For many of us outside the scientific circles and for those who were still in school trying to learn the stuff that scientists had been debating about for a decade or so it seems, Pluto's demotion to a dwarf planet came as a surprise. But talks about the definition of what a planet should be had already been underway since the 1990s. Only until about twelve years ago did the International Astronomical Union voted to reclassify Pluto.
Less than 5 percent of the membership of the IAU voted and the room was full of astronomers like me, with no particular expertise on planetary science. The new IAU rules said that a planet must meet three criteria: it had to orbit the Sun, it had to be massive enough for gravity to make it round, and it had to have “cleared out” its orbital neighborhood. Pluto stumbled at the third criterion.
Current debate centers on the third criterion: that a planet must have cleared out material in its orbit. In other words, there should be no object of comparable size at that distance. The rule is hard to apply consistently since it depends on details of the formation process. Planetary scientist Phil Metzger reviewed the research literature and found only one instance in the past two hundred years where orbit-clearing had been used to classify a planet. He said of the IAU rule: “It’s a sloppy definition.”
Now, as we continue to discover new worlds, galaxies, and planets beyond our own, it's possible that these rules would continue to change and many other exoplanets, asteroids, and other space objects would be reclassified. Although, the more we see of the universe, the more difficult it might be to apply consistency in the rules that we create to make things more systematic. In the case of Pluto, however, it doesn't seem like it will be reconsidered anytime soon.
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