Definition of a Planet

After having to deal with this question countless thousands of times, I thought I'd write down what I think a good definition of the word planet would be.

A planet is an object that satisfies both of the following two rules:

That's it!

The word planet has come to be one of those fundamental words we use to label some of the objects in the universe. To be useful, I think this definition needs to be kept as simple as possible. The historical development of this word basically boils down to a label for large things that could be places we could visit. By this definition, we can imagine technology that would let humans interact directly with anything called a planet. Of course we can also visit smaller things but they seem to fall in a different category becuase of their extremely low surface gravity. In the end, that's really what I'm using as the underpinning of my definition and that's the mass and density and thus gravity of an object.

There are some interesting consequences of my definition which a sharp reader will have already considered.

This definition is not universally accepted in the astronomical community but I think it is a good definition that can be defended and could actually be useful. I have tried to come up with something that captures the spirit (not the law) of past usage of the word while being mindful of what know about the universe around us. If this definition were to be accepted and still useful (and used) 10,000 years from now by our distant descendents (or any other beings for that matter), then I would consider this definition to be a success.

Back to my home page -o- Back to my Pluto page

Marc W. Buie, Southwest Research Insitute, March 2005