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 Dwarf Planets Overview. How do dwarf planets differ from the eight planets?
  4. How are dwarf planets similar to the eight planets?
  5. Explore the links to the first five recognized dwarf planets (Ceres, Pluto, Eris, Makemake, and Haumea). Which one is most interesting to you? Write a paragraph explaining why.