Free floating "gas dwarf"?

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p1t1o
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Free floating "gas dwarf"?

Postby p1t1o » Thu Mar 22, 2018 4:21 pm UTC

So i just (re)read https://what-if.xkcd.com/99/

Could we build a stable ball of gas in space? (lets say, orbiting the sun somewhere between Earth and Mars, or at 1AU at one of the lagrange points)

A ball whose own gravity prevents it from dissipating, but small enough that it all remains gaseous - no liquid air (lets make it out of air), no metallic oceans, just a literal ball of gas.

I know this cannot form naturally, as space-gas is diffuse enough that a cloud large enough to collapse naturally will be large enough to form a solar system in its own right.

But if you could manipulate arbitrarily large amounts of gas - lets say we have an inexhaustible supply of gas cylinders - could you release enough of it in small enough volume, in a short enough amount of time, that its gravity held it in a stable ball?


Slightly simpler form:

Ok lets relax the conditions about the core.

How small can we make a gas-giant planet? Does the minimum size require liquid air at the core?

And if it was constructed from air, could we live anywhere in its volume? What would it be like?

If we built it at 1AU from the sun, would it be warm? Would it absorb enough sunlight? I know there might be heat released as it sought gravitational equilibrium, but its not going to be nearly as much as in other cases right?


Actually now that I try and visualise it, living there sounds terrifying - if I assume there is a "band" of habitable temperature/pressure somewhere in the middle-ish strata...every direction I look is blue-sky, except down which would be a infinitely deep pit of black.

Sunsets/rises would probably be spectacular though.

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gmalivuk
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Re: Free floating "gas dwarf"?

Postby gmalivuk » Thu Mar 22, 2018 4:38 pm UTC

Considering that the surface of Venus is already far too hot and pressurized for habitation, I don't think habitability questions really depend on whether there's a solid core or not. The habitable zone is going to be up somewhere near the top of the atmosphere regardless.
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p1t1o
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Re: Free floating "gas dwarf"?

Postby p1t1o » Thu Mar 22, 2018 4:53 pm UTC

gmalivuk wrote:Considering that the surface of Venus is already far too hot and pressurized for habitation, I don't think habitability questions really depend on whether there's a solid core or not. The habitable zone is going to be up somewhere near the top of the atmosphere regardless.


Ah, good point.

So we'd be living probably about as close to the surface as we do now.

Are there any obvious stand-out physical problems with the concept?

Would such a small ball of fluid be stable enough? Could gravitational perturbations that a rocky planet barely notices, cause fluidic effects that would disrupt it too much maybe? Like blowing on a ball of water in freefall?

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LaserGuy
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Re: Free floating "gas dwarf"?

Postby LaserGuy » Thu Mar 22, 2018 5:22 pm UTC

p1t1o wrote:How small can we make a gas-giant planet? Does the minimum size require liquid air at the core?


Possibly relevant article that may have way more detail than you want here. Short answer is that it is at least theoretically possible to have a stable gaseous planet with a mass of 0.2 Earth masses, but probably not much smaller.

SuicideJunkie
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Re: Free floating "gas dwarf"?

Postby SuicideJunkie » Fri Mar 23, 2018 2:01 pm UTC

From that article, if I understand the intro correctly, it is the non-gaseous core that is 0.2 Earth masses minimum, and the actual atmosphere would be extra on top of that.
It also says proportional to T^1.5, so the small gas planets would have to be bitterly cold, and nowhere near the sun.

I imagine the heat from the gas collapse and planet formation would also be a problem that delays your planet construction, and immigration would be limited to Russians and Canadians.
It could be the last holdout for non-Dutch people after the oceans are drained and all the surfaces in the solar system are stitched together.

jewish_scientist
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Re: Free floating "gas dwarf"?

Postby jewish_scientist » Fri Mar 23, 2018 3:41 pm UTC

You should look into the Roche Limit, which is how close a body held together only by gravity can get to a given body without being torn apart by the given body's gravity.

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LaserGuy
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Re: Free floating "gas dwarf"?

Postby LaserGuy » Fri Mar 23, 2018 6:14 pm UTC

SuicideJunkie wrote:From that article, if I understand the intro correctly, it is the non-gaseous core that is 0.2 Earth masses minimum, and the actual atmosphere would be extra on top of that.
It also says proportional to T^1.5, so the small gas planets would have to be bitterly cold, and nowhere near the sun.


Yes, cold temperatures help a lot. Essentially whether you can have gas trapped depends on the difference between the escape velocity of the gravitational well versus the mean velocity of the gas molecules. So if you lower the temperature, you can trap lighter gases, or trap heavier gases with a smaller mass.

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gmalivuk
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Re: Free floating "gas dwarf"?

Postby gmalivuk » Fri Mar 23, 2018 7:04 pm UTC

jewish_scientist wrote:
You should look into the Roche Limit, which is how close a body held together only by gravity can get to a given body without being torn apart by the given body's gravity.
The Roche limit isn't going to play any part in this, if you're talking about something 1 AU from the Sun.
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moody7277
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Re: Free floating "gas dwarf"?

Postby moody7277 » Tue Apr 03, 2018 1:14 am UTC

u
p1t1o wrote:And if it was constructed from air, could we live anywhere in its volume? What would it be like?

If we built it at 1AU from the sun, would it be warm? Would it absorb enough sunlight? I know there might be heat released as it sought gravitational equilibrium, but its not going to be nearly as much as in other cases right?


Actually now that I try and visualise it, living there sounds terrifying - if I assume there is a "band" of habitable temperature/pressure somewhere in the middle-ish strata...every direction I look is blue-sky, except down which would be a infinitely deep pit of black.

Sunsets/rises would probably be spectacular though.


Reading through this, my mind immediately jumped to Larry Niven's Smoke Ring. Not exactly what you want, it's held together in a really deep gravity well, but the living conditions seem to match what you're thinking of.
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