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Comment by beeworker | Hacker News Reader

> There are dynamical reasons to argue for the fact that a planet has to have cleared it's orbit

Maybe so, but as the wiki article points out in its criticism section, this excludes Earth, Mars, Jupiter, and Neptune from being planets.

I don't think it's so much folly as much as a waste of everyone's time. "Planet" isn't a natural thing like the speed of light, if astrophysicists want to be more specific about which orbiting bodies they're talking about, which "dynamical processes" they're talking about, they should come up with a new term instead of dragging literally everyone (including the children!) into an argument over definitions. It's like a scientist insisting that "sound" can only refer to this vague thing a brain experiences, and it can't refer to generic vibrations in the air, but some people are still on the fence about whether it can refer to vibrations in water.

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From the very next paragraph:

"Most astronomers counter this opinion by saying that, far from not having cleared their orbits, the major planets completely control the orbits of the other bodies within their orbital zone. Although Jupiter does coexist with a large number of small bodies in its orbit (the Trojan asteroids), these bodies only exist in Jupiter's orbit because they are in the sway of the planet's huge gravity. Similarly, Pluto may cross the orbit of Neptune, but Neptune long ago locked Pluto and its attendant Kuiper belt objects, called plutinos, into a 3:2 resonance (i.e., they orbit the Sun twice for every three Neptune orbits). Since the orbits of these objects are entirely dictated by Neptune's gravity, Neptune is therefore gravitationally dominant."

It's not a waste of the astronomers' time, because they need a reasonable taxonomy to communicate with each other. It's clear that the current set of planets are different from the things that are not planets. If we included Pluto in the set of planets, we'd need to come up with a new word to describe the subset that does not include Pluto - so it's much easier to take Pluto out of that set, and continue calling them "planets".

Other people have wasted their own time by having an emotional attachment to what words the astronomers use.

I don't believe the International Astronomical Union has offered an opinion about the Webster's definition of the word, or about how you and your six year-old may use "planet" in ordinary language.

The IAU's definition is a scientific definition, one that pertains to usage among astronomers. Insisting that astronomers make up another word is silly for all the same reasons that (as you pointed out) it would be silly for astronomers to tell us how to use one of our everyday, non-technical words. The same sequence of characters or phonemes can have slightly different meanings in different contexts.

But the IAU definition makes no sense for astronomical use, particularly as we get better at finding extrasolar plane^H^Horbital bodies. We're going to have hundreds of repeats of Ceres/Vesta/Pluto - bodies that we discover, consider to be planets, and then only much later with finer techniques discover that there were other bodies in the same orbit. That's not good for science.

Upvoted, as this is an excellent point. The IAU definition makes serious presumptions about our level of knowledge. Could the IAU definition be used at the time of Pluto's discovery? If not, what does that say about using this definition for future discoveries?

Note that I'm not saying that I think that dwarf planets should be categorized as planets (there are too many), but that the definition shouldn't depend upon us having complete knowledge of every object in a newly discovered potential planet's neighborhood.

I kind of agree with this. Let there be planets and perhaps that should include 'subplanets' and then let there be a next level to their taxonomy where they can be described as 'clear orbit' or not.

I expect that there will be further 'evolutions' of the definition as we learn more and more about exoplanets, there are nearly 2000 of them already.

Seems like the biological sciences have done a decent job, you've got your common names and then Latin taxonomical descriptions which make up formal names.

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