When an alien civilization, with fully advanced technology, is making up its collective mind as to whether to travel to other solar systems, and colonize the best of the bunch, it has certain considerations. One of these is feasibility. Other solar systems are marvelously distant. Just getting a probe there takes a large effort. But even if we grant that they could be determined enough to commit the resources necessary to attempt this, there is the problem of sustainability. A colony in another solar system simply cannot depend on a supply line from the home planet. It is too far and too costly. So, plans have to be made for a finite commitment from the home planet, and after that, the new colony must be able to sustain itself.
This is one reason that origin planets, ones which develop their own life and evolve into an entire ecosystem, would be preferred far above anything else. There may be some incompatibilities between life on the colony planet and life on home planet, such as DNA differences, but these are small compared to the needs of a colony which is just planted on a lifeless rock and expected to generate everything it needs from mining there.
It may be the unfortunate actual state of the galaxy that there are very few origin planets anywhere. This means that an alien civilization has the choice to colonize planets not suited for life on the surface, not possessing the atmosphere needed by the aliens for breathing. This means domes or underground sealed chambers. This adds to the supplies that the original ship or ships must bring to the distant solar system.
One question to ask is, can there be a sustainable colony on a barren planet? Sustainability is not the same as feasibility. With enough resources flowing in from the home planet, the colony could continue to survive. This could happen on a planet or satellite within the home solar system. If the colony there was providing valuable resources, at a cost which was affordable, the colony could be supported as long as the resources kept coming. Having a supply chain within a solar system can be done in some expedient ways, but there is no analog of one for a planet or satellite in another solar system. There is one benefit that a home solar system colony provides: it is a prototype for a barren planet colony in another solar system.
This means that if there is some doubt as to how to do a colony, and the level of engineering on the alien home planet is not sufficient to determine some details of how a colony would function, they can attempt to do one without anything like the cost or risk of a remote solar system venture. By this time in their developmental progress, their engineering skills should be sufficient to answer most of the questions that would arise about such a colony, and their computational capability should likewise be able to determine, or assist in determining, if such a colony could be designed to be self-sufficient.
Consider for a moment what sustainable means in this situation. It means that every resource that is brought for the colony's use by the initial supply vessels must be located on the planet and obtained at a cost that can be accommodated by the energy sources that are found there. Energy can be thought of as the currency of the colony. The myriad materials that are needed for an energy source, such as a simple fission reactor, have to be found, as discrete ore sources, mined, converted, transported, refined, and then fashioned into useful parts and components. This has to be done for a fraction of the energy that these resources will eventually produce. It goes without saying that energy storage or distribution within the colony must also be accomplished within the same energy budget.
One aspect is the amount of fissile materials that are present. If the initial supply vessels brought with them fissile material in sufficient quantity, then simply mining fertile materials might be sufficient. A simple breeder reactor could be assembled using the ship’s fissile material and the newly obtained fertile materials, hopefully leading to a net production of fissile material. If the supply vessel did not bring this, for reasons of intrinsic radioactivity or anything else, then the mining operation would have to locate its own source of fissile materials.
The amount of this material is directly related to the age of the colony solar system. Uranium-235 is deposited in the planets and satellites of the solar system from the amount in the original cloud of dust and gas that formed the solar system, and after than formation, no more is made, any more than other elements are transmuted into existence. In an older solar system, much of the uranium-235 would have decayed into lead, leaving only very weakly enriched uranium-238 behind.
The other quantity that affects the distant colony's chances to become self-sufficient in nuclear energy relates to the amount of uranium in the original gas cloud, which in turn depends on the processes which formed it, notably the number of supernovas which have detonated nearby. This might not be a quantity which could be measured remotely, meaning that one or more probes would have to be deployed. Note that deploying a probe means a delay of centuries in the launch of the colony vessels.
If the solar system is old, or wasn’t bequeathed much uranium prior to its forming a solar system, the prospective colonists might just choose to bypass it, or else rely on some other energy source. There aren’t many to choose from. A barren planet not too far from its star might serve as a good place to collect solar photons. Again, can the materials needed to form such a system be obtained for the net energy produced during the lifetime of the system? We are not really sure yet about this aspect of solar power ourselves, but within a few decades it should become clear if a solar energy system infrastructure can be afforded based on its own power production capability. An alien civilization would of course know this long before they even contemplated interstellar travel and colonization.
One interesting takeaway from this is that we may be thinking a bit about out own interplanetary colonies, and the information we learn from this can be useful in telling us if alien civilizations could manage to pull off colonization. The very understandable return on net power calculations should translate over, with appropriate modifications for a different solar system, and inform us of this. If the answer is negative, we can say we have one very good reason why aliens have not spread throughout the galaxy: Too few origin planets and nothing else is sustainable.
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