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Is it possible (for humans)?

My main concern was heavy gravity because they are much bigger and heavier. But it appears that in our system, the gas giants (but not Jupiter) all have a similar surface gravity as Earth. Actually I'm not sure if by surface they mean the solid core of the planet or something else?

On the page of Saturn: it says that the solid core is between 9 to 22 time more massive than Earth but only twice as large. With these numbers, the gravity could be between 2 to 5 times the gravity on Earth. That is not very human-friendly.

Possible problems for terraforming:

  1. Gravity?
  2. Hostile atmosphere: can we just pump it?
  3. Others?
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  • $\begingroup$ Habitable by whom? (We don't know for sure they aren't already inhabited. ;-) ) Us? (Edit: Sorry, you did say "human friendly" later.) $\endgroup$ – T.J. Crowder Oct 12 '14 at 13:10
  • $\begingroup$ For what we could do on Saturn - en.wikipedia.org/wiki/Accelerando $\endgroup$ – Mithoron Feb 9 '15 at 17:16
  • $\begingroup$ Do you actually want to live on a surface? It seems to me that with the right atmosphere mix you could get a very livable gas giant where all the life floats around. Humans would need antigravity or really big hot hydrogen balloons. You'd need oxygen, which would tend to combine with the hydrogen, so you'd need a lot of oxygen. The atmosphere would have to be mostly helium and higher weight gasses. $\endgroup$ – Zan Lynx Feb 14 '15 at 21:34
  • $\begingroup$ "Can't we just pump it?" To where? $\endgroup$ – JDSweetBeat May 20 '15 at 17:51
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For "ice giants" like Uranus or Neptune, yes.

Larry Niven's book The Integral Trees is set in the "Smoke Ring". This is a ring of gas in orbit around a neutron star, formed by a gas giant which went too close to the star and lost its atmosphere. The central and densest part of the ring is inhabited, but there is no gravity. For this, the gas giant would have to have a lot of oxygen, which not many seem to do.

Gas giants such as Jupiter or Saturn, composed of 90% hydrogen, do have a solid core, but it consists of metallic hydrogen, which can only exist at extremely high pressures - if the atmosphere was lost, the core and the liquified gases surrounding it would evaporate. These can't really be terraformed, and are also bigger than ice giants.

Ice giants have 20% hydrogen, and are mostly other things such as methane, ammonia and water. Uranus is thought to have a hot (5000K) rocky core of 0.55 Earth masses, a "mantle" of "hot" liquid or supercritical water and ammonia, of 13.4 Earth masses, possibly with lots of diamonds forming near the bottom due to high pressure and presence of carbon (from methane). The atmosphere itself has 0.5 Earth masses. Neptune is similar, with a core of 1.2 Earth masses.

It is theoretically possible, but I don't know how you could rip off the atmosphere and mantle. Your civilization would have to be very advanced to find enough energy to remove the atmosphere, and to supply the oxygen which is not present in it. It could have been lost by a catastrophic event (e.g. planet passes too close to large object), or such an event that was artificially induced by some thruster. The thruster needed to move the planet to the habitable zone could operate by ejecting the atmosphere, solving two problems at once.

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    $\begingroup$ And if your civilization was that advanced and had all the energy needed, why not just build a new planet out of all the system moons --- ours solar system alone has like 200 or so. $\endgroup$ – JDSweetBeat May 20 '15 at 17:47
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    $\begingroup$ @DJMethaneMan Because there's not enough mass. The 10 most massive moons only have about 6.2 x 10^23 kg of mass which is about 1/10th the Earth. After the top 10 they rapidly fall off in mass; Iapetus, number 11, has not even 1/1000th the mass of the Earth. Don't go looking to the asteroid belt either, that's only about 3 x 10^21 kg. $\endgroup$ – Schwern Mar 21 '16 at 20:20
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This is really sort of cheating, but it could actually work, so here it goes.

As professorfish says,

It is theoretically possible, but I don't know how you could rip off the atmosphere and mantle.

Very true. Jupiter's atmosphere is about 5,000 km high, and comprises the majority of the planet's mass. That would be pretty hard for a civilization to get rid of. Fortunately, stars can do it for you - if you're willing to move the planet in question.

Hot Jupiters are large gas giants orbiting close to their home stars (0.015 AU and 0.5 AU, as opposed to Jupiter's average orbital radius of about 5 AU). Hot Jupiters are - well, hot because they are so close to the star, and so they suffer from other stellar effects. They most likely form farther out from the star - beyond the frost line - and come inwards, towards the star.

Hot Jupiters often experience strong stellar winds. These winds can, if they are strong enough, rip away the planet's atmosphere via hydrodynamic escape. If the process goes on long enough, the atmosphere can be entirely blown into space; the resulting body is known as a chthonian planet. These hypothetical objects would be similar to terrestrial planets in size. If you could introduce an atmosphere and other materials, you could begin to terraform one.

So a star can convert a gas giant to a terrestrial planet for you. The problem is that the planet has to already be very close to the star. Jupiter would have to have its orbit reduced to a very tiny size for this to be possible. To move it inward, you would have to induce an artificial migration. Perhaps you could attract it with another body - perturbing it just enough to make it begin to move inwards. Or maybe you could build a large object resembling a Dyson sphere in size, and use that to attract it inwards. Both options would be incredibly hard, but still within the realm of possibility.

You still have a problem: The chthonian planet is still not a great place for life. It is too close to the star to be inside its habitable zone, and it's way to hot. To get it further back, perhaps you could use the same mechanism you used to get it inwards: using the gravity of another body to change its orbit, a bit like planetary migration, although stability will, of course, be an issue. Once you have the planet where you want it, you can begin the process of terraforming it.

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One large problem with all the ideas here. Jupiter has a "solid core" only because of the pressure of the rest of the gaseous mass. Kind of like how you highly compress carbon dioxide and it becomes a solid.

As soon as you rip away that big fat gas mass, it's like opening a shaken can of soda. You will not have much left to call a planet, if anything at all. If you have any solids left at all afterwards, I would imagine it would be a useless crumbly pile of useless.

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