Quote from: ProgZmax on Mon 28/08/2006 09:05:21
1. A nonluminous celestial body larger than an asteroid or comet, illuminated by light from a star, such as the sun, around which it revolves. In the solar system there are nine known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.
It's a simple definition agreed, but it has worked for years.
You must have used a different source than I did for the planet definition. This itself shows that there was a problem with that definition, since we came up with two different (albeit very close) definitions. So having a unified definition is actually good, because there is only one way to define a planet.
Anyway, there is no clear-cut definition for asteroids, either.
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Hogwash. Quantify very nearly for me. All of the planets swing in elliptical orbits, whether extreme or not. Nevertheless, Pluto's orbital shape isn't the issue here, it's the overlap with Neptune.
Of course it is not the *shape* that is the issue (I didn't imply it was. Pluto actually has a fairly spherical shape, but that partly comes from the fact that it's a binary system with Charon, so tidal forces help to achieve the sperical shape.)
The issue is the *mass* and the *force of gravity* that awakens from that mass. A planet - according to the new definition - should have enough mass to generate sufficient gravity to form a sphere on its own, without outside help.
This is also the reason for the clause to "clear the neighborhood": if a planet is massive enough, it'll have enough gravity to attract smaller bodies from a distance, thus sweeping an elliptical path in the accretion disk around a newly formed star. A small body would have a lot less gravity (gravity weakens according to the inverse square law), so it would have a lot less chance to do the same.
That's why there are no significant bodies around the orbit of Mercury, Venus, Earth, Mars, etc. - they swept a significant path out by accreting matter from the dust when they formed.
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Note: I think you have a misunderstanding here. I just realized, that when you talk about shape, you talk about the shape of the orbit, right? Ignore this if not.
The shape of the orbit was not even a concern for Pluto - it was the *mass* of the planet that was a issue. The shape of the orbit didn't matter at all - it is not a requirement for a planet (even according to new rules) to have a nearly circular orbit. The requirement is to have a nearly spherical shape for the *planet* itself.
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And I think it is wrong to ask for an actual, quantified rule. Astronomers are well aware that the classification is artificial - it's really there to help sorting different bodies in the sky better. If we started quantifying, we could have 9 categories for planets right away.
So "very nearly spherical" tells us a good rule of thumb to make a decision, especially because we use the mass and the force of gravity as the basis, which are well-known quantities for known celestial bodies.
Mr. Colossal's links are right: there is no overlap between the orbit of Pluto and Neptune. The problem is that Pluto has a tilted orbit, so it implies that it didn't form as part of the planet formation process in the solar system.
When a star forms, a significant amount of matter orbits it, shaped like a wide, narrow disk, in the plane of the star's equator. This accretion disk is the source for the planets' material: there are different chunks of matter in the disk. Larger chunks have slightly greater gravity, so they have a better chance of attracting other (smaller) chunks and become even larger. But this results in planets in the plane of the accretion disk. Pluto could not have formed this way because of its tilted orbit - it must have joined the inner solar system in a different way and got hooked by the Sun's gravity.