Bad Astronomy Blog - Congratulations! It's a planet!
Well, apparently the people whose job it is to decide what constitutes a planet and what doesn't are currently sitting together in Prague and are about to ratify a revised definition.
For some time it has been disputed whether Pluto could be called a planet. The new definition says it is... it's introduced three new classes of planetary object:
1. Small, distant planets called Plutons
2. Small, closer planets called Dwarf-Planets
3. Small objects that don't qualify as planets (including asteroids and comets) called Small Solar System Bodies.
Under these new definitions Pluto counts as a planet, and so does it's largest moon, Charon. Together they're called a Double Planet. (The reason Pluto-Charon counts as a double planet and Earth-Luna doesn't is pretty complicated. But I'll explain it if you like)
Two other objects also count as planets now: the recently discovered 2003 UB313 (I hope they choose a nicer name soon) and the object formerly known as the largest asteroid in the solar system: Ceres.
That brings the total number of planets in our solar system now to twelve.
Astronomers predict that there may be many more Pluton-class planets in the Kuiper Belt (a region of debris beyond Pluto's orbit) that would also classify as planets under the new definition. As detection techniques and observation technology improve, we may eventually be counting the number of "planets" in our solar system in the millions!
Scientifically speaking it doesn't matter whether an object is called a planet or not, it's all a matter of semantics, but I think this is pretty exciting. It means all the astronomy books on my shelf at home are now out-of-date.