Planet Definition

I know you are all wondering what I think about the debate raging right now about the planets in our solar system. Is Pluto a planet? Are there 8, 9, 12 or 200 planets in our solar system? Well, in my opinion, based on what I have learned in school and what conclusions I have drawn on my own, there are only 8 planets in our solar system. The problem is defining what exactly a planet is. Is it an object that under its own gravity causes it to be spherical? Can a planet be an object that orbits around another object other than its parent star? Well, that's hard to use as a definition. Based on the first definition, that would mean there are over 200 planets in our solar system. Do you think your children want to memorize that many planets? Nine planets that we had to memorize as children was difficult enough without having to resort to mnemonic devices. If we decide to break down planets into two categories which is what astronomers have done for years (with Pluto not fitting into either category), this leaves us with eight planets. There are the terrestrial, or Earth-like, planets and the Jovian, or Jupiter-like, planets. Another way of looking at it, the planet is either rocky or gaseous. Pluto is an icy body, much like a comet. In fact, most astronomers believe that Pluto is just a large KBO, Kuiper Belt Object, which is where many comets come from (Halley's is a prime example). It may be that those categories need to be expanded on. You cannot require an atmosphere as being a requirement (Mercury does not have one, and Titan, the largest satellite of Saturn does have one). You cannot require that the planet has at least one natural satellite (neither Mercury or Venus have a natural satellite, Mars' satellites are captured asteroids, and Pluto does have a satellite, Charon). The way I've been looking at it, is to look at the core of the planet. Is the core rocky and metallic and a significant fraction of the planet's volume (the fraction will have to be determined)? Was the planet formed out of the stellar nebula when the star formed, but formed orbiting the star itself, and not orbiting a planetessimal? These are questions that must be asked and answered. There is a lot about planets and planetary systems that we do not know. Up until the mid 1990's, the only information we had about planets was that within our own solar system. But so far, it appears that our solar system is very unique and not the norm. We had always thought that terrestrial planets form near the central star and Jovians had to form farther out due to the heat. However, in planetary systems outside our own, it appears that Jovians can be close to the central star. This may also be a bias in that we can only observe extrasolar planets outside our solar system if they are large and orbit close to the central star. In the future, this bias may be eradicated.

So what have I concluded from what I have learned and read? In a list:
1) There are only eight planets in the solar system.
2) A planet must be (a) either rocky or gaseous and
(b) have a rocky/metallic core that is a significant fraction of the planet's size
(3) There is still a lot we need to learn.

If you have any questions, feel free to comment and ask. Just a reminder, by no means am I a professional astronomer, though I have taught college astronomy and studied astronomy for many years. A lot of things I have said here are my opinion based on my education.

Addendum (24 Aug 2006): Pluto has been demoted. After years of teaching that Pluto was not a planet, I have been redeemed. Of course, my definition is not the same as that of the IAU, but I like it.