Back in elementary school, I learned the following saying "My Very Elegant Mother Just Sat Upon Nine Porcupines" as a way to remember the names of the nine planets. The first letter of each word corresponds to the first letter of one of the nine planets, in order of distance from the sun. Future schoolkids, however, may have to use a revised version of the saying, because there's talk of stripping Pluto of its planethood.

Pluto was discovered in 1930 by American astronomer Clyde Tombaugh. Its status as a planet wasn't in question until 1992 when astronomers discovered a small object, several hundred miles in diameter, orbiting the sun beyond Neptune, in Pluto's neck of the solar system. This was the first hint that there might be more than just Pluto out that far. Astronomers now estimate that there are tens of thousands of these so-called "trans-Neptunian" objects.

So the question that astronomers have been grappling with is: Is Pluto a planet, or is it just the largest of the trans-Neptunian objects? Most astronomers agree on one point - if Pluto were discovered today, it probably would not be classified as a planet.