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Rogue planets are generally not to healthy for us humans to live on. No permanent source of heat or light means that generally they are a pretty cold place. Plenty of other problems arise too, but lets not get into all of that.

How do I make a rogue planet of Earth-like proportions habitable by humans?


My habitants certainly don't need to be strolling around in lush green parks, but I would like them to be able to survive for many centuries.

Assume futuristic technology, limited by incapability to engage in Faster Than Light travel. Also assume that cost is no issue: all of humanity is united in this singular goal.

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  • $\begingroup$ Does the surface have to be habitable? Or is underground fine? $\endgroup$ – Xandar The Zenon Jan 17 '16 at 3:45
  • $\begingroup$ @XandarTheZenon, preferably the surface. Until the surface is livable, my colonists will be underground. $\endgroup$ – Quiquȅ Jan 17 '16 at 3:47
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A "rogue planet" would be a planet that went rogue. Meaning it formed normally orbiting a star, swept its orbit clean, and all that. Just like Earth. And then a close encounter with another stellar object or a gas giant flung it away into the dark and it froze.

So the starting point should be a younger Earth frozen over. And without life. This means there might be significant hydrogen and helium left in the atmosphere, but no free oxygen. Which would allow, in addition to hydrogen, things like ammonia and methane. We can also assume lot's of carbon dioxide since less time would have been available for it to get fixed into carbonates.

The result would be a planet with thin atmosphere of hydrogen and helium, with pressure depending on how much time there would have been for them to escape. Below this would be a surface of frozen water, methane, ammonia, carbon dioxide, and possibly nitrogen. Although it is possible all the nitrogen would be in the form of ammonia given that free hydrogen is available. There would also be lots of impurities from volcanism.

Now what happens if you heat this up somehow? All that ice, and there is lots of it, will turn back to gas. Good thing is will give you a thick atmosphere. Not so good is that it will be mostly composed of things hazardous to humans.

You could fix this by processing the ice in manner similar to what happened on Earth while gradually melting the planet. But given the sheer mass of volatiles you'd need to process and that you'd need get rid of lots of them either by blasting them into space or storing them into the lithosphere deep below the icy surface layer, it would take a ridiculous amount of resources to do. Even with super-science it is hard to imagine it being fast.

This is very different from something like Mars or Moon where you could in theory just gradually add breathable gasses until you have an atmosphere with some pressure and ability to retain heat.

IMHO even if the first generation was totally to terraforming the planet, their descendants would pretty soon start to question the need to use ridiculous amounts of resources to convert the home they and their ancestors have lived on for generations to resemble some mythical place none of them has ever seen. Who knows if "the Dirt" or whatever even was a real place? Certainly the stories must be exaggerated. Who would want to live on a planet where air conditioning is so bad it leaks liquid or even solid water on people and can't even maintain a stable temperature?

And this is before you consider the actual problem of heating the planet and keeping it warm. Warming up the entire planet would really not make sense to people used to living in artificial habitats. Why spend resources warming up and insulating space you do not actually need? On a planet with a sun you get energy for free if you boost the greenhouse effect or reduce albedo, both of which you could get for free while building the atmosphere and hydrosphere. On a rogue planet, nothing would be free.

So I think your people would live in insulated habitats buried deep enough into the ice to protect them from radiation but shallow enough that the ice surrounding their habitats could resist the pressure and be stable with minimal supports. The ice can be mined simply by heating it and allowing it to escape above the surface so building would be simple. By simply making the habitats slightly smaller than the dug tunnel you could get proper insulation. And if you got a leak, it would be easy to spot by the effect it has on the ice. A larger leak could melt and even gasify enough volatiles to create counter-pressure.

You would also have plentiful supply of all the volatiles you might want. Getting access to the lithosphere for mining would be trickier. While digging down is not in itself any harder, the pressure of the ice would add to the pressure of the stone. So you would be deep mining before even reaching the stone. They'd probably have very good mining technology. Using organics instead of metals or silicates and mining at high mountains where the stone is closer to surface would help.

Energy could be gotten by nuclear power or geothermal. Efficiencies would be fairly good since the power plants could have the cold end in a lake of liquid nitrogen. Really, why would anyone want to live on a uselessly warm planet?

Note that the planet would be much better than living in space as you would be safe from radiation, have abundant supply of air and water ("some processing required"), and natural gravity. And you are not surrounded by vacuum. Leaks would be slower and even if your life support totally fails, with proper protection you can walk to safety. Driving around in the tunnels would be perfectly practical.

Only real downside is that the habitats would need to be built, maintained, and repaired. This would lead to slow and carefully planned expansion and strict control of things like population growth and construction. Security services would also have pretty strong authority to stop people who even look like they might think about damaging the habitats. Which historically has been extended to detaining people who question the authority and decision of the powers that be. So at least some aspects of the society would be fairly authoritarian. That is the price of living in artificial habitats.

Still the aspects that need tight regulation would be fairly stable over time, so after a few generations people would have adapted to it. Allowing other aspects to be fairly free.

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  • $\begingroup$ +1 for considering the situation from the point of view of people used to living in space stations/spaceships $\endgroup$ – celtschk Jan 17 '16 at 12:59
  • $\begingroup$ @celtschk I actually didn't, but that is a very good point. They had to reach the planet somehow and have some reason to live there and not on already habitable planet. So it is fairly safe to say they would already be used to living in artificial environments and would primarily be looking for solutions to practical problems with living in space. Such as lack of gravity, catastrophic consequences of equipment failures, and difficulty of securing volatiles. So even the first settlers might have no interest in terraforming. They'd just want a better place to build their habitats. $\endgroup$ – Ville Niemi Jan 17 '16 at 13:24
  • $\begingroup$ Not sure if there would be hydrogen or helium left.The internal heat of the planet would probably keep the surface warm enough for it to stay gaseous, letting it escape. $\endgroup$ – Eth Feb 28 at 10:34
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I have a problem with the term “rogue planet” since it's actually an oxymoron: it is lacking the defining characteristic. While I suppose free-floating planetary-mass objects can come in any size including small rocky bodies (ejected from forming solar systems), we normally think of them as sub-brown dwarfs down to Jupiter sized, probably because those are the ones we can detect. We detect them because they are, in fact, warm. A Jupiter-sized mass would have lost half its heat by now, which is not as cold as you might have thought. A sub-brown dwarf might still be red hot!

It's hard to put any kind of settlement on a gas giant. So look at the moons. In fact, look at Io in particular. It gets its geothermal energy not from the sun at all! This situation could just as well exist without the sun.

Consider a planemo several times more massive than Jupiter, which would still be the diameter of Jupiter, and accompanied by a large number of moons, including a set in orbital resonance. Io can't simply get pushed away because pushing it pushes all the moons, their being in resonance. So it has a great deal of inertia keeping it in that particular orbit, even though it extracts a lot of energy while doing so.

You have a number of large bodies, including icy and rocky moons, for colonies. You have massive geothermal power. You have a small amount of heat coming from the planet itself, which might be enough to make some of the icy moons more liquid; you might have tidal heating of a moon with sufficient water to make a liquid ocean, where the amount of heating is just right.

Life on Europa or Enceladus might be completely oblivious to the sun, and that might be the more "normal" situation, compared to Earth!

In short, it's not the cold place you would suppose.

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They could potentially make structures out of a transparent material, with metal framing and strips of lights built into the frames. Then they could funnel some kind of heated material such as magma through these frames and beneath the city, providing heat. You might have to have some kind of filter that limits what type of energy is allowed into your structure. For example, in an artificial night, you might block most light and turn off the artificial lights.

This would allow plants, animals, and humans to all live self sustainingly in these structures. They could even be constructed to imitate an existing biome. These structures could be expanded to accommodate rising populations. However, I don't think that the ceiling of these structures could be too high, otherwise it would not heat the area effectvely.

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The main problem would be energy. Without a star as energy source, you will have to produce all the energy yourself. You'll need a lot of energy, so probably fusion is the right source (note that in the end, stars are also nothing but huge fusion reactors). I'll assume by then humanity has managed controlled fusion of standard hydrogen (as opposed to our current attempts which still need deuterium), so there is no lack of fuel.

The next problem is that without a star to heat the planet, the atmosphere will be frozen (apart from a few local places with geothermal activity). So you'll probably start settling on the surface of the frozen atmosphere, and gradually work down to the surface. This has a big problem: Most of the oxygen is probably below the frozen atmosphere (as that planet would not have life, the atmosphere would not contain oxygen), either bound in water (ice), or bound in rocks. So you'll need to get to the ground before the oxygen supply you brought with you is no longer sufficient (you may need to apply strict population control in order to prevent oxygen needs to grow too fast). I'd build the first power plants as satellites around the planet, so they are not affected by the later melting/boiling of the atmosphere.

But as soon as you have secured access to the planet's actual surface, it's really just a matter of how much energy you can produce. So build enough fusion reactors to keep the atmosphere warm (and produce enough greenhouse gases to keep losses to space low), and split enough water top produce oxygen (you probably have to do that anyway to get the hydrogen for your fusion reactors), and assuming the planet is sufficiently earth-like, the rest should be simple: Plants can grow under artificial light in hydroponic greenhouses (assuming the people of the time actually eat plant matter, not artificially created food). Advanced technology should be able to build all the substances you need using the elements found on the planet and the energy provided by the fusion plants.

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Oh, you do have your accepted answer already, but I want to add something for the fluff-factor. When reading this I felt remembered of a never ending space opera from Germany: Perry Rhodan.

Its pretty unknown outside the country, but still... they had there habitated rouge like planet in the year 1965 ready if I recall correctly. Its name was... the 100-sun-world. While it wasn't settled by humans, the bio-robots living there where pretty happy just a hop outside the galaxy without a central star.

Guess what its pseudo-science explanation was? Right: they placed about 100 artifact mini-suns around their planet to get a neverending day and surface condition that allowed the space operas protagonist group to visit it without wearing any protective gear.

So if you feel adventurous, let your guys use a similar solution. At all, there is no hard-science tag attached to your question. Oh, if someone ponder about these robots: their name where "Posbi" I think. They may still be around, because that space opera didn't come to an end until now.

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It is my understanding that it is plausible for a "Rogue" planet (one not in orbit around a star) to be heated by other celestial means, such as Dark Matter or a Quasar. The latter of which you might not need to be even that close to in order to absorb its energy sufficiently. If these premises are true, you could have a planet not orbiting a star sustain life, if you could control some of the other factors mentioned by others here, including a strong magnetosphere to insulate against radiation, gamma and cosmic rays.

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