# Thread: 1000 Earth mass terrestrial planet

1. ## 1000 Earth mass terrestrial planet

What would be the characteristics of a planet ~3 Jupiter masses, but composed 99% metal, not hydrogen or helium? Would it implode into degenerate matter?

2. Member
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Jul 2010
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how do you get it to be 99% metal?

Mark

3. Originally Posted by holmes4
how do you get it to be 99% metal?
From the "not hydrogen or helium" bit, I assume this is the astronomical use of the word "metal".

Grant Hutchison

4. Originally Posted by grant hutchison
From the "not hydrogen or helium" bit, I assume this is the astronomical use of the word "metal".

Grant Hutchison
Affirmative

5. Originally Posted by Tom Mazanec
What would be the characteristics of a planet ~3 Jupiter masses, but composed 99% metal, not hydrogen or helium? Would it implode into degenerate matter?
No, it wouldn't implode into degenerate matter. That takes way more than planetary masses, like a white dwarf star.

6. The centre of such a world would be very compressed, but not degenerate. Here's an article about unfeasibly large terrestrial planets by Anders Sandberg.
http://aleph.se/andart2/megascale/wh...bitable-world/
Taking the constants (table 4) corresponding to iron gives a maximum radius at the mass of 274 Earths, perovskite at 378 Earths, and for ice at 359 Earths. We should likely not trust the calculation very much around the turning point, since we are well above the domain of applicability. Still, looking at figure 4 shows that the authors at least plot the curves up to this range. The maximal iron world is about 2.7 times larger than Earth, the maximal perovskite worlds manage a bit more than 3 times Earth’s radius, and the waterworlds just about reach 5 times. My own plot of the approximation function gives somewhat smaller radii:

7. Using that graph, I think the radius of a world with 1000 times Earth's mass would be about 3.2 times Earth's radius; about 20390 km. Note that this would be smaller than a planet with only one Jupiter mass, since the compression factor would start to kick in for planets more massive than 378 Earths.
Last edited by eburacum45; 2017-Dec-20 at 11:46 PM.

8. OK, so what's the surface g on those planets?

9. Order of Kilopi
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Originally Posted by DaveC426913
OK, so what's the surface g on those planets?
Relevant article here:

http://nova.stanford.edu/projects/mo...-surfgrav.html

10. Originally Posted by DaveC426913
OK, so what's the surface g on those planets?
Higher than I would like...roughly 100-300 Gs, depending on composition and exact mass.

11. Originally Posted by Tom Mazanec
Higher than I would like...roughly 100-300 Gs, depending on composition and exact mass.
I don't understand what the problem is? Are you world-building a sci-fi planet? If so, and you want less gravity, you're not obligated to have the mass of a Super-Jupiter in a terrestrial world. If you only want ten gees, you can do that. Adjust the mass and density of the planet accordingly. If you don't want extreme gravity, what's the extreme mass there for?

12. Originally Posted by SkepticJ
I don't understand what the problem is? Are you world-building a sci-fi planet? If so, and you want less gravity, you're not obligated to have the mass of a Super-Jupiter in a terrestrial world. If you only want ten gees, you can do that. Adjust the mass and density of the planet accordingly. If you don't want extreme gravity, what's the extreme mass there for?
No problem, except I would not want to live there!

13. Order of Kilopi
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Tom- check this (700 g) SF classic reference:

https://en.wikipedia.org/wiki/Mission_of_Gravity

14. Originally Posted by Tom Mazanec
No problem, except I would not want to live there!
Not to worry, you wouldn't have to live there for very long.

15. Originally Posted by Jens
Not to worry, you wouldn't have to live there for very long.
Actually, he would live there the rest of his life!

16. Originally Posted by John Mendenhall
I tried to calculate it, but the calculators don't take into account the compression due to mass, so you get a wrong radius, thus wrong answers.

17. That link is not about calculating a radius based on mass and compressibility, it is about calculating surface gravity for a planet of given mass and radius. If we are talking about eburacum45's hypothetical planet, we can just plug in the mass and radius and get about 100g. Someone else has done the work of plotting those graphs where the radius was calculated from the mass and the compressibility characteristics. Eburacum45 has simply assumed that those properties apply to his hypothetical planet.

18. That's right. I make it about 97 gees. Such a planet would almost certainly have a dense hydrogen and helium atmosphere, since the escape velocity of such a world would be too high for those gases (or any other) to escape). The atmosphere would be much more compact than the atmosphere of Jupiter and Saturn, since the weight of the atmosphere would be fantastic and would compress the gases significantly; there could be an ocean of liquid or even metallic hydrogen above the planetary surface, which would make a Mesklin-like ecosphere unlikely.

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