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Say a technological advanced civilization wants to create an Earth-like planet using materials from the Oort cloud.

  • Could they really be able to create a habitable world resembling Earth using just materials off Oort cloud?
  • Due to the scope of this project what would compel them to construct a planet rather than go planet hunting?
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  • $\begingroup$ I don't think this question can be answered by a reality-check because we know next to nothing about it. Everyone seems to have a different estimates of how big it is and the mass of materials in it. Optimistic estimates indicates we might even be sharing the Oort cloud with other solar systems. $\endgroup$ – Mystra007 Aug 18 '15 at 19:35
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While the Oort cloud, by current estimates, has enough total mass to construct several Earth-sized planets, there are problems.

According to NASA, the Oort cloud is mostly ice comets, made of water, methane, and ammonia. There certainly are rock comets as well, but it is questionable whether there is enough rocky material to make a proper Earth-sized rock planet. You can't make an arbitrarily small habitable planet, because the lower its gravity, the faster it loses its atmosphere to solar wind.

The next problem is the magnetic field. In order to create a magnetic field from its core, the core has to be active, i.e. it needs to rotate within a liquid outer core. That requires heat, which comes from radioactive decay and residual heat from the planet formation. While mushing together the rocks into a planet would certainly generate heat, the radioactive decay might be absent for lack of fissible material. I have no way of determining whether that would be enoguh heat to get sufficient geological activity going for long enough to matter.

In order to support agriculture, significant quantities of topsoil are required. Topsoil, in turn, is mostly microorganisms and organic matter in various stages of decomposition. Those would be impossible to obtain in outer space and would have to be imported from Earth.

Overall, I believe you could get some sort of planet or planetoid going from Oort cloud materials, put water and an atmosphere on it and put it into the habitable zone of Sol, but getting a self-sustaining biosphere going will be hard, if not impossible.

However, even if those problems can be overcome, there is little rational reason to do so. A Dyson sphere, its variants, or similar mega-scale space habitats would most likely be a more efficient use of the available resources.

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  • $\begingroup$ Adding to what @Hackworth has said: most likely this planet would be dominated by water, unless you had a way to carefully select which matter is incorporated into this world. $\endgroup$ – Josh Belmont Aug 18 '15 at 18:04
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Q1: Yes.

The Oort cloud has approximately 5 times the mass of the earth so you could make 5 earths from it.

Q2: Not much.

They might do it for the challenge/the fun of it. They might do it as some sort of vast interplanetary artwork. They might have colonized every single planet in the solar system and are desperate for more living space (although in this case constructing Orbitals would be far more efficient uses of the mass).

If they were after minerals or similar then it would almost certainly be simpler to extract them on site and just ship the minerals back.

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  • $\begingroup$ The Oort cloud is mostly ice comets, so you can't make 5 earths just because the Oort cloud has 5 times Earth's mass. $\endgroup$ – Hackworth Aug 18 '15 at 7:50
  • $\begingroup$ @Hackworth You could make 5 habitable planets though. However I agree your answer is better than mine and have upvoted it. $\endgroup$ – Tim B Aug 18 '15 at 8:27

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