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Bigger Planet, Same Gravity -- Different Density?

  1. Dec 14, 2015 #1
    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.
     
  2. jcsd
  3. Dec 14, 2015 #2
    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.
     
  4. Dec 14, 2015 #3

    Borek

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    Staff: Mentor

    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?
     
  5. Dec 16, 2015 #4
    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!
     
  6. Dec 16, 2015 #5
    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?
     
  7. Dec 16, 2015 #6
    It simply has to average 1.83. So the middle could be heavier and the surface lighter.
     
  8. Dec 16, 2015 #7

    Borek

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    Staff: Mentor

    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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