Question Sheet: Defining Planethood


Before reading:

  1. Name all the planets in our solar system.
  2. What makes something a planet?

During reading:

  1. What five planets have scientists known since ancient times?
  2. How is Pluto different from the other planets?
  3. What is the Kuiper belt?
  4. To a dynamicist, what qualities make an object a planet?
  5. What are two different ways to define a planet?
  6. Sykes says, “Whenever we come up with an explanation for something, we as

    scientists have to be ready for someone to show us that we’re wrong.” What does

    he mean?

After reading:

  1. Do you think Pluto should be considered a planet? Why or why not? See (NASA) and

  2. Why might scientists need to have a definition of a planet? See
  3. The new definition of a planet accepted by the IAU says that a planet must

    have “cleared the neighborhood around its orbit.” What does that mean? Have

    Earth, Mars, and Venus cleared their orbits? See

  4. Where is the Kuiper belt located? What sorts of objects are found in the

    Kuiper belt? Why are these KBOs of interest to scientists? How does the Kuiper

    belt differ from the Oort cloud? See and

    (University Corporation for Atmospheric Research).

  5. Should astronomers by themselves get to decide whether a body in the solar

    system is a planet, or should the general public also have a say? Why?


Who discovered Pluto? When did he find it? How did he find it? How was the discovery of Pluto related to the discovery of Neptune? See (Wikipedia) and

(University of St Andrews).


  1. Write a letter to your local newspaper arguing that either (a) Pluto should

    be called a planet or (b) that Pluto should not be called a planet.

  2. Write a paragraph or two that your science textbook might use to describe

    the history and controversy surrounding Pluto.


It takes Pluto 248.54 Earth years to orbit the sun. Neptune orbits the sun in 164.88 Earth years. How many times longer does it take Pluto to orbit the sun than Neptune? Pluto was closer to the sun than Neptune was from 1979 to 1999. What percentage of Pluto’s orbital period was spent closer to the sun?