VT-16 wrote:I just think this last "extra" criteria sounds like it was slapped on at the final moment. What exactly is "ability to clear its own orbit of debris"?
How distant must the debris be to the planet? How concentrated must the debris be to be counted?
It should actualy ba a fairly well defined cut off point with only a thin grey area between planet and dwarf planet. If the planet doesn't have enough mass then however much junk it sweeps up in one orbit the band which is cleared is so small that by the time the planet came back to its origional position osmosis would have caused as much junk to drift back in as was there origionaly. If there is less than the origional amount of junk, even only slightly, then it should be a simple cumulative afect to clear the entire band. Especialy remembering how much time that cumulative efect would have had by the time life emerges on one of the planets.
Or I could be talking out of my ass. Feel free to corect.
From what I can tell the debris they are talking about has to be in solar orbit, as opposed to orbiting the planet itself.
I'm more interested in how this affects Neptune, since Pluto's orbit intersects Neptune's orbit.
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VT-16 wrote:Also thought of another fun scenario after my last post:
What if we found a planet that might have had a moon be destroyed and now littering it? Would this planet not be a planet until it cleared its surroundings? Hey, to take examples from our system, what about Jupiter, Saturn, Uranus and Neptun? Doesn't their rings count as debris? They're not going to be cleared for a while.
The "sweeps its orbit" thing prevents large asteroids and other semi-large inner-system objects from being classified as planets. It also connotes a certain size and gravity.
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I'm more interested in how this affects Neptune, since Pluto's orbit intersects Neptune's orbit.
Isn't that only the case if you look at a 2D, top-down or profile representation of the orbit? Pluto's orbit is far off the plane of the eight planetary orbits.
I'm more interested in how this affects Neptune, since Pluto's orbit intersects Neptune's orbit.
Isn't that only the case if you look at a 2D, top-down or profile representation of the orbit? Pluto's orbit is far off the plane of the eight planetary orbits.
It does have a 17 degree angle to the rest of the planetary orbits, though it cuts across the plane of the orbits of the rest of the planets for obvious reasons...
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Lord Zentei wrote:I'm more interested in how this affects Neptune, since Pluto's orbit intersects Neptune's orbit.
Exactly the objection Bill Jefferys raised, in private communication. That's one bona fide astronomer not liking this definition.
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What really gets me is this "it's not a planet, but it's a kind of planet". That's basically been common knowledge ever since we learned about Pluto's real size: it's a planet, but it's really fucking small.
Why even call these things "dwarf planets" when that somehow means they're not planets after all? :S
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I don't see why not, classification of Pluto as a planet was by comittee, wasn't it?
This is just stupid, I can't believe they dropped Pluto. What about Tradition, History?
Well, if you get a piece of compacted space junk flying around the sun in a different manner to the rest of the planets and all the rest of the differences between Pluto and the proper planets and the similarity with KBOs, well, a tradition that's under a century old has to take back seat to clinical consistency.
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And how does Mercury rate as a planet? Its patheticaly small and I doubt it sweeps much more material then Pluto.
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Alyeska wrote:And how does Mercury rate as a planet? Its patheticaly small and I doubt it sweeps much more material then Pluto.
Because it's just about half the mass of Mars and is over 25 times the mass of Pluto (And something like 4.5 times the mass of Earth's moon.) Its escape velocity exceeds that of Mars. If Mercury had formed where Mars is today, it'd have held onto its atmosphere better, as it, unlike Mars, has a real magnetic field (owing to the fact that it's mostly core, unlike Mars, which has a pretty low density as far as terrestrial planets go.)
If one asks how can Mercury rate as a planet, then they can ask the same question of Mars . . . since they're both pretty close to the same size, and were it not for Jupiter getting in the way, all that lovely main asteroid belt debris we see today would be part of Mars.
I honestly don't care. Whether we call it a planet, planetoid, planetino, or planetoga, Pluto will still be there when the sun rises on Earth tomorrow. And I will always think of Pluto as the ninth planet.
I do, however think it is somewhat ironic that Pluto should be the one planet we haven't sent a spacecraft too, and suddenly, but months after we launch New Horizons to correct this oversight, we decide it's not a planet after all.
Does anyone else think that's a bit odd?
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A good plan for a revised kid's textbook would be to show all 9 planets, but with an asterisk next to Pluto saying "officially a dwarf planet". Then in the lower corner they can also list Ceres and Xena and Quaoar. No need to radically revise our current mental image of the solar system when we can just add an asterisk.
Mike Brown wrote:
Population classification. This definition requires a little more explanation and a little more understanding of the solar system, but, in the end, leads to the most satisfactory definition of "planet". Just like the solar system very naturally divides itself between round objects and non-round objects, it also very naturally divides itself between solitary individuals and members of large populations. The best known example of a large population is the asteroid belt. We call it a population because one region of space contains objects with a continuous range of sizes from one moderately large object (Ceres) to a handful of slightly smaller objects (Vesta, Pallas, Hermione) to a huge number of extremely small objects (rocks, dust particles). The solitary individuals are much different. In their region of space there is only them (Earth, say) and then a collection of much much smaller objects (the near-earth asteroids), with no continuous population in between. A single example helps to dramatize the difference between a continuous population and a solitary individual. Ceres, the largest asteroid, has a diameter of 900 km. The next largest asteroid, Pallas, has a diameter of 520 km. After that is Vesta at 500 km, and Hygiea at 430 km, and the list continues on down. The jump in size between asteroids is never more than a factor of two. In contrast, the earth has a diameter of about 12,000 km, while the largest other object in the earth's vicinity, the asteroid Ganymed, has a diameter of about 41 km, a factor of 300!
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune all count as solitary individuals by this definition. Pluto and Quaoar do not. Pluto is clearly a member of the Kuiper belt population, as can be seen from the fact that there are objects in the same vicinity slightly smaller than Pluto (Quaoar, 2004 DW, Varuna), and then even a larger number slightly smaller than that, and then on down.
It seems that the decision making process has essentially gone:
1. Let's keep Pluto as a planet, and make all the other Pluto-like bodies planets too.
2. Later... Agh! Too many planets, let's call all these other ones Dwarf Planets.
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LMSx wrote:A good plan for a revised kid's textbook would be to show all 9 planets, but with an asterisk next to Pluto saying "officially a dwarf planet". Then in the lower corner they can also list Ceres and Xena and Quaoar. No need to radically revise our current mental image of the solar system when we can just add an asterisk.
Or hell, just add the mention of pluto being a 'dwarf planet' due to its erratic orbit and small size when teaching from the old books. Then bring it up again in a brief talk of the Solar System's planetoids like Xena and Ceres. The change isn't big enough to throw out books just because the last planet is mis-labeled (in fact I'd wager this makes some districts' text books suddenly accurate again ). But it's not like most people even retain any semblance of knowledge about other planets either way; they'll remember it's location, general size, and general environment when it's presented in 3rd and 10th grade, then go back to not knowing it in the same way they've never heard of Libya.
I've always been a fan of astronomy since childhood, but I think it's perfectly good they finally sorted out Pluto's awkward categorization, and promoted other noteworthy objects to the same category. Just like with biological taxonomy, If we were content to continue lumping things into the same old, simplistic categories purely for the sake of tradition and personal feelings, it wouldn't be science.
Winston Blake wrote:It seems that the decision making process has essentially gone:
1. Let's keep Pluto as a planet, and make all the other Pluto-like bodies planets too.
2. Later... Agh! Too many planets, let's call all these other ones Dwarf Planets.
It's not as if there is any specific cut off point in nature, just a smooth scale of sizes of celestial bodies. The problem is the human insistance to lable and compartmentalise everything. That's why there's the problem with the Plutoids: they're nearly planets but not quite, and neither 'planet' or 'not planet' fits them.
On a side not I wonder if the definition of planet could also be defined by the streangth of the given stars gravity at any given distance. Could it be that Pluto is so far out and the suns influence so small that its orbit would be inherantly unstable? I meen if the centripetal force of the sun is that week then the orbits of the debris can be more easely afected by the debris around them, making the cloud of orbiting debris more spread out and less likely to coeless. So could it be that having a true planet that far out is imposable?
Then in the lower corner they can also list Ceres and Xena and Quaoar.
Sigh. Why not have them on the illustration as well if they're the same thing? Making Pluto "special" even though it doesn't deserve it any more than say, Ceres, is ridiculous as well.
Either way, I'm gonna start treating Ceres, Pluto, Charon and "Xena" as planets from now on. "Dwarf planet" is a kind of planet like a "dwarf elephant" is a kind of elephant, that's my new rule. :P
I hate to "me too" but I have to say that it's about time that they took Pluto off the list. It may have been special and planetary back in 1930, but now it's just another spherical Kupier Belt object. Like we've discovered more of.
Pluto=Not a planet. Finally.
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Agh! Too many planets, let's call all these other ones Dwarf Planets.
). But it's not like most people even retain any semblance of knowledge about other planets either way; they'll remember it's location, general size, and general environment when it's presented in 3rd and 10th grade, then go back to not knowing it in the same way they've never heard of Libya.