Video:What Is the Definition of a Planet?

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Transcript:What Is the Definition of a Planet?

Hi. I'm Ben Oppenheimer. Associate Curator for Astro Physics at The American Museum of Natural History and I'm here for About.com.

What is a Planet?

Today we're going to discuss what makes a planet a planet. This is a very interesting question because we all know our sun has about eight planets orbiting it and many other things orbiting it. Why are these so different from eavh other? And what is it that makes them planets?

History of the Word: Planet

The word planet has a long history. It was a word that simply merant wanderer. But, along with the earth, there is quite a variety of these objects. But they share something in commin and that has to do with how they formed. The earth and all of the other planets around the sun, we believe formed in a disc of material that coalesced as the sun, itself, was forming.

What Makes a Planet a Planet, and a Star a Star

Other objects, such as stars, don't form in this manner, they don't form around another star. They're all vastly different. The diverasity of what we call planets is huge. One of the ways to classify planets versus stars, versus what we call Brown Dwarfs, which are objects intermediate between planets and stars has to do with the internal physics of these objects. Planets do not fuse any kind of nuclear material.

Nuclear fusion is how stars shine and illuminate their vicinities. With this definition of a planet being something that naturally never fuses, goes through any nuclear fusion reaction in its core, you can actually convert that into a mass – so, that anything smaller then the mass of about 13 times the size of Jupiter would be considered a planet. Anything above that would be a brown dwarf or a star. This is one critical definition.

Now, that definition implies a number of other things. The chemistry of planets is complicated as compared to stars, where it's actually very simple. So, at this point, astronomers have roughly agreed that we would call anything below thirteen Jupiter masses, that's 13 times the mass of Jupiter, anything smaller then that, down to about the size of Pluto will be called a planet. They have to be spherical and orbiting a star. But this is the essence of what we are talking about.

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