Composition of this planet

  1. Lets say there is an exoplanet 50 light years away. The radius of this planet is 2x Earths with 8x Earths mass and a density of 5.52 g cm/3. What is the composition of this planet?
     
  2. jcsd
  3. phinds

    phinds 8,479
    Gold Member

    Stuff.
     
  4. SteamKing

    SteamKing 8,789
    Staff Emeritus
    Science Advisor
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    It's a lot simpler than that. Compare the volume of the exoplanet with the volume of the earth. Anything jump out at you?
     
  5. I know it would be made of lighter elements but would the lighter elements be water/rocky mix or a smaller iron core with a silicate mantle and crust making up most of the mass?
     
  6. D H

    Staff: Mentor

    What is the mean density of the Earth?
     
  7. 5.52 g cm/3
     
  8. That´s precisely why the composition of an exoplanet is unknowable.

    There are 2 observables - the total mass and volume. There are 4 major variables of internal composition: the amount of iron, the amount of rock, the amount of water and the amount of hydrogen/helium. Remember that 2x Earth radius and 8x Earth mass means twice Earth escape speed, and that is in the region of what Uranus has.

    If a body consisted of only 2 variable substances, like iron and rock, or rock and water, or water and hydrogen, then you could solve for an unique composition that fits the observed mass and diametre. If you have 3 unknowns then you cannot solve, because it is underdetermined equation system, and you have degenerate solutions. A body with tiny iron core and mainly rock, like Moon, can have the same density as a body with a big iron core but also a large amount of water on top. And as stated, you have 4 unknowns, not even just 3.
     
  9. Lets remember that Uranus has a density of only 1.32 a radius of 4x Earth and has a small rocky core, water/ice mantle and hydrogen rick atmosphere. I would think that a planet with a density of 5.52 with 2x Earth radius would have most of its mass made into a iron core, rocky mantle and crust while being much more water rich than Earth. I mean take Mars for example it is made of less dense materials on average.
     
  10. SteamKing

    SteamKing 8,789
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    What would happen if you took the earth (with the same composition of minerals, water, air, etc.) and scaled it up so that its radius doubled? What would be the mass of the larger planet?
     
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