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What is a Planet? Definition, Debate, Dwarf Planets, Kuiper Belt - Windows to the Universe

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Earth is much bigger than Pluto. It is also much bigger than Eris, another dwarf planet. Even Earth's moon, Luna, is bigger than the dwarf planets. This picture shows some other objects, Sedna and Quaoar. Do you think they are planets?
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Original artwork by Windows to the Universe staff (Randy Russell) using images courtesy of NASA/JPL-Caltech/R. Hurt and NASA, ESA, and A. Feild (STScI).

What is a planet?

Do you know what a planet is? Guess what... astronomers are not quite sure what a planet is!

Mercury, Venus, Earth, and Mars are the planets closest to the Sun. They are definitely all planets. They are a lot like each other. They are called "rocky planets" or "terrestrial planets".

Jupiter, Saturn, Uranus, and Neptune are farther from the Sun. They are all planets, too. They are much bigger than the rocky planets. Jupiter and Saturn are called gas giant planets. Some people call Uranus and Neptune gas giants, too. Other people call them ice giants, because they are so cold.

Then there's Pluto. It is way out on the edge of our Solar System. It is much smaller than Mercury, the smallest rocky planet. Is Pluto a planet? Some people say yes. Other people say no.

There are many other large ice worlds in our Solar System. They are called Kuiper Belt Objects (KBOs). The KBOs are a lot like Pluto. Some people say Pluto is just another KBO. They say that KBOs are too small to be planets.

Some people say Pluto is a planet. Some of them say other KBOs are planets, too. Some people say that any KBO that is as big as Pluto or bigger is a planet. We may have 20 or more planets in our Solar System some day. Astronomers are finding new KBOs all the time.

A group called the International Astronomical Union is trying to make up a good definition for "planet". Until they decide, different people will call different objects planets. What do you think the definition of "planet" should be?

Last modified November 30, 2007 by Randy Russell.

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