Jupiter sized Super Earth

[fasquardon]

As a student, I once played around with the numbers, and seem to remember that by assuming a composition of the lightest silica rocks, a planet could have about 121 times the surface area of Earth, with a surface gravity of 1 G.

I thin this is far less than the size of Jupiter, double the radius means far more than double the surface.

I had to assume even density through the planet though. And there would be practically no iron etc except meteoric on the planet. Internal processes would probably not match earths anyway. So no continents , continental drift or ocean basins.

Note that you can have a planet that is a diamond with some slagging on top.

A planet made out of pure carbon of Earth mass or greater wouldn't have a diamond at its core - it would have a denser amorphous carbon core. The diamond layer would probably start mid-mantle or so.

And this is the problem with the light silica planet as well - even such light minerals will be crushed to a significant density by the sheer mass of stuff above.

And a giant iron poor planet could well have quite familiar processes - even if it didn't have any radioactive elements in the core and mantle, stored heat from planetary formation would drive some sort of active crustal processes for quite a long time. For a planet with 121 times the earth's surface, the stored heat would probably drive plate tectonics for longer than the planet's star would last.

fasquardon

 

[Umbral]

Its is a good point about the stored heat, but remember that 121 times the earths surface does not mean 121 times the radius! Roughly 3 times the radius, ottomh.

As for the diamond planet, I assumed a different planetary formation. Basically a captured planet that survived the end of its previous system. Very old.

Gas gigant with a carbon core, loses its atmosphere to the end of its sun, and is flung free of its orbit. Spends a couple of billion years as a cooling rogue planet, then gets recaptured into a younger system.

 

[January First-of-May]

Its is a good point about the stored heat, but remember that 121 times the earths surface does not mean 121 times the radius! Roughly 3 times the radius, ottomh.

121 times the volume is about 5 times the radius. 121 times the surface is 11 times the radius (almost exactly the size of Jupiter, incidentally).
OTL Saturn, ironically, has almost the same surface gravity as Earth - but its average density is less than that of water.

As far as I can tell, chances are it's 12.1 times the surface and you just forgot the decimal point that's just about the closest approximation anyway.

 

[fasquardon]

Its is a good point about the stored heat, but remember that 121 times the earths surface does not mean 121 times the radius! Roughly 3 times the radius, ottomh.

As for the diamond planet, I assumed a different planetary formation. Basically a captured planet that survived the end of its previous system. Very old.

Gas gigant with a carbon core, loses its atmosphere to the end of its sun, and is flung free of its orbit. Spends a couple of billion years as a cooling rogue planet, then gets recaptured into a younger system.

Well, a planet doesn't lose heat over its radius, it loses head over its surface, so it is the surface/volume ratio that really matters for driving plate tectonics.

As for the different planetary formation you propose, that doesn't help the carbon atoms resist the forces of compression at the core. Bluntly, diamond isn't strong enough to form the core, so the carbon needs a stronger structure - that of the denser amorphous carbon.

Here's a nice picture from wikipedia which shows the importance of compression on planetary structure:

http://upload.wikimedia.org/wikipedia/commons/f/f6/Planet_sizes.svg

fasquardon

 

[B_Munro]

Yeah, gravity only goes to zero as the thickness of the sphere approaches zero...

So how large, anyway, could an earth with a similar mix of elements to Earth get before the surface gravity became unbearable? (Assuming humans could adopt to, say, double Earth's gravity)

 

[Dirk]

Yeah, gravity only goes to zero as the thickness of the sphere approaches zero...

So how large, anyway, could an earth with a similar mix of elements to Earth get before the surface gravity became unbearable? (Assuming humans could adopt to, say, double Earth's gravity)

What I started to write:

"Well since gravity follows without exception an inverse square rule when it comes to "

Then I realized that I wasn't taking volume (and thus mass, and thus the equal density) into account and thought "whoah, what a cool problem!"

So, since the volume of a sphere is a function of r^3 and equal density is proportional to volume, g=G*m1*m2/r^2 becomes g=G*m1*k*r^3/r^2 (where k is 4/3*pi*density with proper units), so g=G*m1*k*r. Thus, any change in the radius of a sphere, with density remaining equal, will change its gravity by the same multiplicative factor. *Drops mic, exits left*

EDIT: Well, so this means that the Earth could double in radius, or quadruple in surface area, or octuple in volume, and we could still "adapt to it", if you say so.

EDIT2: And, like the non-idealist and non-expert I am, I forgot to take into account that greater radius means masses that I can't even comprehend compressing the material underneath, increasing density...or making the radius less, but with the same mass. Somewhere between will be the balance for double gravity. So it's less than double radius. So then mass is a function of the radius in its own right, not simply by proxy through volume. Well shit.

 

[mr.dickens]

Jack Vance touched on this subject with his book series about the Big Planet. It was not Jupiter sized, but it was a huge terrestrial planet with lower density and almost no metals.

 

[CalBear]

Jack Vance touched on this subject with his book series about the Big Planet. It was not Jupiter sized, but it was a huge terrestrial planet with lower density and almost no metals.

TEN YEARS?!

Don’t do that!