Still Debating Pluto

Astronomers are still debating whether or not to classify Pluto as a planet. Some argue it is far smaller than any of the eight current planets, its composition is different than any of them, its orbit is different, and it is clearly one of many similar objects in the Kuiper Belt. If Pluto is a planet, they say, the number of planets will explode because those similar objects will also have to be called planets.

Some on the other side are fine with that. They argue, basically, that any object in orbit around the Sun that is large enough so that its gravity has pulled it into a spherical shape is a planet. That would include the largest asteroid, Ceres, for example, plus an unknown number of Kuiper Belt inhabitants. If that ultimately balloons the number of planets into the dozens, or hundreds, they say, it simply does.

With the exception of Ceres, the argument comes down to where the Kuiper Belt fits in the overall scheme of the Solar System. There seems to be four realms in the System-- that of terrestrial worlds out to the Main Belt of asteroids, that of the gas giants from the Main Belt to Neptune, the Kuiper Belt beyond Neptune, and the Oort Cloud beyond the Kuiper Belt. The Cloud, home of long period comets, may in fact stretch halfway to the next star.

So, how should we classify Pluto? That debate might go on for a while.