What is a planet?

We all live on a planet, and most of us can name most of the other planets in our solar system. But what is a planet? This is in some sense a real philosophical question. That is, it is a question that does not depend on the scientific facts of the matter, but rather on a set of definitions, and a "conceptual analysis" of what planet is. Again, it is not a scientific question, but a philosophical one.

It is good to see philosophy of science making the news, and it is good to see that astronomers are managing this question just fine without philosophers of science, and have a tentative definition, albeit not without its own troubles. This should be fun to watch.

As far as I know, there is little literature on the philosophy of astronomy, and hopefully this will change things.

The debate hinges on the following points: There are thousands of things orbiting our sun. They are of various sizes. They are of varied distances from the sun, and they have different types of orbits. They have different shapes too. They are effected by different celestial forces.

It will be important to doa few things. First, the number of planets, whatever it amounts to, is kept small. It would be too much of a mess to call everything a planet, and it would seem to miss the point. Second, everything we end up calling a planet has to have certain features in common. If not, we cannot have a real definition. Third, the traditional planets should be retained as planets. We do not want Earth to suddenly not be a planet because of our new definition.

Some earlier proposed definitions had eight planets, and excluded Pluto. And here is where politics infringes on science. Standard earlier definitions exclude Pluto as a planet . But Pluto is the only planet (of the three that have been discovered) that were discoverred by an American. Americans have a lot of say in science. American scientists, and Americans in general will want to make sure that there is at least one planet that is forever associated with American science. So we can be assured that as long as the US is a superpower, Pluto will be a planet.

Again, ultimately this is a philosophical question, one that nothing serious hinges on it. Some debates in science are much more significant, and the outcomes do make a difference, Some debates in the philosophy of science are serious and make a difference in setting a scientific framework, like the debate between cladists and pheneticists in biology.

We will likely realize in the long run that no two celestial bodies have that much in common, and we will use extensional definitions of planets instead of intensional definitions. We will just call a planet anything that we have been calling planet, and anything that the term planet catches on for, instead of trying to come up with a clear definition of what a planet is. If philosophy of sience has taught us anything, it is that a definition that includes all the cases we want, and excludes all the cases we don't want is never forthcoming. Nature is just not that clean.