Bigger Planet, Same Gravity -- Different Density?

AI Thread Summary
A hypothetical planet three times the size of Earth, with the same gravitational pull, would need to have a specific gravity of about 1.83, indicating it must be one-third as dense as Earth. This could be achieved with a combination of a heavy rock core and lighter materials on the surface, potentially allowing for solid ground suitable for life. Gaseous compositions or lighter materials like pure water would not suffice for a solid surface. The discussion suggests that a smaller moon could provide a stable solid surface, while a larger moon would face gravitational challenges. Overall, the planet's structure would need careful consideration of density and material composition to support Earth-like life.
scifiwriter888
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I'm doing some research for a sci-fi story that I'm writing and hoping that some of you chemists can help me answer a quick question about the material of a hypothetical distant planet.

If a distant planet were three times the size of Earth with the same gravitational pull, what would the planet's material be like?

Would it have to be gaseous? Is there any situation in which it would be firm enough for earth-like life forms to walk on? Is there a version where the material is something like quicksand?

Please excuse my ignorance! Thanks in advance for your help.
 
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It would have to be one-third as dense as Earth - or a specific gravity of roughly 1.83.
At it would have to be able to retain that sg under high pressure - so light clays or plaster probably would not work.

Pure water would be too light.

How about Beryllium, Cesium, Phosphorus, epoxy, fiber glass, or a mixture. Of course, you can give it a heavier core and allow lighter oceans on the surface.
 
Jupiter has density around 1.3 g/mL, Staurn even below 1 g/mL, so definitely some combination of heavy rock core and gas could produce density you are looking for. But I doubt in the solid surface then. Perhaps solid surface on the planet moon would suffice?
 
.Scott said:
It would have to be one-third as dense as Earth - or a specific gravity of roughly 1.83.
At it would have to be able to retain that sg under high pressure - so light clays or plaster probably would not work.

Pure water would be too light.

How about Beryllium, Cesium, Phosphorus, epoxy, fiber glass, or a mixture. Of course, you can give it a heavier core and allow lighter oceans on the surface.

Thanks Scott. What do you mean when you say "of course, you can give it a heavier core and allow lighter oceans on the surface"? Sorry I don't understand this stuff as well as you do!
 
Borek said:
Jupiter has density around 1.3 g/mL, Staurn even below 1 g/mL, so definitely some combination of heavy rock core and gas could produce density you are looking for. But I doubt in the solid surface then. Perhaps solid surface on the planet moon would suffice?

Hm interesting. The planet moon would still have to be smaller in order to keep that gravity/solid surface, right? If the moon were three times the size of earth, I'd still run into the same problem?
 
scifiwriter888 said:
Thanks Scott. What do you mean when you say "of course, you can give it a heavier core and allow lighter oceans on the surface"? Sorry I don't understand this stuff as well as you do!
It simply has to average 1.83. So the middle could be heavier and the surface lighter.
 
scifiwriter888 said:
The planet moon would still have to be smaller in order to keep that gravity/solid surface, right? If the moon were three times the size of earth, I'd still run into the same problem?
Not sure what you mean. Too large moon means actually a double planet, I am not sure if such systems are stable in a long run, especially when other planets are present. Smaller and more dense moon would have a solid surface without problems, just like our Moon does, but the lower mass definitely means lower gravity on its surface.
 

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