First off, Pluto was discovered back in the 1920's by Clyde Tombaugh an American amateur astronomer (who went on later to get a degree at the University of Kansas). Part of the reason it was immediately billed as a planet was because of the nationalistic pride of Americans. It was the first new object of this size ever discovered by an American.
However, by the late 1990's astronomers were discovering some new objects out past Pluto that were nearly as large. Until this point, there had never been enough intermediate objects between "planets" and "asteroids" to ever worry about the definition. But with the discovery of these new objects (Senda, Quaoar, Xena...), it became clear to astronomers that we needed to actually decide what the exact definition of a planet should be.
The decision of definitions of things in astronomy is determined by an internation group of astronomers named the International Astronomers Union (IAU). Early this year, they decided that, under the new definition of planet, Pluto shouldn't make the cut.
The reasons: It was too small and has much more in common with a class of non-planetary objects known as Kupier Belt Objects (KBOs). It also did not have a nearly circular orbit like all other planets do. Additionally, it's orbital inclination is very noticably out of the plane of the solar system.
Thus, it was decided that Pluto should not be allowed to retain the title of planet and needed to be reclassified. Many people are upset by this but fail to realize that this isn't the first time that this has happened. the largest asteroid in the solar system (Ceres) was originally classified as a planet when it was discovered. However, astronomers began to discover the presence of many more smaller objects in the same vicinity and realized that Ceres was just a large member of what we now call the asteroid field.
Similarly, when Pluto was first discovered, it was unique. But now we realize that Pluto isn't special. It's just one among a much larger collection of objects from the Kupier Belt. As such, it has been relabeled appropriately.
This of course, begs the question, to many, of why we should care what the label is. No matter what we call it, Pluto is still Pluto, so why not just let it retain the title for historical purposes? After all, there's a great deal in science (astronomy especially) that's funny because of historical purposes. For example, the system of measuring brightnesses of stars (known as magnitudes) runs backwards with brighter stars being negative.
But ultimately, labels are extremely important. The reason for this is that classification systems allow us to find common traits and understand systems of objects.
One example of this is the development of our entire understanding of stellar properties during the early 1900's. Originally, we classified stars based on their spectrum. But we had no idea how it progressed. As such, stars were placed in classifications rather randomly and no further progress was made. Since we had stars in classification systems with no meaning, we couldn't figure out the common trends.
But by the 1930's, a female astronomer named Annie Jump Cannon devised a new classification system and it was quickly realized based on this that the spectral classifications she derived would allow us to understand the progression of effects that temperatures has on stars.
Thus, with a proper classification system, we can learn things. Without, we stumble in the dark. This reveals the importance of classifying Pluto properly and getting over the historical impetuous to allow it to retain the title. Astronomers need accurate classifications. The layperson does not.
So as an astronomer, I ask those of you who are not, to get over your nostaligic view of things.
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