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Forum » SpaceEngine » Science and Astronomy Discussions » Defining Planethood (What sets planets apart?)
Defining Planethood
HarbingerDawnDate: Saturday, 12.12.2015, 12:14 | Message # 46
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steeljaw354, essentially what you're saying is that science should be disregarded entirely, and definitions should be based upon your own personal feelings. Your reasoning is extremely weak: "I don't agree with the IAU, therefore anything goes". That makes no sense. If you read the rest of this thread, you'll see that regardless of what the IAU says, it makes no sense to put Pluto in the same category as Neptune.

Quote JackDole ()
I am not referring to the Kuiper Belt as such, I was referring to Pluto, and Pluto crosses Neptune's orbit

No. Pluto's semi-major axis is greater than Neptune's. As I said before, it doesn't matter if individual objects cross paths with the governing body (Neptune in this case), what matters is the semi-major axis of the orbits, and whether the objects fall into resonances with the governing body. Pluto is in a 2:3 orbital resonance with Neptune, so its orbit is governed by Neptune's gravity. Pluto is not an exception to the rule, it is a perfect example of the rule.





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Edited by HarbingerDawn - Saturday, 12.12.2015, 12:28
 
steeljaw354Date: Saturday, 12.12.2015, 13:19 | Message # 47
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Neptune shouldn't be a planet, because there are so many objects that haven't been cleared including pluto, that doesn't make sense. Does this look clear to you? Defintion should be like this instead, a bit simpler than previous defintions. http://www.skyandtelescope.com/wp-cont....360.jpg

1. Must be round
2. Must not be fusing deturium or hydrogen. Therefor the Primary planets are the "8" and the secondary planets are the dwarf planets the ones that are in orbital resonances with neptune.


Edited by steeljaw354 - Saturday, 12.12.2015, 13:29
 
JackDoleDate: Saturday, 12.12.2015, 13:38 | Message # 48
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I do not want to make Neptune to a dwarf planet, and I accept Pluto's classification as a dwarf planet. It bothered me the reasoning that Pluto did not clear his path.

A potentially better definition might be:
When an object is affected in its orbit by a more massive object that is no sun, then it is not a planet!
I do not know if all Kuiper belt object are in resonance with Neptune, but for many this seems to be the case.

Simply put:
An object that orbits a planet is a moon; an object which is in resonance with a more massive object, is a dwarf planet or an asteroid.

I do not insist on this definition. I could imagine that two objects that are only slightly different in their masses, such as Venus and Earth are in resonance with each other. But I do not know if that's possible, and if it's possible, whether such objects can have stable orbits.

What the semi-major axis, however, has to do with it, I do not understand. Are there any objects that have the same semi-major axis as Pluto?



Why is Sedna defined in 'Space Engine' as Asteroid?





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Edited by JackDole - Saturday, 12.12.2015, 13:45
 
steeljaw354Date: Saturday, 12.12.2015, 13:52 | Message # 49
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Even if the planet is earth sized or larger? Don't use location as a definetion use size! Eris and sedna aren't in resonances with neptune so eris is a planet and sedna is but pluto isn't, pluto is almost the same size as eris

Edited by steeljaw354 - Saturday, 12.12.2015, 13:54
 
HarbingerDawnDate: Saturday, 12.12.2015, 14:39 | Message # 50
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steeljaw354, the graphic you linked is rather misleading. It shows the positions of objects, not their orbits. A more informative graphic would be this one: https://upload.wikimedia.org/wikiped....ces.gif

As you can see, Neptune most certainly has dominance over its orbit. All of those objects are beyond Neptune or in trojan orbits. Only a few objects exist inside of Neptune's orbit (Centaurs), and their orbits are unstable due to the gravitational dominance of the outer planets.





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steeljaw354Date: Saturday, 12.12.2015, 15:02 | Message # 51
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why classify by location?
 
JackDoleDate: Saturday, 12.12.2015, 15:13 | Message # 52
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Quote steeljaw354 ()
sedna aren't in resonances

Sedna is not a Kuiper Belt object. Sedna is probably an object of the inner Oort cloud and certainly much too far out to be influenced by Neptune.





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steeljaw354Date: Saturday, 12.12.2015, 15:16 | Message # 53
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So it's a planet?
 
JackDoleDate: Saturday, 12.12.2015, 15:32 | Message # 54
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Quote steeljaw354 ()
Even if the planet is earth sized or larger?

The size does not matter, an object the size of Earth, orbiting a gas giant, still would be a moon. Although dwarf planet is perhaps not a happy expression for such a large object. The question is whether such a large object could resonate with Neptune, or whether it would be catapulted away.

Quote steeljaw354 ()
So it's a planet?

As I said, I did not insist on the definition. dry





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FaceDeerDate: Saturday, 12.12.2015, 17:29 | Message # 55
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Quote steeljaw354 ()
So it's a planet?


Sedna's tricky to classify at the moment. We don't know its mass (it has no known moons) and it's so far out from the Sun that we can't really do a thorough survey of what else is in its orbital neighborhood in order to calculate its Soter planetary discriminant.

We know it's approximately 1000 km in diameter and has a semimajor axis of 524 AU. We could use the density of similar bodies, such as Charon, to make a rough guess as to Sedna's mass. Using Charon's density of 1.7 that gives Sedna a mass of about 0.0012 times that of Earth.

From this we can work out the Stern-Levison parameter relative to Earth's; 1.2 * 10^-10. Comparing this to the values for known large solar system objects puts Sedna very far down in the "dwarf planet" category. It's dwarfier than all other known dwarfs by an order of magnitude.

So I think I'm feeling pretty comfortable with not classifying Sedna as a planet. It's way too small and its orbit is way too large for it to dominate that space.


Edited by FaceDeer - Saturday, 12.12.2015, 17:32
 
steeljaw354Date: Saturday, 12.12.2015, 18:05 | Message # 56
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What are your thoughts on eris being a planet? I feel comfortable with eris and pluto as planets but no more.

Edited by steeljaw354 - Saturday, 12.12.2015, 18:08
 
HarbingerDawnDate: Saturday, 12.12.2015, 18:46 | Message # 57
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Quote steeljaw354 ()
What are your thoughts on eris being a planet?

Eris is every bit as much a member of a distinct population as Pluto and Ceres are. It makes no sense to arbitrarily lump it in with Mercury-Neptune.

Quote steeljaw354 ()
I feel comfortable with eris and pluto as planets

Why?

Quote steeljaw354 ()
but no more

Why not?





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steeljaw354Date: Saturday, 12.12.2015, 18:52 | Message # 58
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To have more than of each kind of planet Rock,Gas,Ice. Even if you did have more the list would grow larger than anyone could remember all the names. You can't just throw pluto and eris out just because they are small and made of ice, It'd be like walking up to someone who is short but the same age and saying "you aren't a human"

Edited by steeljaw354 - Saturday, 12.12.2015, 18:55
 
HarbingerDawnDate: Saturday, 12.12.2015, 20:22 | Message # 59
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I really don't know what else to say...





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steeljaw354Date: Saturday, 12.12.2015, 20:24 | Message # 60
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Thats offensive Trolling or stupid.... Saying my opinion in a thread is considered trolling or stupid?

Edited by steeljaw354 - Saturday, 12.12.2015, 20:30
 
Forum » SpaceEngine » Science and Astronomy Discussions » Defining Planethood (What sets planets apart?)
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