But what is a planet anyway? Well, the word 'planet' comes from the Greek planetes, meaning wanderer (it's also the name of an excellent Japanese science fiction comic), and it was a name given to those 'objects that moved in the sky with respect to the background of fixed stars', as the IAU press release I link to below puts it. Obviously, that's a seriously dodgy definition, given that we can now observe loads of objects that fit that description.
What it comes down to is the fact that the nine things that we call(ed) planets aren't the only interesting things in the solar system. They are a good way to define the solar system in a big stroke - here are the significant bodies, most of the rest of the stuff is either orbiting them or in the asteroid belt here or the Kuiper belt here. The reason that Pluto isn't relevant in this broad definition is that we now know it to be only one example of a large number of similarly sized objects in the outer reaches of the solar system (the aforemention Kuiper belt in fact) - possibly as many as millions.
This poses a bit of a problem. It seemed that either we would have to accept that there are an enormous number of planets, a number that we can expect to keep growing indefinitely, or we would have to exclude the interloper from the club. Given that the first option turns a 'planet' from something significant to something that is mostly found in the Kuiper Belt, and the second, as Neil deGrasse Tyson reminds us gleefully, provokes squeals of horror from everyone under the age of twelve, it is perhaps not too surprising that the International Astronomical Union has attempted to take the middle road, as described in this press release:
If the definition is approved by the astronomers gathered 14-25 August 2006 at the IAU General Assembly in Prague, our Solar System will include 12 planets, with more to come: eight classical planets that dominate the system, three planets in a new and growing category of "plutons" - Pluto-like objects - and Ceres. Pluto remains a planet and is the prototype for the new category of "plutons."
The Planetary Society has an excellent news item on the history and reasoning behind this decision, which you can find here, and I highly recommend (with the power of the colour red) that you check it out if you want to be well informed on this subject.
Bad Astronomer Phil Plait, on the other hand, has an excellent criticism of the definition of a planet that the IAU is using:
The problem here is simple, really: we’re trying to wrap a scientific definition around a culturally-defined word that has no strict definition. Doing this will only lead to trouble. Why? For one thing, it’s divisive and silly. How does a definition help us at all? And how does it make things less confusing than they already are?
There has been a lot of controversy about this, mostly in the media and the public, since most astronomers don’t care all that much.
In a similar vein, Sean of Cosmic Variance reminds us that we are not doing anything more than coming up with a definition for a word:
We are not doing science, or learning anything about the universe here. We’re just making up a definition, and we’re doing so solely for our own convenience. There is no pre-existing Platonic nature of "planet-ness" located out there in the world, which we are trying to discover so that we may bring our nomenclature in line with it. We are not discovering anything new about nature, nor even bringing any reality into existence by our choices.
To an extent, I am ambivalent about this. On the one hand, I strongly felt that we should have just knocked Pluto back and set the number at eight. But on the other hand, I like the fact that this draws attention to all the other interesting bodies in the solar system. I wonder how many people had even heard of Ceres before now?