Planet size: how big can they get ?

[captainjack23]

So, I'm tossing this out here in the hopes of some input. How big can a non-gas giant planet get ? I know that GGs don't get much bigger than Jupiter, as gravity increases to the point where more mass just increases the density, not the diameter (until and if it goes pop, obviously).

I'm assuming the same would be true for rocky worlds - so how much bigger than "A" could they get ? Is there an upper limit ? I assume that a dense enough core would essentially become a GG after a certain point (mass/grav > escape function of H2 at that temp,perhaps) ? What might that point be ? Any idea what the likely max grav, if any for a non-GG rocky superplanet ?

 

[EDG]

So, I'm tossing this out here in the hopes of some input. How big can a non-gas giant planet get ? I know that GGs don't get much bigger than Jupiter, as gravity increases to the point where more mass just increases the density, not the diameter (until and if it goes pop, obviously).

I'm assuming the same would be true for rocky worlds - so how much bigger than "A" could they get ? Is there an upper limit ? I assume that a dense enough core would essentially become a GG after a certain point (mass/grav > escape function of H2 at that temp,perhaps) ? What might that point be ? Any idea what the likely max grav, if any for a non-GG rocky superplanet ?

Beyond about 3 earth masses (roughly size 9 for a silicate/metal world in the habitable zone like Earth) it starts to become more likely that the planet can hold onto hydrogen and helium, at which point it's more likely to snowball during accretion and become a gas giant.

Above 4-5 earth masses (size 10-11), a GG is much more likely (though sometimes you'd be able to get panthalassic worlds if their H2/He gas envelope is blown off prematurely).

Beyond about 6 earth masses (size 11+) and you're pretty much guaranteed to end up with a GG.

All that is complicated by star type and orbital distance and world density though.

 

[Infojunky]

Wish, i had an answer.......

 

[EDG]

Wish, i had an answer.......

Um, I just posted the answer. Right above your post.

 

[Infojunky]

Wish, i had an answer.......

Um, I just posted the answer. Right above your post.

Um, yes, we must have been posting at the same time........

 

[Bygoneyrs]

The best rule of thumb is to build a world via looking at the UWP and designing a world stats to fit what you want and need. Forget this random stuff, put on your GM hat and design out what you want /need for your campaign

Penn

 

[captainjack23]

So, I'm tossing this out here in the hopes of some input. How big can a non-gas giant planet get ? I know that GGs don't get much bigger than Jupiter, as gravity increases to the point where more mass just increases the density, not the diameter (until and if it goes pop, obviously).

I'm assuming the same would be true for rocky worlds - so how much bigger than "A" could they get ? Is there an upper limit ? I assume that a dense enough core would essentially become a GG after a certain point (mass/grav > escape function of H2 at that temp,perhaps) ? What might that point be ? Any idea what the likely max grav, if any for a non-GG rocky superplanet ?

Beyond about 3 earth masses (roughly size 9 for a silicate/metal world in the habitable zone like Earth) it starts to become more likely that the planet can hold onto hydrogen and helium, at which point it's more likely to snowball during accretion and become a gas giant.

Above 4-5 earth masses (size 10-11), a GG is much more likely (though sometimes you'd be able to get panthalassic worlds if their H2/He gas envelope is blown off prematurely).

Beyond about 6 earth masses (size 11+) and you're pretty much guaranteed to end up with a GG.

All that is complicated by star type and orbital distance and world density though.

Thanks. assuming the atmosphere is blown off , could one find a silicate type bigger than B ?

Hmmm. At a suffuicient density will gravitational warming become an issue ?

 

[captainjack23]

The best rule of thumb is to bild a world via looking at the UWP and designing a world stats to fit what you want and need. Forget this random stuff, put on your GM hat and design out what you want /need for your campaign

Penn

No random stuff involved.....UWP only goes to A, which is likely an upper limit of habitability simply by gravitation (if not convenience) . Just curious what the physical upper limit was.

 

[EDG]

Thanks. assuming the atmosphere is blown off , could one find a silicate type bigger than B ?

Size 9 and A is pushing it. B is just about possible, depending on world density and other circumstances (Panthalassics do tend to be lower density than Earth despite having larger radius, because they're about 1-2 earth masses of high density rock/metal and 1-2 earth masses of lower density water/volatiles). Anything bigger would essentially be a very small gas giant regardless of density, with a few thousand km of atmosphere over a solid surface.

Hmmm. At a suffuicient density will gravitational warming become an issue ?

It won't be an issue in a body that's mostly solid. Heating due to gravitational contraction is only an issue with proper gas giants, with tens of thousands of km of atmosphere.

 

[Infojunky]

No random stuff involved.....UWP only goes to A, which is likely an upper limit of habitability simply by gravitation (if not convenience)

Naw, go with convenience......

Marc wants to add A+ sized worlds........

 

[Dave Chase]

So, I guess my world that's 1 million miles in diameter is non reality.


But, I am keeping it anyway.

Dave Chase

 

[dreamingbadger]

So, I guess my world that's 1 million miles in diameter is non reality.


But, I am keeping it anyway.

Dave Chase

Jupiter is 88,759 miles in diameter and has a denisty of about 1.4 times water and 1/4th the desnisty of earth... and a mass of about 1.899 × 10^27kg. If you increase it's mass by 80 times it would have been a sun. By increasing to 1m miles, you would get an increase in volume of 1430 times and a proportional increase in mass.

I think you are increasing your planets mass to the point where a fusion reaction is certain, at that size and density you would have a sun.

 

[EDG]

So, I guess my world that's 1 million miles in diameter is non reality.

But, I am keeping it anyway.

A million miles diameter is 1.6 million km, which is bigger than the Sun! There's absolutely no way it could be solid, it'd have to be a hollow artificial shell, and then you run into all the problems with dyson spheres but on a smaller scale (and with no star in the middle).

 

[Dave Chase]

So, I guess my world that's 1 million miles in diameter is non reality.

But, I am keeping it anyway.

A million miles diameter is 1.6 million km, which is bigger than the Sun! There's absolutely no way it could be solid, it'd have to be a hollow artificial shell, and then you run into all the problems with dyson spheres but on a smaller scale (and with no star in the middle).

Shssh, my old players might be reading this forum. :lol:

What happened is that it gathered 'dust' and other things from some of the systems it traveled through (and distrupted those systems pretty badly too.) Over time it got caught up in tri-star configuration and began orbiting (in a werid kind of way.)

After many eons, it's surface developed a desert like texture. After having different races visit from time to time and leaving small creatures, insects and other things behind, a sort of introduced life started.

A comet crashed (skipped over the surface) and introduced some water (and other gases) in to the 'soil'.

After some more eons, small groups of 'humans took up residence. Now (in my Futura game) it has been over a million ears since this planet has been captured in the Tri-star system. No (known) living thing can remember this planet not being here.

Scans show large granite and other volcanic like rock at about 5 to 7 miles down. And that this layer is several miles thick. No one has attempted to drill/penetrate it to see what is below that.
(well several have but no one has heard from them again. It is assumed that they died or something and it has been centuries since anyone tried.)

It has been assumed that it was a 4th star (in this 3 star system) that cooled too quickly and 'sucked' up several other planets, asteroid belts and such (since there is only 3 other planets in the Tri-star system), during its massive cool down and instead of collapsing completely into a black hole, it stabilized.

Its surface gravity runs about 1.6G's, has a day that lasts 4.63 earth days, night tempatures down to -10C with day time highs reaching 45C or a bit higher.

Most all cities are domes (Archologies) and there are super highways between most cities besides starports.

Very little surface water (5%) and some desert like plants and other adapted animals/insects.

Besides, reality aside, it is a scifi game.

Or is it?

Dave Chase

 

[EDG]

Well, that's more science-fantasy (or just plain fantasy) than scifi. Which is fair enough for that sort of thing, but I don't think the OP was asking about non-realistic worlds.

 

[Rikki Tikki Traveller]

You might also find larger worlds orbiting around White Dwarfs. These would be the left-over cores after the star blew away the GG atmosphere. The volitiles (water etc) would be gone as well, but there MIGHT be rock/metal that survives the Nova. They will probably be pretty rare though, since if they are farther out, they might survive with atmosphere and if they are too close, they boil away even the rock and metals.

 

[Infojunky]

You might also find larger worlds orbiting around White Dwarfs. These would be the left-over cores after the star blew away the GG atmosphere. The volitiles (water etc) would be gone as well, but there MIGHT be rock/metal that survives the Nova. They will probably be pretty rare though, since if they are farther out, they might survive with atmosphere and if they are too close, they boil away even the rock and metals.

Wasn't that a Poul Anderson story?

 

[captainjack23]

You might also find larger worlds orbiting around White Dwarfs. These would be the left-over cores after the star blew away the GG atmosphere. The volitiles (water etc) would be gone as well, but there MIGHT be rock/metal that survives the Nova. They will probably be pretty rare though, since if they are farther out, they might survive with atmosphere and if they are too close, they boil away even the rock and metals.

Wasn't that a Poul Anderson story?

"Satans world" IIRC. It was in fact, the inspiration for the question.

Well spotted !

 

[Infojunky]

You might also find larger worlds orbiting around White Dwarfs. These would be the left-over cores after the star blew away the GG atmosphere. The volitiles (water etc) would be gone as well, but there MIGHT be rock/metal that survives the Nova. They will probably be pretty rare though, since if they are farther out, they might survive with atmosphere and if they are too close, they boil away even the rock and metals.

Wasn't that a Poul Anderson story?

"Satan's world" IIRC. It was in fact, the inspiration for the question.

I was thinking Lodestar myself.......

I believe Satan's world was a captured rogue on a large eccentric orbit...

 

[captainjack23]

Wasn't that a Poul Anderson story?

"Satan's world" IIRC. It was in fact, the inspiration for the question.

I was thinking Lodestar myself.......

I believe Satan's world was a captured rogue on a large eccentric orbit...

Lodestar ? Hmmm. Is it a shortstory ?

And, yeah, satan's world was a rogue. Can't remember if they suggest it wandered off from a remnant star or formed as a small microprotosunlet thingie.