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Assume you have access to some very large amount of energy but there are no exceptions to currently known laws of physics (for example, you can't go faster than the speed of light).

Is it possible to turn gas giants like Saturn or Jupiter into planets with solid ground and desired atmospheric composition?

If so what would be the best way to achieve this with minimal assumptions on technological advancement? This last criterion is important: assume a minimal amount of technological advancement!

So if you are thinking about deploying nanorobots running pico quantum computers with superhuman AI so they can re-engineer each molecule, that's not a good satisfying answer.

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  • $\begingroup$ Related: How can I destroy a gas giant planet? $\endgroup$ – a CVn Jan 14 '17 at 12:08
  • $\begingroup$ Gas giant is probably entirely anything but solid, as ur journey deeper pressure and temperature builds up quickly and even turn hydrogen metallic think mercury. Therefore I'll vote for no... $\endgroup$ – user6760 Jan 15 '17 at 7:44
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Is it possible to turn gas giants like Saturn or Jupiter into planets with solid ground and desired atmospheric composition?

Yes and no. The main problems will be surface gravity, temperature and irradiance.

With the energy and resources required to pseudoTerraform a gas giant, you could probably tear it apart, discard everything except the material required to build another Earth, and do so.

The first step is to transform a sizeable portion of the planet into some composites with very high compressive strength and very low density. With this, you expand the planet until its surface gravity is around 1G. You need to have enough material left to cover the results with a thick rocky crust.

In the case of Saturn this is actually not so inconceivable, because its "surface" gravity is already close to that of Earth. You might "only" need to dredge up enough rock to build the new surface and the structures required to make it buoyant above the remaining atmosphere. Carbon and silicon aerogels might do. So you end up with about 80% of the old planet, plus a layer of churned waste materials, covered by a thick aerogel crust, covered by rock, soil, and finally Earth-like atmosphere.

Except that at this point you need some heat source, because at Saturn's distance you're going to experience quite a chilly climate, and greenhouse effect can only do so much.

You could use a lot of the hydrogen as energy source for fusion plants, and use humongous helium ion engines to slow down the planet and bring it closer to the Sun. It would probably require thousands of years, though.

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Move them

You have lots of energy. Move the planet closer to the sun. You'll lose some of the hydrogen during the move, and once you are closer, the solar wind will push away more of it.

Use it

The problem with gas giants is that they are gassy. You want to get rid of hydrogen, which requires the extra gravity just to keep from flying off into space. We use hydrogen as fuel for fusion, as reaction mass in rocket engines, and as ingredients in things like water and hydrocarbons (plastics). So extract hydrogen and use it.

Fusion

Fusion takes hydrogen and converts it into bigger atoms. The bigger the atom, the more energy it takes to trigger and the less it produces. Hydrogen alone is a gas at any reasonable temperature, but larger atoms like carbon can anchor liquids and solids at Earth temperature.

Short on helium? Fusion makes that. Given sufficient energy, you can also make carbon and oxygen, which appear often in our ecosystem. Even heavy elements like iron can be made, although that requires a lot of energy up front.

Stages

Note that there are different stages here. Post fusion, you have a bunch of cooling elements. Harvest those and move them to the rocky core that needs them. For example, Mars is short of atmosphere. So take various elements from pre-fusion (hydrogen) and post-fusion (oxygen, nitrogen) gas giants and make it an atmosphere. Maybe even bulk it up a bit, since the gravity is low there.

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  • $\begingroup$ This is very close: One can set up fusion process on gas giant that does two things: (1) convert gases to solids (2) use the generated energy from the fusion process to move it closer to the Sun. Once desired solid size and atmosphere is achieved, the planet than can be broken up so we have may be dozens of smaller planets in orbits. This would solve the gravity issue as well. $\endgroup$ – Shital Shah Jan 15 '17 at 9:58
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This might not be what you meant, but probably the best option for creating a livable surface on a gas giant would be a shell world. As explained in the video this is an active support structure enclosing the gas giant at the radius where gravitational acceleration is equal to Earth gravity.

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  • $\begingroup$ Could you please summarize what she’ll world is? $\endgroup$ – Shital Shah Jan 25 '18 at 6:46
  • $\begingroup$ "shell" not "she'll" $\endgroup$ – Lex Jan 25 '18 at 7:05
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Tricky, in large part because they are so massive.

Probably the gas giants are solid in the middle. Stuff that hits them must go to the middle. Probably they are liquid partway down, because of the tremendous atmospheric pressure makes the atmosphere liquid. As regards habitability even if you built a dome to exclude nonbreatheable gases I think the gravity would make things difficult for humans, regardless of atmosphere and rock or no. Wikipedia says Saturn has 95 times the mass of earth.

Maybe you could wave hands hard and assert that the rocky core is small, and the mass of atmosphere above you and below (on the far side of the core) canceling out such that G forces for people on the core are tolerable. That leaves atmospheric pressure which would still be immense. You would need to get rid of a lot of mass. That means either converting it to energy and radiating it away (in which case you have made a little star) or having a driveby from a massive object that makes off with it - a near miss from a baby black hole might do. Good luck steering one of those.

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    $\begingroup$ Yes 95 times the mass of Earth, but what’s the radius at the equator? That is, what is the actual gravity? You made a lot of statements without looking up anything. $\endgroup$ – JDługosz Jan 14 '17 at 18:46
  • $\begingroup$ I guarantee I looked up that 95. If I had made that up I would have put 91. $\endgroup$ – Willk Jan 20 '17 at 0:17
  • $\begingroup$ Then you should know that the surfqce gravity is 1.065 × that of Earth, so why “ I think the gravity would make things difficult for humans” ? $\endgroup$ – JDługosz Jan 20 '17 at 6:29
  • $\begingroup$ I did not know that. It is a big surprise to me! I see it several places on the web. I see the reason stated is that Saturn is very massive but the mass is gas and somehow, therefore, it exerts less gravitational pull than manlier types of mass . That can't be right; mass is mass. My thinking: vectors cancel out / treat the center of mass as a point / inverse square etc. All true - I think for Saturn the deal must be that the point is really far away from a hypothetical person on the surface. Worth more reading, Thanks for taking the time to point out my error. $\endgroup$ – Willk Jan 21 '17 at 14:51
  • $\begingroup$ So now you understand my original comment? You can edit your answer, you know. $\endgroup$ – JDługosz Jan 21 '17 at 15:57
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As has been pointed out, there is probably plenty of solid matter in a gas giant. The trouble is that it is an insignificant portion of its mass.

So, you would have to blow away the gas without blowing away the bits you want to keep. This will be difficult depending on how much circulation there is in the deep regions of the planet. The interior is generally hot enough that "solid" matter will likely be melted into a liquid or boiled into a gas. If you are lucky enough to have low circulation, the solid bits may have clumped into the center but if the circulation is high enough, the solid matter will mix with the rest and you will have a filtering job.

Though if you strip off the outer layers, you can probably get the solid bits to condense out once the pressure has dropped.

The big question though is: why?

We have lots of solid bits that we can get to that don't take nearly that much work: planets, moons, asteroids, oort cloud.

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Sure, this is the prevailing theory of the formation of.planets from a ball of gas that splits into several balls of matter. It just takes a long time. If you can remove the rules of physics and somehow increase the rate of fusion, hydrogen and helium can become iron and other minerals. The atmosphere would be tricky as where would it come from?

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    $\begingroup$ There's a huge difference between forming rocky planets from a protoplanetary nebula and forming rocky things from gas giants. I think most of this answer could use some scientific justification. $\endgroup$ – HDE 226868 Jan 14 '17 at 15:47
  • $\begingroup$ @HDE226868 What about compressing the star into a solid? $\endgroup$ – johnny Jan 15 '17 at 1:04

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