4. How are dwarf planets similar to the eight planets?
5. What led to the decision to demote Pluto? In 2005, NASA reported the discovery of a "tenth planet". Why did the new planet's discovery spell doom for Pluto's place in the solar system?