Professors Iwan Williams and Jocelyn Bell, both members of the International Astronomical Union (IAU), can explain the whole thing much better than I ever could, so I’ll let them do the real work and I’ll try and translate it into English for you. The article that they’ve written is a cross between a first-hand experience geared at others in the Astronomy field (since they were at the actual IAU meetings) and some gentle explanations of what really went on during the General Assembly last August.
For a little bit of context, Pluto was discovered in 1930 and became an instant cultural phenomenon. Not only was there an element and a cartoon dog named after it, the newly-classified planet captured the imagination of early-Depression era America. In 2001, a team of astronomers discovered other Pluto-like bodies in the same general area, leading to the conclusion that there was a large asteroid belt beyond the reaches of Neptune. And they were right! The Kuiper Belt (as it’s called) is the second asteroid belt in our solar system and Pluto holds the honor of being the first Kuiper Belt object ever discovered… which unfortunately demotes it from its planetary status.
Now, while the media simply hyped up on the amount of siblings our planet had—be it 12, 8, or 9—the real issue under the knife at the IAU General Assembly was the concrete definition of a planet… because there wasn’t one. Ever since the word “planet” was created by ancient Greek astronomers there has never been a concrete definition of what types of celestial bodies the term really encompassed. Until now.
There were three definitions coming into play during the meeting: the first was based entirely on size, the second suggested that there be a very loose and informal definition of a planet (according to what people felt like calling an object), and the final definition based planetary status more on how an object affected the other objects around it. The definition that won was, as we all know, the first which claims that anything smaller than Mercury (or somewhere in the neighborhood of 4000km) cannot be a planet. But since Pluto has enough gravity to form in a spherical shape, it is classified as a “dwarf planet” along with numerous other recently discovered Kuiper Belt objects.
Now honestly, even looking at the orbit of Pluto, it’s obvious that there’s something screwy with the “planet.” Defining it as a dwarf planet and Kuiper Belt object just seems to make more sense. And really, it’s just a big abnormal hunk of rock surrounded by ice. Despite the cultural attraction we have to this eccentric oddball, it’s not like we’ve really lost lone Pluto way out there in the reaches of space. We’ve just given him a few friends. And we may have lost a planet but we gained a completely new classification of astronomical bodies. How cool is that?
Williams, Iwan, Jocelyn Bell. “What it takes to make a planet.” Astronomy & geophysics : the journal of the Royal Astronomical Society. 47.5 (2006): 5.