In 2006, the International Astronomical Union (IAU) voted to change Pluto's classification as a planet. This move was controversial, especially by the general public that largely opposed the demotion of Pluto. For instance, there was a recent news report about revisiting this question. Why did astronomers decide to reclassify Pluto? The move was driven by several factors. The eight other planets easily fell into one of two groups, the terrestrial and Jovian planets. The terrestrial (meaning Earth-like) planets are the four innermost planets, and the Jovian (meaning Jupiter-like) planets are the four outermost planets. However, Pluto doesn't fit into either group. Astronomers had long known that Pluto was far smaller than Mercury, the smallest planet if Pluto were excluded. Since 1992, astronomers have found more than 1,500 other objects orbiting the sun beyond Neptune and near Pluto's orbit. Eris, discovered in 2005, is one of these, and it is slightly larger and more massive than Pluto. If Pluto is a planet, why isn't Eris a planet? Many other objects orbiting the sun near Pluto are nearly as large as Pluto and Eris, so why aren't they planets too?
A number of astronomers had favored this change for some time, but out of respect for Clyde Tombaugh, the discoverer of Pluto, this topic was not discussed much during his lifetime. Following Tombaugh's death in 1997, astronomers increasingly began to reconsider the issue. This culminated in the IAU's decision in 2006.
Complicating the issue was that there never was a standard definition for a planet, because until recently nearly everyone sort of knew what a planet was — a planet was a large object that orbited the sun. Objects that orbited the sun but were much smaller than planets were either asteroids or comets. However, recent discoveries have made the old distinctions less clear. For instance, the difference between asteroids and comets once was based upon composition, but now that difference is blurred. Perhaps the kind of orbit that a body has may be more significant in determining whether a small solar system object is an asteroid or a comet. And with the discovery of many large asteroids orbiting the sun near Pluto's orbit, the question of how large a body must be to qualify as a planet came up. Ultimately, the IAU formally adopted a definition for a planet for the first time. According to that definition, a planet must meet three criteria:
- A planet must orbit the sun
- A planet must be spherical
- A planet must have significantly cleared its orbit of other objects