Question Sheet: Planets on the Edge


Before reading:

  1. Name all the planets in our solar system. 
  2. Have you ever looked through a telescope at the night sky? If you have, describe your experience.

During reading:

  1. What is a planetoid? 
  2. Where did the name Sedna come from? 
  3. What is the difference between the Oort cloud and the Kuiper belt? 
  4. What are the difficulties of spotting objects in the Kuiper belt? 
  5. How do astronomers account for the fact that they found fewer objects in the Kuiper belt than they expected to find? 
  6. How might the launch of the New Horizons spacecraft help research on the Kuiper belt?

After reading:

  1. How does research on other planets help us understand Earth’s formation and history? 
  2. What might account for Sedna’s strange orbit? 
  3. The article says that, if Sedna is unusual or an exception, its discovery may not contribute much to increasing our understanding of the solar system. However, if there are many objects like Sedna, scientists are missing “something essential.” Explain why finding a one-of-a-kind object might not be as helpful as finding lots of similar objects. 
  4. Make an argument why NASA should retire or not retire the Hubble Space Telescope.


  1. Make up a 10-word sentence in which the first letter of each word stands for a planet, giving the order of the planets from the sun to Sedna.
  2. Imagine that you have found another planet in our solar system. Write a description and draw a picture of this planet and argue why you think it is, in fact, a planet and not a planetoid. Offer a suitable name for your planet. 
  3. There are many things in space that are not yet known or explained. If you were an astronomer 30 years from now, what might you know about Sedna that isn’t known now? Write a report on how the discoveries were made, noting how astronomical studies changed during those 30 years.


If temperatures on Sedna drop to –240 degrees Celsius, how cold is that in Fahrenheit degrees?