Science Scoops: Pluto: A Planet or Not?

there is a cartoon of a small green planet pointing at a larger blue planet

Poor Pluto. It's such a little pipsqueak of a planet, with a diameter of only about 1400 miles. (Compare that to Earth's 8000 miles!) It's even smaller than seven of the moons in the solar system, including Earth's. Pluto doesn't behave much like the other planets either. The path it travels around the Sun has a funny tilt and an unusual oval shape. And Pluto is made mostly of ice and rock—more like a comet than a planet. No wonder many scientists think it doesn't deserve to be called a planet.

To make matters worse, in October 2002, astronomers announced the discovery of Quaoar, a planet-shaped body with a diameter of about 800 miles. Quaoar is the biggest thing anyone's found in our solar system since, well, Pluto. But no one's calling Quaoar a planet. It's a KBO (short for Kuiper Belt object).

KBOs are small bodies that orbit the Sun in a region beyond Neptune called the Kuiper Belt. More than 600 KBOs have been identified since 1992, and scientists think there could be billions more. Some of them even have moons. Until now, the only real difference between KBOs and Pluto, which lies within the Kuiper Belt, has been size. Many of the visible KBOs are less than 100 miles in diameter.

there is a picture of pluto over the United States.

This shows how big Pluto is compared to the U.S. Can a planet be smaller than a country?

But Pluto is not that much bigger than Quaoar. And scientists predict that more large KBOs will be found, perhaps some even larger than Pluto. Many astronomers argue, however, that even if we do find a Pluto-sized KBO, we should still consider Pluto a planet. What do you think?


  1. What is a KBO?
    [anno: A KBO is a Kuiper Belt object.]
  2. How big are KBOs?
    [anno: KBOs vary in size from small objects about 100 miles in diameter to objects about 800 miles in diameter. There might be even larger KBOs that have not yet been discovered.]
  3. Where do you think KBOs came from? Explain your answer in a sentence or two.
    [anno: Answers will vary but could include that KBOs came from the collision of planets when the solar system was forming. Students might also answer that KBOs came from unstable planets or moons breaking apart.]