How big can a rocky planet get?

inzane · 2007-11-12 16:28:52

How big can a rocky planet like the earth get to grow to in our universe. Is their a size/weight/mass limit?

Thanks

cIclops · 2007-11-13 03:33:57

Maybe not much bigger than Earth. A rocky planet forms by sweeping up all the debris in the dust disk surrounding a new star. Once it has cleared all the dust in its path it stops growing. Rocky planets in different orbits would sweep their orbits too, if one was too close it would collide (as is thought in the case of the Earth) and both would be broken up again.

noosfractal · 2007-11-13 05:23:07

For some reason 14 Earth masses gets bandied about, but I don't know why. We don't really understand planet formation very well.

At some point gravity will become so high that the planet will be a featureless sphere.

Neptune is about 17 Earth masses, with current theory giving a breakdown of: 1 Earth mass rocky core, 10-15 Earth masses water/ammonia/methane ice "ocean", the rest being the hydrogen/helium/methane atmosphere.

At about 13 Jupiter masses (~ 4100 Earth masses) deuterium starts to fuse and the planet becomes a brown dwarf star.

cIclops · 2007-11-13 06:15:54

inzane asked about rocky planets, Earth is the largest one in the Solar system.

Jupiter has a core of about 10 earth masses, but it's not clear if this is rocky or something exotic like metallic hydrogen.

noosfractal · 2007-11-13 10:17:33

http://en.wikipedia.org/wiki/Mu_Arae_d

inzane · 2007-11-13 10:20:07

Hi, yes, planets - made from accretion discs and then planets that continue to form from collisions with other similar bodies in early solar system formation, molten metal cores in the gas giants Great! thanks.
But seriously - How big can a rocky planet be?

I like this - "At some point gravity will become so high that the planet will be a featureless sphere. "
So a bigger planet may have less mountain ranges/features etc. But then thinking about it that would depend on the internal workings of that particular planet - plate tectonics/volcanics etc - Again I suppose this depends on the size of the planetoid.
So can I assume that thre is no size limit because at high gravity the planet will either break up or become 'something else' like a ball of metal.

My original question was inspired by thinking about how big a rocky plant similar to our own could be.

Also could an increased 'spin' on a heavy planet give enough centrifugal force to significantly mask the planets 'true' gravity?

When I say true' gravity? I mean hypothetically - if our own Earth stopped spinning everything would become slightly heavier wouldn't it? To what degree I don't know, I wouldn't think its much - But the effect would exist nether the less....right?

inzane · 2007-11-13 10:39:36

http://en.wikipedia.org/wiki/Mu_Arae_d

Thanks for that. Very interesting.

noosfractal · 2007-11-13 14:30:53

My original question was inspired by thinking about how big a rocky plant similar to our own could be.

So I think that is the 14 Earth masses figure. Apparently above that you get runaway gas accumulation during solar system formation and become a gas giant (or at least, that is the current theory).

Also could an increased 'spin' on a heavy planet give enough centrifugal force to significantly mask the planets 'true' gravity?

Yes but very high spin is unstable (bits fly off, which lowers the gravity, so more bits fly off, etc).

When I say true' gravity? I mean hypothetically - if our own Earth stopped spinning everything would become slightly heavier wouldn't it? To what degree I don't know, I wouldn't think its much - But the effect would exist nether the less....right?

Yes, you'd weigh about a third of 1% more. (Obviously, your mass wouldn't change, just the force you experience).

inzane · 2007-11-14 04:52:00

I think thats covered everything, thanks

nickname · 2007-11-29 04:15:14

What about internal heat as a limiting factor for rocky planet size?

If Earth was 2 or 3 times its current makeup and size wouldn't the internal heat turn it into a molten planet ?
Those bigger worlds should also have bigger atmospheres with more heat trapping abilities, so they won't cool like Earth has.

At some point well before 14 Earth masses of similar to earth material do we get to that?

Antius · 2007-11-29 06:13:20

What about internal heat as a limiting factor for rocky planet size?
If Earth was 2 or 3 times its current makeup and size wouldn't the internal heat turn it into a molten planet ?
Those bigger worlds should also have bigger atmospheres with more heat trapping abilities, so they won't cool like Earth has.
At some point well before 14 Earth masses of similar to earth material do we get to that?

An interesting set of points. The habitable zone around a star would depend upon the size and properties of the planet, as well as sunlight levels. If Earth or Venus were at Mars' distance from the sun, their thicker atmospheres would probably allow liquid water to exist in some locations.

A larger planet with more significant geothermal heating could maintain liquid oceans with little or a lot less sunlight. It would also have a continuous stream of greenhouse gases entering its atmosphere. Under these conditions, the habitable zone of our sun might stretch out to the asteroid belt.

nickname · 2007-11-29 14:15:51

Antius,

Seems like a good probability that real big earths around the universe are going to be pretty warm places.

Once we start moving them further out from the host star to accommodate the extra internal heat and thicker atmospheres.
The more likely small fluctuations in sunlight amounts will have bigger effects on any life trying to grow on them.
Guess at some point the really big earths will be similar to the surface of the moon Io.

Earth might turn out to be a very unusual just so place, rare even in astronomical terms.
A little to big in a warm location and its a warming runaway like Venus.
To small and it looses its gas to space like Mars.
A few times Earth size and it has to be so far from the star that life is a very slow process with little sunlight energy.
Many times Earth and internal heat becomes a problem.

Location location and star type a big factor in this mix also.
Not much room to move Earth around in our solar system before we get an ice block or overheating problem.

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#1 2007-11-12 16:28:52

Re: How big can a rocky planet get?

#2 2007-11-13 03:33:57

Re: How big can a rocky planet get?

#3 2007-11-13 05:23:07

Re: How big can a rocky planet get?

#4 2007-11-13 06:15:54

Re: How big can a rocky planet get?

#5 2007-11-13 10:17:33

Re: How big can a rocky planet get?

#6 2007-11-13 10:20:07

Re: How big can a rocky planet get?

#7 2007-11-13 10:39:36

Re: How big can a rocky planet get?

#8 2007-11-13 14:30:53

Re: How big can a rocky planet get?

#9 2007-11-14 04:52:00

Re: How big can a rocky planet get?

#10 2007-11-29 04:15:14

Re: How big can a rocky planet get?

#11 2007-11-29 06:13:20

Re: How big can a rocky planet get?

#12 2007-11-29 14:15:51

Re: How big can a rocky planet get?

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