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Forum » SpaceEngine » Science and Astronomy Discussions » Defining Planethood (What sets planets apart?)
Defining Planethood
HarbingerDawnDate: Sunday, 13.12.2015, 21:34 | Message # 76
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Quote Wicker1M ()
Some objects greater than 13 Jupiter masses are considered as planets. Why is that?

Usually because their true mass is unknown, and could be lower than 13 Jupiter masses.





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steeljaw354Date: Sunday, 13.12.2015, 22:51 | Message # 77
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why 13? just round it to 10 or 15
 
HarbingerDawnDate: Sunday, 13.12.2015, 23:09 | Message # 78
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Quote steeljaw354 ()
why 13? just round it to 10 or 15

Because 13 Jupiter masses is the threshold at which fusion can occur. It is not an arbitrary boundary, but one dictated by physics.





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WatsisnameDate: Monday, 14.12.2015, 03:22 | Message # 79
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It is a little bit of a fuzzy boundary, as it varies with metallicity, but it works well enough that the astronomical community has largely agreed on it as the dividing line between gas giants and brown dwarfs.

Quote steeljaw354 ()
Whats your defintion of a planet?


Well, I've been talking about my views on it throughout the whole thread, but here's a quick summary:

I call an object a planet if it meets the following criteria:

1) It is massive enough to assume a shape by hydro-static equilibrium. (It is round, not potato-shaped.)
2) It is not so massive that it is undergoing nuclear fusion in its interior. (It is not a star.)
3) It is gravitationally bound to a star. (It orbits a star.)
4) It dynamically rules its orbital region. Equivalently, it is unique within its orbital region, rather than belonging to a population of similar bodies.






 
steeljaw354Date: Monday, 14.12.2015, 11:07 | Message # 80
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So anything that doesn't orbit a star isn't a planet? (not moons, planemos) Even gas giants?
So anything that doesn't rule it's region even if it's earth sized or larger isn't a planet?
I'm afraid your a bit to logical on this one. Dwarf planets still have the name "planet in them" to me that kinda means they are "half planets" As in the ones smaller than 2000KM. But we won't list them until we know more about them. Or until we can get better sizes on them.

Heres mine
1. Doesn't start fusing
2. Is round
3. Orbits a star, or is rouge
4. Size limit on 2000KM for planet anything below is dwarf planet. Pluto and Eris are planets by my 4 rules for a planet. Anything else under 2000KM is a dwarf planet unless it's a potato asteroid. Jupiter,Saturn rule their orbits,Uranus maybe,Neptune Doesn't it is sitting in a field of asteroids.

Earth has asteroids that cross it's orbit and so does almost every other planet, Mercury doesn't dominate it's orbit and mars doesn't. Examples of the orbital domination test, Failed.

Attachments: 5910698.jpe(57Kb)


Edited by steeljaw354 - Monday, 14.12.2015, 20:13
 
SpaceEngineerDate: Monday, 14.12.2015, 21:48 | Message # 81
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Quote steeljaw354 ()
Earth has asteroids that cross it's orbit and so does almost every other planet, Mercury doesn't dominate it's orbit and mars doesn't. Examples of the orbital domination test, Failed.

Domination meaning what there are no other bodies of comparable mass. If Jupiter would have Earth-sized bodies near it's orbit, this configuration will be unstable and they quickly will be ejected away or crushed into Jupiter.

Binary systems such as Earth-Moon is not clear in this definition, as long as trojan planets. Trojans are semi-stable though, and only can exist if mass ratio is great (Jupiter can't have another gas giant as trojan planet, or even Earth-mass planet will be unstable there on long timescales).





 
steeljaw354Date: Monday, 14.12.2015, 21:50 | Message # 82
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Doesn't say "orbital domination" on the IAU one it says "orbital clearing" which certainly isn't in the image I provided. There are no known objects near sedna, it's round orbits the sun, so it's a secondary planet by my defintion. Or "dwarf planet" by my defintion. Orbital clearing by the IAU isn't defined.

Edited by steeljaw354 - Monday, 14.12.2015, 21:54
 
HarbingerDawnDate: Monday, 14.12.2015, 23:54 | Message # 83
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Quote steeljaw354 ()
Doesn't say "orbital domination" on the IAU one it says "orbital clearing" which certainly isn't in the image I provided.

Why are you debating the IAU's definition when nobody here is advocating it? I think everyone here agrees that the IAU's definition isn't great. To replace the arguments made here with those made by the IAU is not a very honest way to conduct a discussion like this.





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steeljaw354Date: Tuesday, 15.12.2015, 00:30 | Message # 84
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So your saying there are 8 planets? When sedna is out there on it's own (as we know) being round and all, not being a star, no bodies anywhere near it's size. Something pushed it out there, rouge planet, maybe Planet X? Anything else that has a "similar" orbit could have been pushed away from where it was from planet x (if it exists) There are currently 2 known sednoids Only 2 and the one smaller than sedna is probably a very tiny rock.
Attachments: 4559514.png(8Kb)


Edited by steeljaw354 - Tuesday, 15.12.2015, 00:36
 
HarbingerDawnDate: Tuesday, 15.12.2015, 01:07 | Message # 85
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Quote steeljaw354 ()
There are currently 2 known sednoids Only 2 and the one smaller than sedna is probably a very tiny rock.

I remember when the second was discovered, and added it into SpaceEngine at the time, and I can assure you it was no tiny rock.





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WatsisnameDate: Tuesday, 15.12.2015, 04:07 | Message # 86
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Quote steeljaw354 ()
So anything that doesn't orbit a star isn't a planet? (not moons, planemos) Even gas giants?


Read my further explanation.

Quote
So anything that doesn't rule it's region even if it's earth sized or larger isn't a planet?


Correct. After reading the further explanation, can you see my motivation as to why?





 
FaceDeerDate: Tuesday, 15.12.2015, 06:29 | Message # 87
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Quote steeljaw354 ()
Dwarf planets still have the name "planet in them" to me that kinda means they are "half planets"


Sea lions are not actually lions. A guinea pig is not a pig or from Guinea. English can be weird that way sometimes.

Where did you get the value of 2000km as the cutoff threshold diameter for planethood? Is there any physical reason for it, or is it just a number that "felt right" for subjective reasons? There doesn't seem to be any reason why there'd be a natural gap in the size of solar system objects at that scale.

You've been given links several times now to various methods of objectively measuring an object's orbit-clearing capability, and none of them indicate that Mercury, Earth, Mars or Neptune have failed to clear their orbital neighborhoods. Where are you getting your method of determining orbital neighborhood-clearance? It seems like you're just making subjective calls on that, too.

Unless you have some actual process for determining these things that other people can run through and check the results of for themselves I don't think your definition is very useful. Particularly in the context of Space Engine, were we can travel to innumerable other solar systems and see all sorts of arrangements of objects orbiting stars. If I spin the wheel and go to some random solar system out there, how can I determine whether a body has cleared its orbit using your method in such a way that anyone else can go there and get the same answer from the same data?
 
steeljaw354Date: Tuesday, 15.12.2015, 11:15 | Message # 88
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Quote FaceDeer ()
Sea lions are not actually lions. A guinea pig is not a pig or from Guinea. English can be weird that way sometimes.


Well why do we call things that way then? What do you mean it was no tiny rock? was it bigger than sedna?


Edited by steeljaw354 - Tuesday, 15.12.2015, 11:16
 
HarbingerDawnDate: Tuesday, 15.12.2015, 11:57 | Message # 89
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Quote steeljaw354 ()
was it bigger than sedna?

No, it's about half the size of Sedna, but that's still quite large, certainly large enough to be round.





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steeljaw354Date: Tuesday, 15.12.2015, 20:08 | Message # 90
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Any proof that it is round? [Citation needed]
 
Forum » SpaceEngine » Science and Astronomy Discussions » Defining Planethood (What sets planets apart?)
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