Why Is Pluto Not a Planet? - Lesson for Kids

Instructor: Diane Sieverson

Diane has taught all subjects at the elementary level, was the principal of a K-8 private school and has a master's degree in Measurement and Evaluation.

For a long time, Pluto was considered to be the 9th and tiniest planet in our solar system. However, after scientists made some new discoveries, they decided it really wasn't a planet after all. Come learn about Pluto and why it is no longer called a planet.

What is Pluto?

Imagine riding in a rocket through space. After zipping past the Sun, you fly by Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Eventually, a small, round object comes into view. You get very excited because you know what planet comes next. Suddenly, mission control calls with breaking news. That planet you're about to pass by isn't really a planet after all! It's Pluto!

Pluto is a dwarf planet that used to be considered the farthest and smallest full-sized planet from the Sun. You may have even learned songs or sayings that helped you remember all the planets in our solar system in order, including Pluto, until it got a demotion.

A Little About Pluto

Pluto is smaller than the Moon we see in our sky at night and it takes 248 of our years on Earth to go around the Sun once. That means if you were 10 Pluto-years old, you would be 2,480 years old on Earth!

Photograph of Pluto taken by NASA
Pluto photographed by NASA New Horizons

As you might guess, there aren't enough jackets, gloves, or heaters to keep you warm while hanging out on Pluto. Because it is so far away from the Sun, it's about 390 degrees below zero and scientists think that it is covered in ice.

The New Horizons space probe
New Horizons Space Probe that Took Photo of Pluto

Gravity is not as strong on Pluto as it is on Earth. If a scale says you weigh 80 pounds here, you would only weigh about 5 pounds on that same scale if you were visiting Pluto.

First You're a Planet, Then You're Not

Since nothing about Pluto has changed since its discovery, you might wonder why it went from being called a planet to a dwarf planet. How scientists look at planets changed when new discoveries were made, and that changed their definition of a planet.

Since it was first seen in 1930, everyone thought Pluto was hanging out alone, with nothing else past it in our solar system. In 2003, a scientist found an object lurking further out behind Pluto that was bigger and he named it Eris (pronounced EAR-is). This discovery sent other scientists into orbit! They started talking about what defines a planet, and this is where Pluto ran into trouble.

Because of the discovery of Eris, a group of expert scientists decided that a planet must meet three conditions:

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