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## Main Question or Discussion Point

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?

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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?

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Stuff.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?

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SteamKing

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What is the mean density of the Earth?

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5.52 g cm/3

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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.

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