The International Astronomical Union (mirror) has finally defined the meaning of the term "planet", and much to my dissapointment Pluto doesn't fit the bill. The part of the definition that it fails to satisfy is that a planet must have "cleared the neighbourhood around its orbit."
This brings to mind certain news stories that pop up once in a while about killer asteroids that might hit Earth. NASA has an active program to track "Near-Earth Objects". According to their website, there are at least 803 potentially-hazardous asteroids, and that doesn't even count the NEO's that aren't PHA's. Or maybe there are 4170 known NEO's. But regardless, I wonder, has Earth really "cleared the neighbourhood around its orbit" enough to fit the definition of a planet?
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"Cleared its orbit" does not include those asteroids, which I think are usually either objects in highly elliptical solar orbits or asteroids that get knocked out of the asteroid belt.
Having said that, I think the definition sucks ballz. As much as I wanted to see Pluto go from the planet list, I think the definition should have clear NUMBERS dictating things such as:
1) Ellipticity of the orbit.
2) How "round" the planet needs to be (for instance, how ellipsoidal vs. spherical the object is).
3) Inclination of the orbit (did the planet form from the original solar accrection disk or is it something else?)
I don't think numbers will help overcome the fact that what's going on ("gone on" if nobody argues too much) is a hair-splitting word game that does little if anything to advance science.
I would be interested in numbers if they were motivated somehow by the origin of the solar system (I hadn't thought about that until I saw fede's comment above), but I admit that such a criterion is arbitrary.
Consider this: The mass of the Sun is 2e30 kg (forgive my C numerical notation) and its radius is 7e8 m. The mass of Jupiter is 1.9e27 kg and it orbits 7.8e11 m (semi-major axis) from the Sun. This places the center-of-mass of the Jupiter-Sun system at 7.4e8 m from the center of the Sun, on average. So Jupiter doesn't actually orbit the Sun. They both orbit a common point in between. It should be named something other than a planet.
Regarding definitions and numbers and hair-splitting, to me, Science contains a process of defining and categorizing. At first, there are only a few examples, so we make arbitrary definitions to match our observations, then fit the stuff into the definitions. When we do this, we build in what we want our definitions to cover. Then, as more examples show up, the definitions get refined. This means (1) the definition they made contains the bias that they wanted to call Pluto a non-planet and (2) is vague so that the definition won't (hopefully) turn out to be wrong, just vague.
I don't know how the definition could turn out to be wrong, which is why it's semantics, not advancment of scientific knowledge. I say the same about taxonomy.
That's not to say that either is unimportant. Grouping things is a part of science because we need to have a shorthand for our sanity-- it would be annoying to list all the planets when we spoke of them collectively instead of using the word planet to mean all of them. But the grouping process usually collects what we know rather than increasing our scientific knowledge. We need the language so that we can talk about what we know, but it doesn't usually make us know more.
Since there are likely no other big objects between the Sun and Neptune left be discovered, I imagine that the refinements in the end will come from finding things we'd never call planets. The definition of "planet" could very well eventually end up being "planet: one of Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. pl. planets."
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