Celestia Forums

Imagine a rock planet twice the size of earth. What could the interior look like? What would be required for it to suport life as we know it?

First we need to know what you mean by twice the 'size'. Do you mean twice the mass, or twice the radius? I think we could probably work something out for either case.

Twice the radius of Earth seems rather large for a rocky planet - according to this paper, such a planet would be about 10 Earth masses, more massive still if it has an iron core. This is the kind of mass range where it is expected you end up with either runaway accretion to a gas giant, or a Uranus-type planet if the planet forms too late or in a region of the disc depleted in gas.

Also I believe a planet with of this size would also have a very thin crust and a much more molten core and mantel. The larger the planet and mass increases the amount of radioactive elements which in hand helps generate heat in the core. A planet like this even as old as the Earth may have a very thin crust and be explosively volcanic. Now if it had enough water and a think atmosphere it would probably end us a Panthalassic world.

Now the only way around this is if the planet was made up of lighter materials. think of a Mars like world that size.

Don. Edwards

I have described a 10 Earth-mass rocky planet here
http://www.orionsarm.com/worlds/Sisyphos.html
it is a Chthonian type planet, which used to have a thick gas giant atmosphere but lost it because of heating by the star and by interactions with the stellar wind.
The gravity on such a world would be 2 gees or more, depending on whether there is a substantial metal core.

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On the other hand a 2 Earth mass planet would be somewhat more hospitable. If it had a minimal or absent metal core it would have a radius of about 1.3 Earths, and a gravity of about 1.18 gees. Not too excessive; but most Earth-like planets would have at least a small metallic core, making the planet a little denser and the gravity a little higher.

Yes, I meant diameter.

Don. Edwards wrote:Also I believe a planet with of this size would also have a very thin crust and a much more molten core and mantel. The larger the planet and mass increases the amount of radioactive elements which in hand helps generate heat in the core. A planet like this even as old as the Earth may have a very thin crust and be explosively volcanic. Now if it had enough water and a think atmosphere it would probably end us a Panthalassic world.

Now the only way around this is if the planet was made up of lighter materials. think of a Mars like world that size.

Don. Edwards

What would be the gravitational pull of that mars like world twice the radius of earth? Could it have two earthlike moons? More importantly, could it be terraformed?

PlutonianEmpire wrote:Yes, I meant diameter.

What would be the gravitational pull of that mars like world twice the radius of earth? Could it have two earthlike moons? More importantly, could it be terraformed?

Well, Mars does have an iron core, but if we assume a coreless silicate planet twice the radius of Earth, you get a mass of about 10 Earth masses (as I mentioned above).
The acceleration due to gravity at the surface is calculated by [tex]g=\frac{GM}{r^2}[/tex], which is made very simple on Google calculator, which handily has the values of G, m_earth and r_earth as built-in constants.

So to put this in Google calculator, you do a Google search for

G*10*m_earth/(2*r_earth)^2

and you get the answer.