The scientific community has been arguing about that recently, actually.

Pluto is a member of the Kuiper Belt, which is a class of icy objects that range around the orbit of Neptune and out we don't know how far. Most of the objects in the Kuiper belt are the size of large asteroids, but at least three, these being Triton, Pluto, and the recently discovered Eris, are quite a bit larger.Triton, the biggest, has been captured by Neptune and is currently one of the largest moons in the solar system. Eris is the next-largest, followed by Pluto.Pluto's moon Charon is also a Kuiper object.

The discovery of the Kuiper Belt and of Eris in particular has prompted an argument on what a planet is. Eris is bigger than Pluto, and is farther out, so by the traditional system, it should be the tenth planet from the Sun. However, it and Pluto very clearly belong to this belt of objects that has been likened to an icy version of the asteroid belt between Mars and Jupiter. I don't think that there is widely-held agreement on whether to call the larger Kuiper objects "planets" or not. Many, including myself, think that it doesn't really matter because the planets that we do recognize are very different objects anyway: the four rocky inner planets have much more in common with asteroids than they do with the four giant planets in the outer solar system, of which Jupiter at least could effectively be described as afailed star.

This is one of the big mysteries of our solar system.Pluto and its moon Chiron both appear to be made out of rock, but they are barely bigger than asteroids. In fact, many astronomers don't consider Pluto a planet anymore, but call it a "minor planet" instead. There are at least 4 other minor planets in the solar system that are about the size of Pluto, and astronomers believe there are many more out there waiting to be discovered. Here is another unusual thing about Pluto: its orbit is tilted compared to the other planets. All the other planets orbit in the same plane, like wheels on the same axle. And its orbit is also not round. Sometimes Pluto is closer to the Sun than Neptune is.