Earth is much bigger than the dwarf planets Pluto and Eris. Even Earth's moon, Luna, is bigger than these dwarf planets. This picture also shows Sedna and Quaoar, two other objects that might be dwarf 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? If so, you are doing better than professional astronomers! Right now astronomers aren't quite sure how to define a planet.

Maybe you've heard that some astronomers think Pluto is a planet, but others think that it isn't. Pluto is a lot smaller than the other eight planets, even Mercury, which is the second smallest. In recent years, astronomers have discovered a whole bunch of new objects like Pluto. Most of these new worlds are in the Kuiper Belt, a part of our Solar System near Pluto. These Kuiper Belt Objects (KBOs) are big balls of rock and ice, like Pluto. One of them is probably even larger than Pluto.

Some astronomers think that Pluto and all the KBOs are not planets. Other astronomers think that any KBO at least as big as Pluto should be called a planet. There may be more KBOs bigger than Pluto out there that we haven't found yet. We might have 20 or more planets in our Solar System some day! Some people think that we should still call Pluto a planet, because we've been calling it a planet for many years. Many of those people don't think other KBOs are planets, though.

Astronomers agree about two things. To be a planet, an object must orbit a star. If it orbits something else, like another planet, it is a moon instead. The second point is that the object must be big enough that gravity makes it into a sphere. Many asteroids and comets have odd shapes. They are definitely not planets.

A group of astronomers called the International Astronomical Union is working on an official definition for planets. Until they decide, different people will have different definitions. What do you think the definition of "planet" should be?

Last modified November 30, 2007 by Randy Russell.

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