As Michael Martin-Smith has written, the solar system has tripled in size within the past ten years. With the recent findings of Kuiper Belt objects -- one of which is now confirmed to be larger than Pluto -- our solar system is a much different place than before. The fact that Pluto is not alone, and belongs to a new asteroid/comet belt, has raised questions as to whether or not Pluto should be classified as a planet, or not.
...our own solar system has been enlarged at least 3 fold, with the discovery of large planetoids up to three times further from us that Pluto; indeed Pluto is now regarded as the "ambassador" of a whole new class of objects -neither typical planets nor yet asteroids or comets -from which our system was probably assembled in its first 50-100 million years.This coming September, the IAU will hold a meeting to decide how to define a planet. I've believed that the way that we classify solar system objects is obsolete. As I've read somewhere else, our current classification scheme leaves the impression that there are 9 important bodies in the solar system, while the others are non-important. This scheme works for Earth-based observation through a telescope, but for people actually settling space, this scheme falls short. I believe that Luna and Mercury should have a similar classification because they are similarly "dead" rocky bodies with a negligible atmosphere, very cratered, and of a similar size. For people wanting to use (read: settle, study, or doing some sort of economic or recreational activity) these bodies, such a classification system would work very well.
As if this were not enough, we find comets that behave like asteroids, from exhaustion of their surface ices, and asteroids in elliptical comet-like orbits beyond their normal habitat between Mars and Jupiter- out to the reaches of Saturn and Uranus. Asteroids that regularly cross the orbit of Earth have been found while comets have been seen to break up and shift their orbits in a historical instant.
The Jovian planets are easy to categorize, and there are only four bodies of this type in the solar system. Any body large enough to be round should be considered a planet. Earth would be classified with Mars, Venus, and the Saturnian satellite Titan. Europa, Ganymede, and Callisto would have their own classifications because it is believed that they have subsurface water oceans. Space.com has articles on sub-surface oceans on Ganymede and Callisto. Triton, Pluto, the "Tenth Planet" and the large Kuiper Belt objects would also have their own classifications. Io would have its own category.
If any body is too small to be circular, then it should be classified as a planetoid. One class of planetoids would be the metal asteroids. Another would be the stony asteroids. Those made primarily of Carbon (and some volatiles) would be the Carbonaceous planetoids. Then there would be further classifications as the percentage of volatiles increase, depending on how much of each volatile is on the object.
Such a scheme for classification would significantly aid in the economic development and settlement of space.