@Draugnar: That definition isn't as helpful as you might expect, because planets (and other objects) don't have stable orbits. They change dramatically over time. If you looked at our solar system a few billion years ago, you'd very likely have seem at least one additional gas giant planet, for example. There's about a 0.1% chance that Mercury doesn't survive the next few billion years.
Really, there are several very different types of objects in our solar system. There are four objects that have properties very similar to Earth. There are four that have properties very similar to Jupiter. There are asteroids, and those are often very different from each other, but often similar in size and orbit. There are asteroids that co-orbit with the Earth, too, I might add, and many more that co-orbit with Jupiter. Some of these are quite large. Charon is a substantial fraction of the size of Pluto.
So, I think the problem is that planet is a pretty poor term to begin with, and as we continue to learn more both about the solar system and objects orbiting other stars, we come to realize both that we've tried to make a category out of several things which are quite different and that we've tried to make a sharp distinction when there isn't really a large gap.
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