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Forum » SpaceEngine » Feedback and Suggestions » Planet density
Planet density
migratingmynahDate: Saturday, 02.03.2013, 01:12 | Message # 1
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Have you ever realized Gas giants and terrestrial never have the diameter of 3 times of Earths?

From what i learned

9.99 Earth masses = not a gas/ice giant
over 9.99 Earth masses = Ice/Gas giant

Probably because gravity causes the planet to compress itself.

Is there any chance for future versions to feature not very dense planets? Like the moon, made of Earth's crust.
(I hate it when science textbooks say the moons gravity is weaker then Earth's because it's smaller... I really feel like correcting the science teacher)





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HarbingerDawnDate: Saturday, 02.03.2013, 02:27 | Message # 2
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migratingmynah, this is incorrect. Your mass figures are accurate, but your diameter figures are way off. Almost EVERY gas giant in SE is greater than 3 Earth diameters, usually much greater.

And the Moon's gravity is weaker because it's smaller than Earth, for the most part. Even if the Moon had the same density as Earth it would not have anywhere near Earth's gravity.





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WatsisnameDate: Saturday, 02.03.2013, 02:49 | Message # 3
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Gas giants most definitely do have diameters over 3 Earths... I imagine you just meant terrestrial planets. The maximum theoretical size of terrestrial planets is actually about 4 to 5 Earth radii, depending on the composition.

Gravity does cause large planets to compress a bit, increasing the density more than what they would otherwise be. This can be seen from this figure, where the radius doesn't quite follow the expected mass1/3 relation, especially for larger masses.
Source of figure for interested readers

Terrestrial planets tend to only be up to 8 to 10 Earth masses because at that point they become capable of collecting and retaining the nebular gas from their environment -- turning them into ice or gas giants, assuming they formed far enough from their stars so that gas/ice is available. The formation mechanism of gas giants may also be completely different than terrestrials, via the direct collapse of the nebular disk.

Quote (migratingmynah)
Is there any chance for future versions to feature not very dense planets? Like the moon, made of Earth's crust.
(I hate it when science textbooks say the moons gravity is weaker then Earth's because it's smaller... I really feel like correcting the science teacher)


The surface gravity of an object equals 4/3*pi*G*r*density; i.e. it is proportional to size and density to the first power. The moon's radius is 27% of Earth's. The moon's density is 61% of Earth's (it's actually the second-densest moon in the solar system, after Io). Therefore it is the Moon's smaller size, not density, that factors in more for its weaker gravity. So your science teacher isn't really wrong. smile





 
migratingmynahDate: Saturday, 02.03.2013, 07:52 | Message # 4
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Thanks a lot. I'm much younger then you think
Quote (HarbingerDawn)
Almost EVERY gas giant in SE is greater than 3 Earth diameters, usually much greater.


What i meant was between 3-4 Earth masses.I should've phrased it like that.





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Edited by migratingmynah - Saturday, 02.03.2013, 07:52
 
HarbingerDawnDate: Saturday, 02.03.2013, 08:18 | Message # 5
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Quote (migratingmynah)
What i meant was between 3-4 Earth masses.I should've phrased it like that.

100% of gas giants have more than 3-4 Earth masses...





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migratingmynahDate: Saturday, 02.03.2013, 08:48 | Message # 6
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Quote (HarbingerDawn)
100% of gas giants have more than 3-4 Earth masses...

OH MY ENGLISH...

Sorry between 3 to 4 Earth diameters

The only gas giant which has 3 to 4 Earth diameters i found and know is Neptune





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Edited by migratingmynah - Saturday, 02.03.2013, 08:50
 
HarbingerDawnDate: Saturday, 02.03.2013, 09:36 | Message # 7
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So wait, you're saying that you want to see smaller gas giants? In your opening post you seemed to be expressing a desire for less dense planets, which means that they would have to be larger. I don't think I understand what you're trying to say.




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migratingmynahDate: Saturday, 02.03.2013, 10:26 | Message # 8
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Quote (HarbingerDawn)
So wait, you're saying that you want to see smaller gas giants?

Yup i guess...Sorry for my ignorance.





Bye

Edited by migratingmynah - Saturday, 02.03.2013, 10:27
 
SalvoDate: Saturday, 02.03.2013, 15:00 | Message # 9
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I had quickly realized it, maybe because I'm not native English biggrin

Anyway I noticed it too, usually Super-Earths have this size, an ice giant must have very low density to have a radius between 3 and 4 EA wacko





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SpaceEngineerDate: Saturday, 02.03.2013, 16:29 | Message # 10
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According to picture what Watsisname posted (I used equations from that article in SE), solid super-earths have radiuses of 1-2 Earths, only extremely massive ones, >10 Earth masses, may have radiuses of 3-4 Earths. But in SE such planets are determined as ice giants. I may make overlapped ranges of masses for solid and gas planets, so solid planets will be say up to 50 Earth masses, and giants will be 7 Earth masses and less.




 
HarbingerDawnDate: Saturday, 02.03.2013, 16:59 | Message # 11
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Quote (SpaceEngineer)
I may make overlapped ranges of masses for solid and gas planets, so solid planets will be say up to 50 Earth masses, and giants will be 7 Earth masses and less.

This is a good idea to have overlapped ranges. But maybe the solid worlds should not go higher than ~15 Earth masses? As far as I know no solid worlds have been detected that are much greater than 10 Earth masses, and the trend is for mass/radius of worlds larger than that to be more consistent with a non-terrestrial planet.





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SpaceEngineerDate: Saturday, 02.03.2013, 23:15 | Message # 12
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We know nohting about exoplanets composition, only upper limit for their masses or radius, or (in better case) true mass and radius together.




 
HarbingerDawnDate: Sunday, 03.03.2013, 03:43 | Message # 13
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Quote (SpaceEngineer)
We know nohting about exoplanets composition, only upper limit for their masses or radius, or (in better case) true mass and radius together.

I meant that we don't know of any large worlds that have a mass/radius consistent with being terrestrial worlds, and that worlds larger than 10-15 Earth masses have values that are more consistent with dominantly icy or gaseous bodies.





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Edited by HarbingerDawn - Sunday, 03.03.2013, 03:44
 
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