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Defining Planethood
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| midtskogen | Date: Wednesday, 08.06.2016, 19:49 | Message # 136 |
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| It was IAU's proposal. It must have a mass, size and composition that create hydrostatic equilibrium.
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| spacer | Date: Wednesday, 08.06.2016, 19:50 | Message # 137 |
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| i am not sure pluto and other big kaiper belt objects is massive enough to have clear orbit and have nothing to disturb them.
pluto disturbed by neptun, the others probably disturbed by planet 9, as you can see thier orbits are not perfect round.
and pluto is smaller than some of the big kaiper objects so why declare pluto as planet becuase of history but not the bigger objects?
"we began as wanderers, and we are wanderers still"
-carl sagan
-space engine photographer
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| Watsisname | Date: Wednesday, 08.06.2016, 21:13 | Message # 138 |
 Galaxy Architect
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| That's actually one thing you've proposed so far that I find fairly reasonable. Let's not assume planet 9 exists until we have even more compelling evidence. That doesn't necessarily need to be in the form of an image, however. We don't need an image of a black hole to have enormous confidence that they exist, for example.
As for determining the lower bound for planetary definition, size alone is a terrible metric. Especially if you're just going by "1000km" or whatever. This is an arbitrary choice in many ways. It's arbitrary because you are imposing a base-10 number on a universe that does not know or care about base 10. You are imposing "kilometers" as a distance measure on a universe that does not know or care about the definition of a meter. There is nothing in nature that makes 1000km a special dividing line in the context of astrophysical bodies. Why use it?
Let us instead be motivated by something physical, allowing nature itself to guide us in defining how to classify things. Observe that objects assume a shape by hydrostatic equilibrium when they get large enough for their composition. Apply physics to determine why that should be and calculate the conditions for which it happens.
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| steeljaw354 | Date: Wednesday, 08.06.2016, 21:29 | Message # 139 |
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| Or we could do this idea which I have seen a few times, I have taken a liking to it. "Wanderer classification"
All are classified into the same category called Wanderers, we have 2 subset.
Planets: Mercury,Venus,Mars,Jupiter,Saturn
Dwarf Planets: Ceres,Uranus,Neptune,Pluto,Eris
How you classify, Planets are objects that constantly appear in the sky and have a magnitude above magnitude 5 they are rounded objects and not man made. Dwarf Planets are objects that have a magnitude below 5 and are round in shape, not man made. Moons are objects that orbit planets and dwarf planets and not man made, Dwarf moons are objects that orbit asteroids as they are very small, not manmade.
Earth isn't classified into any of these because it isn't visible in the sky.
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| HarbingerDawn | Date: Wednesday, 08.06.2016, 23:46 | Message # 140 |
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| Quote steeljaw354 (  ) "Wanderer classification"
This is without a doubt the most useless classification system you've proposed yet. It doesn't take the intrinsic or contextual characteristics of any of the bodies into account. It also calls Neptune a "dwarf", but not Mercury. It's arbitrary and insane. It's EXTREMELY unscientific.
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| steeljaw354 | Date: Thursday, 09.06.2016, 00:30 | Message # 141 |
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| Neptune is a dwarf in this classification because it is very dim, size isn't taken into account, only apparent brightness in this classification.
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| FaceDeer | Date: Thursday, 09.06.2016, 01:50 | Message # 142 |
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| A classification based on apparent magnitude is, quite frankly, the worst thing I've heard yet. The Moon and the Sun become planets under it as well, along with a whole bunch of artificial satellites in low Earth orbit. And of course it's not at all applicable to exoplanets, and someday if we've got colonies on other worlds they'll have an entirely different set of objects that count as "planets" to them.
I'm getting a little baffled here. Why do you dislike the IAU's definition so much that you'd cast this far afield looking for alternatives?
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| Ostarisk | Date: Thursday, 09.06.2016, 03:36 | Message # 143 |
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| Quote FaceDeer ( ) Why do you dislike the IAU's definition so much that you'd cast this far afield looking for alternatives?
I agree. using apparent magnitude isn't much help for a new planet classification system, as that would mean that stars like Betelgeuse will become planets when they go supernova, as (according to many theories) any red supergiants (In my case, red giants the form from blue stars) which should go supernova within our time here in the universe, will light up the sky, outshining the moon and making night become like day, illuminating the sky. This would also mean that meteors that appear bright in the sky, such as the one that lit up New Zealand skies, will also become planets. However, I do believe the IAU's planet classification is wrong.
According to the IAU, a planet is a planet when it's orbit is cleared of debris, which means the planets such as the Earth, Jupiter, and Saturn, are dwarf planets, as Jupiter obviously has its trojan asteroids, meteors can be seen falling through earth's sky constantly, and at times, Saturn can be observed having objects falling into the planet. This means that only Mercury, Venus, Mars, and Uranus will be the only actual planets in the system (If I can recall correctly).
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Edited by Ostarisk - Thursday, 09.06.2016, 03:48 |
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| spacer | Date: Thursday, 09.06.2016, 04:26 | Message # 144 |
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| ohh and i need to fix myself, when i wrote circle orbit i meant to say perfect orbit like earth etc that doesnt have really strange Aphelion
and Perihelion. sorry 
"we began as wanderers, and we are wanderers still"
-carl sagan
-space engine photographer
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| FaceDeer | Date: Thursday, 09.06.2016, 04:53 | Message # 145 |
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Space Pilot
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| The debate is starting to loop. As I explained in my post one page earlier, the presence of any particular little bit of detritus in an object's orbital zone is not sufficient to disqualify that object for planethood. That would just be downright silly - the IAU would not propose a definition that excluded all of the major bodies of the solar system. And indeed, the IAU's definition has a footnote that explicitly calls out the eight known planets as being examples of planets under it. So if you read the IAU's definition and think "according to this Jupiter is not a planet" that means you've misunderstood something about the IAU's definition, not that the IAU has misunderstood something about it.
I'll try to summarize it again.
There exist several different formulae that one can use to produce a number that represents either an object's capability to clear other objects from its orbit (the Stern-Levison parameter is one such) or that represents the degree to which the object actually has cleared other objects from its orbit (the Soter planetary discriminant).
This table shows the actual numbers for various large bodies in our solar system. These numbers account for things like the Trojan asteroids, Earth-crossing meteors, and other such bodies. It turns out it doesn't matter too much which specific formula you use, they all produce a strikingly similar pattern.
Compare the degree to which Earth or Jupiter has cleared their neighborhoods to the degree to which Pluto or Eris have cleared their neighborhood. None of them have cleared them perfectly, as you point out. But the planets have cleared them to a degree five or six orders of magnitude greater than the non-planets.
Furthermore, as Watsisname illustrated with this excellent histogram, the numerical degree of orbit-clearing for these two groups of bodies shows a clear pattern of clustering. Planets have a similar amount of orbital cleanliness, and non-planets have a similar amount of cleanliness, and there's a huge region in between those two groups that is unoccupied. A bimodal distribution like this is an excellent basis for a division in a classification system, you see this sort of pattern throughout all scientific disciplines.
The difference between planets and non-planets in this regard is large, clear, meaningful, and easily demonstrated. This is not an ambiguous measure, there're no weird "gotchas" or edge cases or ambiguities, the IAU didn't somehow accidentally declare that there's no such thing as a planet or that Earth is not a planet or anything silly like that.
The eight planets really have effectively cleared their orbits of other debris. The other major solar system objects really haven't done so. The numbers are right there in black and white, the method of calculating them and the theoretical justifications for choosing those methods is available for scrutiny in the referenced papers, you can check the results yourself if you don't believe them. If you wish to criticize the methods by which those formulae operate then by all means explain how they've gone wrong. But please be finished with insisting that they give results that they don't actually give.
Edited by FaceDeer - Thursday, 09.06.2016, 04:55 |
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| midtskogen | Date: Thursday, 09.06.2016, 05:33 | Message # 146 |
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| The geocentric model was abandoned centuries ago, but philosophers in the antiquity would agree with steeljaw354: They held that there were 7 planets: The Sun, the Moon, Mercury, Venus, Mars, Jupiter and Saturn.
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| JackDole | Date: Thursday, 09.06.2016, 07:52 | Message # 147 |
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| Quote midtskogen ( ) They held that there were 7 planets: The Sun, the Moon, Mercury, Venus, Mars, Jupiter and Saturn.
That's what I've said here:
Quote JackDole ( ) The term planet means something like wanderer, and in ancient times all the objects were so called, who are wandering across the sky. So I think only the planets that were known in ancient times, should be called planets: Sun, Moon, Mars, Mercury, Jupiter, Venus! All other would then be 'dwarf planets' because they are so small that they can not seen! 
 
Perhaps it is this that gave steeljaw354 the idea. Then it's my fault! 
Don't forget to look here.
Edited by JackDole - Thursday, 09.06.2016, 07:55 |
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| midtskogen | Date: Thursday, 09.06.2016, 09:35 | Message # 148 |
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| Yes, "planet" is Greek, and the Latin term is "stella errans" (as in the old meaning of English "to err" - "to stray off"). In modern languages: "wandering star", "Wandelstern", "vandrestjerne", as opposed to "fixed star", "Fixstern", "fiksstjerne" (Latin "stella fixa"). These terms come from the geocentric worldview.
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| steeljaw354 | Date: Thursday, 09.06.2016, 10:16 | Message # 149 |
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| If planet 9 is really real, it should be named Pluto because it is a "destroyer" and "controller" and we should rename Pluto to Cupid because it has a heart on it. Problem solved? And I also think uranus should be renamed to "Caelus" to avoid the issues with the name "uranus". It is pronounced "Say-less" instead of the rather awkward name that uranus has.
Edited by steeljaw354 - Thursday, 09.06.2016, 10:32 |
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| midtskogen | Date: Thursday, 09.06.2016, 10:30 | Message # 150 |
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| A heart?
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Edited by midtskogen - Thursday, 09.06.2016, 10:33 |
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