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.

Good News/Bad News

The good news is is that I'm back home in Pittsburgh after a wonderful visit with my family (only two days too long - more on that later). Besides spending time with Mom and Dad, I got to see Tom, Joanne and Maggie and 6 of their combined 8 children (Tom's daughter, Joanne's two youngest, and Maggie's three) and my sister-in-law Chris and Maggie's husband Josh. Aunts Dorothy, Gina and Judy also came by on Saturday and we had a great time (when I was awake - again, I'll explain later). I went to the Nelson for the first time since I was a senior in high school, and attended a double header that the Royals swept over the A's. I have some pictures, but they will have to wait till my camera arrives (again, I'll explain it later). All in all, a great time.

Now the bad news. I drove to Kansas City in my beautiful 1993 Corolla whose color my brother-in-law Birch referred to as "Zinfadel". It was an easy drive to Kansas City. I left Pittsburgh at 5 PM on the 13th of August and got into KC at 7 am the next morning. I was sore as all, but that happens the older you get. I drove to the museum with dad and did some shopping (shot glasses for my boss and some t-shirts for my girlfriend). So basically, the week was great. I was coming back from Kauffman Stadium after the double header and fireworks, and got stuck in traffic on I-70W just before the I-670 ramp to Kansas. Unfortunately, a woman behind the car behind mine wasn't paying attention. She rear-ended the car behind me which in turn pushed that car into mine. Needless to say, when I got home after the accident, I had trouble sleeping, so that is why I fell asleep on Saturday with everyone there (amazing considering how many kids were there). So here I am stranded in KC without my car, far from home. Luckily, I was there visiting family, so I was able to stay with Mom and Dad a little longer, but by Sunday, I was ready to go home. Hence, two days too late. My car was towed to a body shop in Kansas City, MO but I still had to get back home. I ended up buying a plane ticket to Pittsburgh for Tuesday night (which I will get reimbursed for), but still don't have a car. So I'm in limbo till I know what they are going to do with my car, either fixing it (which means I have to fly back to KC to pick it up), or totalling it (which is what I prefer, though I'll have to buy a new car). I'm just glad school starts on Monday, so I can hopefully concentrate on something else for a little bit instead of worrying about my car or lack thereof.

Addendum (24 Aug 2006): My insurance company called me today. My car will be totalled which means I do not have to fly back to KC, but I will have go out and buy a new car.

Going out of Town

I'm heading here tomorrow.I'm going to eat barbeque, watch the Royals, and try to stay cool. See you all when I get back. Except those that live in the KC area.

Bridge to Terabithia

They are making this into a movie. I remember my 6th grade teacher, Mrs. Higgins, reading this book to us, but she couldn't finish it. For those who have never read it or heard of it, something occurs in the book and Mrs. Higgins couldn't bring herself to finish it. I might have to go see this movie as I remember the book, but never found out how it ended. I might have to go pick it up and read it myself.