What Is a Planet, Anyway?

Name: _________________________________       Date: _____________

The familiar list rolls easily off our tongues: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Shockingly, the list suddenly shrank from nine to eight after astronomers "kicked out" Pluto in August, 2006. Most of us probably assumed the list of planets would never change. But astronomy, like every other science, is constantly changing. As telescopes look deeper and probes travel further out into space, new observations lead to new conclusions.

What exactly is a planet, anyway? Before the decision about Pluto, there was lively debate among astronomers about adding other solar system bodies and/or subtracting Pluto from the list. In this Planet Diary activity, you'll find out how new discoveries fueled this debate. You'll also learn the new definition of a planet and why Pluto no longer qualifies.

1. In 2005, NASA reported the discovery of a solar system body called 2003 UB313, nicknamed Xena by its discoverer. In 2006, it was officially named Eris and was classified as one of three "dwarf planets." Soon after it was discovered, however, NASA called it the tenth planet, clearly expecting it would soon be added to the list of nine. Compare 2003 UB313's distance to the Sun with that of Pluto.
   
   
   
   
2. What is the Kuiper Belt?
   
   
   
   
3. Why did Dr. Brown think 2003 UB313 should be added to the list of planets?
   
   
   
   
4. How do astronomers use reflectance to determine the size of an object in the solar system?
   
   
   
   
5. Here's another report from 2005 on the discovery of 2003 UB313. Explain why it wasn't found sooner.
   
   
   
   
6. Why did Dr. Brown argue in the past to kick Pluto out of the solar system?
   
   
   
   
7. Why did Dr. Brown change his mind about Pluto?
   
   
   
   
8. At the time this article was written, how did Dr. Brown define a planet?
   
   
   
   
9. Scroll to No definition for planet. What does Brian Marsden say about calling 2003 UB313 the 10th planet?
   
   
   
   
10. The debate about what a planet is has actually raged on for a decade. What earlier discoveries (before 2003 UB313) forced astronomers to rethink the definition of a planet?
    
    
    
    
11. Finally, read the August, 2006 Planet Diary journal entry Pity Poor Pluto: It's Been Demoted. What are the IAU's three new requirements for a planet?
    
    
    
    
12. Explain why Pluto no longer qualifies as a planet and is considered a dwarf planet instead.
    
    
    
    
13. Name and describe the other two dwarf planets.