Questions for ‘Pluto is no longer a planet — or is it?’

image of pluto

Some scientists argue that Pluto’s fascinating geology should qualify it as a planet.


To accompany “Pluto is no longer a planet — or is it?


Before Reading:

  1. Research the definition of the word “planet.” Is Pluto a planet? Why or why not? 
  2. Why might it be useful for scientists to have a specific set of criteria to determine what counts as a planet?

During Reading:

  1. When and why was Pluto reclassified from planet to dwarf planet?
  2. According to Jim Bell, what criteria should determine what counts as a planet?
  3. What are some features of Pluto’s geology?
  4. During Galileo’s time, what counted as a planet?
  5. When and how was Pluto discovered? Who made this discovery?
  6. What is the Kuiper Belt? Why did the discovery of the Kuiper Belt cast doubt on Pluto’s status as a planet?
  7. How did Mike Brown get his Twitter handle @plutokiller?
  8. What is Ceres? Why was it determined not to be a planet? Is that similar to or different from the reason Pluto was deemed to not be a planet?
  9. When and why did people stop considering moons to be planets? When and why were moons later called planets once more?
  10. What types of solar system bodies should be called planets, according to Philip Metzger? Why?
  11. What are the potential benefits of using a more inclusive definition for “planet” than the IAU’s definition?
  12. What are the potential benefits of using the IAU definition for the word “planet”?

After Reading:

  1. Think about the many ways the word “planet” has been used over the course of history. Which of its past uses surprised you? Explain why.
  2. If the definition of the word “planet” were up to you, how would you define it? What might be the benefits or drawbacks to your definition?