Composition of this planet

  • #1

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?
 

Answers and Replies

  • #2
phinds
Science Advisor
Insights Author
Gold Member
2019 Award
16,086
6,061
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?
Stuff.
 
  • #3
SteamKing
Staff Emeritus
Science Advisor
Homework Helper
12,798
1,666
It's a lot simpler than that. Compare the volume of the exoplanet with the volume of the earth. Anything jump out at you?
 
  • #4
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?
 
  • #5
D H
Staff Emeritus
Science Advisor
Insights Author
15,393
683
What is the mean density of the Earth?
 
  • #6
5.52 g cm/3
 
  • #7
1,469
156
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.
 
  • #8
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.
 
  • #9
SteamKing
Staff Emeritus
Science Advisor
Homework Helper
12,798
1,666
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?
 

Related Threads on Composition of this planet

  • Last Post
Replies
6
Views
708
  • Last Post
Replies
1
Views
4K
  • Last Post
Replies
5
Views
2K
  • Last Post
Replies
14
Views
2K
  • Last Post
Replies
3
Views
8K
Replies
11
Views
3K
  • Last Post
Replies
5
Views
4K
Replies
14
Views
8K
Replies
0
Views
2K
Replies
9
Views
998
Top