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Old 03-08-2016, 01:41 PM   #31
Urwumpe
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Originally Posted by TMac3000 View Post
 But "It has clouds" combined with "It orbits the Sun" is quite convincing. That wouldn't demote any of the classical planets, and also would not promote anything in the Asteroid belt or Kuiper Belt (unless Ceres or Sedna could be shown to have atmospheres).
What about comets?
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Old 03-08-2016, 02:19 PM   #32
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 In other words: if it orbits the sun, it might be a planet (Ceres isn't). If it has an atmosphere, it might be a planet (Titan isn't). If it meets both, it is certainly a planet.
That definition appears to be strictly based in sentimentality. If there's any scientific logic behind it, you aren't explaining it well enough. Again I ask, why should clouds influence the definition at all? Titan isn't a planet because it doesn't function like a planet. In the path it takes around the sun, it comprises a minute fraction of the mass and directly controls almost nothing. Saturn is the dominant body here, affecting planetary/gravitational dynamics strongly enough to be comparable only to other planets. Saturn has clouds but it isn't a planet because of that, it's a planet because it is one of the gravitationally dominant bodies of the solar system, whereas Titan is not.

Like I said before, Pluto doesn't even come close to affecting other bodies the way a planet does. It is controlled by Neptune. Their orbital resonance is locked in by Neptune's influence on Pluto, not Pluto's influence on Neptune. Just as it would be ridiculous to call Triton or Titan planets, it's ridiculous to call Pluto a planet, as it just is not functionally a planet at all.

And like you said yourself, the definition falls apart as soon as you look at Mercury, requiring you to make an exception (which defeats the point of a definition), or you drop the "clouds" requirement and just make it anything that orbits the sun, and we know that that won't work either.
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Old 03-08-2016, 04:17 PM   #33
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 What about comets?
A coma is not the same as an atmosphere: it is not gravitationally bound to the comet.

---------- Post added at 11:17 AM ---------- Previous post was at 11:03 AM ----------

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Again I ask, why should clouds influence the definition at all?
As I said, by itself it doesn't. But when a gravitationally bound atmosphere is combined with a solar orbit, in my opinion the rest of the requirements should be waived and the object should be considered a planet. Mercury is a planet because, while it does not have an atmosphere, it meets the IAU requirements that excluded Pluto.

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Titan isn't a planet because it doesn't function like a planet.
No...Titan isn't a planet because it orbits Saturn, not the sun.

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In the path it takes around the sun, it comprises a minute fraction of the mass and directly controls almost nothing. Saturn is the dominant body here, affecting planetary/gravitational dynamics strongly enough to be comparable only to other planets.
I don't see why any of those things should be a factor.

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Like I said before, Pluto doesn't even come close to affecting other bodies the way a planet does. It is controlled by Neptune. Their orbital resonance is locked in by Neptune's influence on Pluto, not Pluto's influence on Neptune. Just as it would be ridiculous to call Triton or Titan planets, it's ridiculous to call Pluto a planet, as it just is not functionally a planet at all.
I would be a smart-aleck here to accuse you of suggesting that Pluto orbits Neptune. We all know better than that, and I don't think you are saying that. But...what exactly are you saying with that?

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And like you said yourself, the definition falls apart as soon as you look at Mercury
Umm...no. While I mentioned that anything that meets both of those conditions (orbit the sun and have an atmosphere) should definitely be called a planet, I did not say that a body that is missing one or the other should not be called a planet. Hence, my assertion is, first of all, not a definition, and secondly, does not exclude Mercury.

Now, I say my assertion is not a definition because 1), it is not an AND gate, otherwise it would exclude Mercury, and 2) it is not an OR gate, otherwise it would include Ceres and Vesta. It is merely an exception to the IAU requirements.

Last edited by TMac3000; 03-08-2016 at 04:32 PM.
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Old 03-08-2016, 04:25 PM   #34
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 A coma is not the same as an atmosphere: it is not gravitationally bound to the comet.
And the atmosphere of Pluto is really different? Its only 0.3 Pa surface pressure and it is constantly blown away by the weak solar wind.
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Old 03-08-2016, 04:36 PM   #35
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 And the atmosphere of Pluto is really different? Its only 0.3 Pa surface pressure and it is constantly blown away by the weak solar wind.
True--but it's still gravitationally bound while it's there, and it is replenished by the planet itself. That makes it different from the pseudo-atmosphere of Mercury, which is nothing more than a veil of solar wind.
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Old 03-08-2016, 04:41 PM   #36
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 True--but it's still gravitationally bound while it's there, and it is replenished by the planet itself. That makes it different from the pseudo-atmosphere of Mercury, which is nothing more than a veil of solar wind.
Well, what about that then:

http://news.nationalgeographic.com/n...space-science/

In some years Pluto might be like Eris and have no atmosphere at all because it is too cold.
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Old 03-08-2016, 04:47 PM   #37
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Then it would be suitable to demote Pluto from planet. Until then I see no reason not to keep it.

Last edited by TMac3000; 03-08-2016 at 04:49 PM.
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Old 03-08-2016, 04:49 PM   #38
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 Then it would be suitable to demote Pluto from planet. Until then I see no reason not to keep it.
So, you mean it should be a "perihelion-only-planet"?
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Old 03-08-2016, 04:59 PM   #39
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Hmmm...no. Now that is a good point there

Look, I'm not emotional or sentimental about this. I just think that, while it's silly for people to complain about Pluto being demoted, it's equally silly to heap ridicule on anyone who might have a difference of opinion with the IAU and have some reasoned arguments to back it up, and lump them in with moon-hoaxers and 9/11 truthers. I'm not saying that you did that--you certainly haven't--but still...
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Old 03-08-2016, 05:05 PM   #40
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The whole planet/no planet dilemma seems to stem from the human compulsion to categorize. At heart we all appear to be autists who get uncomfortable whenever we discover continuous transitions in formerly neatly separate species.

Given that the planet yes/no separation is necessarily arbitrary and therefore meaningless, why not allow a subjective component, giving up generality in favour of individual freedom:

"A body is a planet if you feel it should be"
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Old 03-08-2016, 05:08 PM   #41
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Alan Stern gives a more detailed account of his reasons for calling Pluto a planet in this interview from 2011:

Pluto's Planet Title Defender: Q & A With Planetary Scientist Alan Stern.
By Mike Wall, Space.com Senior Writer | August 24, 2011 12:00am ET
http://www.space.com/12710-pluto-def...interview.html

The key point is the "clearing the neighborhood" argument would also fail for an Earth-sized planet at Pluto's location in the Solar System. The reason is it is too large an area to clear out that far from the Sun.

By the way, Stern is not opposed to the term "dwarf planet", in fact he first proposed it. But he says the term "dwarf" should only be an adjective to describe its size. It should still be considered a planet. It would be analogous to "rocky planet" being used to described Earth-sized planets, which are of course still planets.

The key question is do we want to have a scenario where a star system, including our own, might have hundreds of planets? We may very well find other systems with Earth-sized or larger planets at Pluto-distances from their star. In that case they would also not clear their neighborhood but it would be hard to say they are not planets.


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Old 03-08-2016, 05:11 PM   #42
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 As I said, by itself it doesn't. But when a gravitationally bound atmosphere is combined with a solar orbit, in my opinion the rest of the requirements should be waived and the object should be considered a planet. Mercury is a planet because, while it does not have an atmosphere, it meets the IAU requirements that excluded Pluto.
Why should they be waived, other than your opinion that Pluto should be a planet? There doesn't appear to be any scientific basis to this.

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No...Titan isn't a planet because it orbits Saturn, not the sun.
The reason Titan orbits Saturn and not the other way around is because Saturn is the gravitationally dominant body, and that's the important part here.

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I don't see why any of those things should be a factor.
Because that's what defines a planet, a body orbiting the sun that has enough gravitational influence to "clear the neighbourhood", ie. completely control everything within its orbital path, either by locking objects into a resonance, capture them as a satellite, or boot them into a different orbit. Saturn does that, which makes it a planet. Titan is incapable of clearing the neighbourhood because Saturn controls Titan and holds it as a satellite. Thus, Titan doesn't affect planetary dynamics at all, it's just an extension of Saturn. We're talking about tiers of influence here. The sun gravitationally controls everything in the solar system. Then the planets control everything within their orbits. Then smaller bodies, like Pluto, and moons, can only control things in their immediate sphere of influence, and even then, they still "answer" to the planets.

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I would be a smart-aleck here to accuse you of suggesting that Pluto orbits Neptune. We all know better than that, and I don't think you are saying that. But...what exactly are you saying with that?
Pluto was locked into its current orbit by Neptune. The dominant body in this case is Neptune. Pluto isn't just there because it felt like it, Pluto is there because Neptune put it there. Pluto doesn't need to be a satellite of Neptune to be controlled by it. Orbital resonances derive from the same cause, Neptune controls everything in its orbit, satellite or not, and that includes Pluto. If Pluto were to somehow stray from its current orbital resonance (say a rogue planet came into the solar system and flew by Pluto briefly), Neptune would either lock it back into an orbital resonance, capture it as a satellite, or kick it into a much higher or lower orbit. This makes Neptune a planet, and Pluto, being incapable of doing any of that, is not.

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Umm...no. While I mentioned that anything that meets both of those conditions (orbit the sun and have an atmosphere) should definitely be called a planet, I did not say that a body that is missing one or the other should not be called a planet. Hence, my assertion is, first of all, not a definition, and secondly, does not exclude Mercury.

Now, I say my assertion is not a definition because 1), it is not an AND gate, otherwise it would exclude Mercury, and 2) it is not an OR gate, otherwise it would include Ceres and Vesta. It is merely an exception to the IAU requirements.
The idea here is to have a scientific definition of what exactly qualifies as a planet, and adding arbitrary exceptions is unscientific. You can't add exceptions to a definition. This defeats the point of a definition. There's nothing about an atmosphere that makes something more or less of a planet, which you clearly admit by saying that Mercury is a planet despite its lack of one. So in that case, why include "has an atmosphere" in the definition at all? It's pointless, and clearly intended to qualify Pluto as a planet despite its obvious lack of requirements to be one, a decision which, like I said above, is rooted in sentimentality, not science.
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Old 03-08-2016, 05:57 PM   #43
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You got
I conceded the argument a couple of posts ago
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Old 03-08-2016, 06:18 PM   #44
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 You got
I conceded the argument a couple of posts ago
Yeah, haha. It takes me a good while to write posts while at work.
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Old 03-08-2016, 08:54 PM   #45
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 "It has clouds" is a terrible reason to define something as a planet.
I agree. Heck, even George Lucas would agree. I mean, Endor was a forest moon. It not only had an atmosphere, it was covered with massive speeder bike-eating redwoods for crying out loud, and it was still classified in the Star Wars universe as a moon.
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