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Forum » SpaceEngine » Science and Astronomy Discussions » Defining Planethood (What sets planets apart?)
Defining Planethood
HarbingerDawnDate: Friday, 14.06.2013, 09:01 | Message # 1
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[Split from here]

Quote (titanousbont)
It is a planet, it is just a dwarf planet, it is still a planet.

Incorrect. "Planet" as a class of objects refers to large worlds which do not fit into any other large category of objects. Pluto does not fit this description as it clearly does belong in another category (Kuiper belt object). It was rejected from planet status for the same reason that Ceres was in the mid-19th century: other similar objects were discovered in similar orbits and it became clear that these objects formed a distinct population.





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Edited by HarbingerDawn - Friday, 14.06.2013, 22:31
 
WatsisnameDate: Friday, 14.06.2013, 18:03 | Message # 2
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^What Harbinger said.

Objects like Pluto and Ceres fail to meet what is perhaps the most powerful criterion for what we now define as a planet -- that they dominate their orbital space.

Quote (smjjames)
we don't have close up pictures of Pluto like we do for the rest of the planets.


Eagerly awaiting 2015! smile





 
HarbingerDawnDate: Friday, 14.06.2013, 19:01 | Message # 3
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Quote (Watsisname)
that they dominate their orbital space.

I prefer to avoid using that as a criterion. How do you define dominating its orbital space (or "clearing its neighborhood" as the IAU phrased it)? All of the planets share orbital space with other populations of bodies. I think it is simply sufficient to judge whether a given object falls into another category or not. If it does not (and it is large enough to be round) then it is a planet.





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WatsisnameDate: Friday, 14.06.2013, 19:42 | Message # 4
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Quote (HarbingerDawn)
I prefer to avoid using that as a criterion. How do you define dominating its orbital space (or "clearing its neighborhood" as the IAU phrased it)? All of the planets share orbital space with other populations of bodies.


Of course they do, the solar system is a busy and chaotic place.

By 'dominating the orbital space' I don't mean there isn't other stuff in the orbit. I mean that the object consists of the majority of the total mass of the population that occupies the region. What defines 'majority'? It's arbitrary. But such is the trouble of trying to categorize things in nature that don't form fundamentally distinct groups. That said, this works fairly well for the solar system as we know it.

Quote (HarbingerDawn)
I think it is simply sufficient to judge whether a given object falls into another category or not.


How does one do this without having a set of criteria to define different categories, such as the example above?





 
HarbingerDawnDate: Friday, 14.06.2013, 20:52 | Message # 5
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Quote (Watsisname)
How does one do this without having a set of criteria to define different categories, such as the example above?

You class objects into categories by their orbital and physical characteristics. If an object does not fall into any other category (and is large enough to be round, though not necessary actually round) then it is a planet. In a funny sort of way, "planet" is the miscellaneous category of planetary astronomy. The only thing that all planets have in common is that they have nothing in common with any other category of objects. Unlike all other categories of object planets do not share orbital or physical characteristics with each other in most cases.





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WatsisnameDate: Friday, 14.06.2013, 21:43 | Message # 6
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That's what I'm getting at though. If Pluto dominated its orbital region, then it would not be a member of a large population of similar bodies. The two methods we're describing here are based on the same principle, just described in a different way.

Planets are unique from other categories of objects in the solar system because they became the most dominant ones -- they 'won the accretion game', and so for their orbital space there are no comparable bodies. And the planets are unique from one another because they are massive enough for compositional differences and geological processes to become important (for terrestrials) or the creation of thick gaseous atmospheres (for giants). I think this best describes what makes a planet special. smile





 
HarbingerDawnDate: Friday, 14.06.2013, 21:46 | Message # 7
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I'm still not sure that's the best way to phrase it though, since that would be confusing to the layman, and since the phrasing of "dominating its orbital space" or "clearing the neighborhood around its orbit" is somewhat ambiguous. I don't argue that those are wrong, only that they're not the best way to describe it.




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WatsisnameDate: Friday, 14.06.2013, 22:10 | Message # 8
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I agree, it could be phrased much better. Ultimately it requires some mathematical expression; such as a ratio of the mass of the object to the sum of everything else in that region, and setting some boundary. Problem of course still is that it's an arbitrary boundary, both in terms of the mass ratio and the volume of space you consider. That being said I believe that when our solar system is considered in this manner we find distinct and unambiguous regimes (I don't know the actual numbers) separating the 8 planets from everything else.

Edit: Was there ever a thread discussing this subject (I searched but didn't see one)? Perhaps we can split or make a new thread for it if we want to continue.





 
HarbingerDawnDate: Friday, 14.06.2013, 22:27 | Message # 9
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I don't think that it necessarily can be expressed mathematically, or if it can it would be some expression with a lot of variables. I can imagine circumstances where this might not be applicable. That's why I like to establish well-defined conceptual criteria without going into the black and white of mathematics, it's ultimately more likely to result in a good categorization for a given object.

Quote (Watsisname)
Perhaps we can split or make a new thread for it if we want to continue.

Agreed, I'll do that now.





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Crashman1390Date: Saturday, 15.06.2013, 01:15 | Message # 10
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Quote (Watsisname)
2015!

WOW! NASA added on one year! CHEST BUMP! WOOOOO!





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HarbingerDawnDate: Saturday, 15.06.2013, 01:20 | Message # 11
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Quote (Crashman1390)
NASA added on one year!

What are you talking about?





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WatsisnameDate: Saturday, 15.06.2013, 02:13 | Message # 12
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Apparently there's some profound hack for the laws of celestial mechanics we weren't aware of...




 
SpaceEngineerDate: Saturday, 15.06.2013, 09:00 | Message # 13
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Quote (HarbingerDawn)
I prefer to avoid using that as a criterion. How do you define dominating its orbital space (or "clearing its neighborhood" as the IAU phrased it)?

It must clear out its orbit off other bodies of similar size. So Jupiter do not have other gas giants or terrestrial planets on its orbit, just few trojan asteroids in Lagrange points. Moons does not count.

Quote (Watsisname)
By 'dominating the orbital space' I don't mean there isn't other stuff in the orbit. I mean that the object consists of the majority of the total mass of the population that occupies the region. What defines 'majority'? It's arbitrary. But such is the trouble of trying to categorize things in nature that don't form fundamentally distinct groups. That said, this works fairly well for the solar system as we know it.

This is another good method. It works well in Solar System, but does not work in young systems that have trojan planets, ie planets with the same orbit in Lagrangian points of each other (early Earth and Theia as example). This configuration is unstable, but may exist somewhere.





 
HarbingerDawnDate: Saturday, 15.06.2013, 09:58 | Message # 14
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Quote (SpaceEngineer)
It must clear out its orbit off other bodies of similar size

I understand that, Watsisname said similar things. I just think that those terms I quoted are not the best way to describe it, especially to people not very knowledgeable about planetary astronomy.
Quote (HarbingerDawn)
I don't argue that those are wrong, only that they're not the best way to describe it.


Quote (SpaceEngineer)
but does not work in young systems that have trojan planets, ie planets with the same orbit in Lagrangian points of each other (early Earth and Theia as example). This configuration is unstable, but may exist somewhere.

What if a 6 Jupiter-mass planet had an Earth-mass trojan. Would that be stable because of the large mass difference, or are all trojan orbits unstable on long timescales? (I feel silly for not knowing the answer, I really should wacko )





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WatsisnameDate: Saturday, 15.06.2013, 16:37 | Message # 15
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I'm not certain of the answer myself. Trojan orbits occupy the L4 and L5 Lagrange points, which are stable. I'm fairly sure that this stability depends on the mass ratio (ideally considering gravitational test particles, for which asteroids relative to a gas giant is a good approximation), but I don't know at what ratio the stability begins to fail. It ought to be discussed in some scholarly article somewhere, I'll take a look when I have the opportunity.

I wonder what would happen to our understanding/definition of planethood if we were able to view other solar systems in arbitrary detail (resolving even asteroid size bodies). Do you think it would become more or less coherent with such a greater sample size? I honestly don't have a clue. wacko





 
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