The Pluto Debate: The International Astronomical Union Defines a Planet

From the introduction:

Since its discovery in 1930, Pluto has always been considered an odd planet. It is smaller than the Earth’s Moon. Its orbit is highly eccentric and is tilted outside of the orbital plane of the other eight planets. In the mid-1990’s, however, astronomers began to find other objects like Pluto, with similar sizes and orbits. Astronomers pondered whether or not these new objects should also be called planets. The fact that they were all slightly smaller than Pluto was used as justification for keeping them out of the planet category.