Astronomers use, or perhaps misuse, the term habitable to refer to a planet which in a near circular orbit inside the zone where water could exist as a liquid. There are many variations and perhaps each astronomer has his own preferred definition. The popular press takes these ideas and talks about new homes for mankind. Perhaps we should change the definition as well as the word.
Let’s generalize to cater to this blog’s penchant for talking about alien civilizations. Here is the proposal. Suppose you are in an alien civilization on some home planet. Your fellow citizens have never developed any colonies on other planets, but you are debating, or rather, thinking through just what you would all like to do. Your science is very far advanced, and you are aware that the planet, the solar system, and the galaxy all have some perils which might impact your home planet and put an end to your happy life there, perhaps even causing extinction of your species.
Let’s assume you don’t like that prospect. According to our classification of the memes which drive civilization, this means you are in category 0, 1a, 1b, or 2. So, which exo-planet gets the honor of being the first clone world. You want to clone your home world so that your species doesn’t go extinct.
It may be that you have already rendered your original species extinct, as you have used genetic engineering to make a better species. That’s the one you don’t want to go extinct. You don’t want to have it reduced to a primitive living level either. You want to maintain your standards of living for whichever of your citizens are alive at any time in the future. You live comfortably, and future generations should also.
Recall some details about your life. Because of the need to conserve materials and resources, you do a very high level of recycling, and that means that almost all of you live in almost closed, almost hermetic cities. You are living on fusion power, from a power station that is possibly located distantly from your city. You have robots and intellos all over your city, and have no problems with disruption from citizens or from the environment.
The debate rages on, and some citizen asks: “Do we need a habitable surface?” This question completely changes the debate. Would it be a big loss to live wholly underground? Having a surface is nice, but it is not necessary in the same degree as all of the infrastructure you have, the knowledge bank, the central control network, the sustenance activities, and so on. This opens up a large variety of possibilities that might have been ignored.
For planet-specific perils, such as basalt floods or strikes by unstoppable asteroids, living on another planet in the solar system provides the backup to ensure your civilization continues. There are solar system perils, such as rogue planets, stellar encounters, solar activity, solar flux changes, and probably more. These latter are not mitigated by having a local planet backup colony. However, compared to starting up an exoplanet colony, having a backup colony inside your own solar system is quick and cheap. It’s not very likely that two planets will occupy the habitable zone in your solar system, so you might have to make compromises. Going outward, towards cooler temperatures, allows you to use your own heat for your colony. If you are underground on some cooler planet, there would be insulation provided by the rock itself. For this type of local colony, requirements are far different. You would need to have the necessary resources on the planet to be able to sustain your civilization. This means, probably, nuclear fuel of the fusion variety, mineral resources of a wide variety accessible from one location, and not much else.
The key threat to your civilization’s deciding to go outward in their solar system is sustainability. The surface of your home world was great to evolve on, and fine to continue to occupy. Without the surface being habitable, in an origination of life sense, your civilization would never have gotten started. But you have already evolved to the peak you want to achieve, and any further changes in your own genetic structure will not be done by further evolution, but by using your knowledge of genetic engineering.
Surfaces were nice back in the primitive days when you had to get food there. But with genetic technology, food can be produced industrially, and after some centuries of doing that, you wouldn’t want to have to go back to the limitations and obstacles that grown food had. Food is superb, and available in as many varieties as several millennia of expert chefs can concoct. There is no demand for a surface that arises from the need to feed the population.
You are already recycling the air in your city. The city’s systems remove everything except the basic constituents and send the rest into the generic recycling bin. After a few millennia of dealing with leak problems, you don’t have any more of them. You have learned, over and over, what it takes to maintain a closed loop system. If you didn’t learn it at home, you learned it by building crewed planetary vessels.
The debate would conclude by deciding to have a backup colony on a local planet, provided that there was a suitable one. There are unremovable restrictions, such as gravity, but not many. Perhaps a large satellite would work as well as a planet. This decision has implications beyond the solar system you live in. Your choice for an exo-planet backup colony, or even a large population world, is not a world with a nice surface such as your home world, but one where there were resources. It may well be that energy resources are the critical deciding factor. If deuterium is your fuel, does the planet have it in a way that is accessible at a reasonable energy cost? If the atmosphere is methane instead of carbon dioxide, nitrogen or oxygen, can you accept it, and get your deuterium from the methane? An atmosphere full of it sounds like a sufficient quantity.
If we are looking around the galaxy for habitable worlds, we will have to remember that habitable in the sense of liquid water does not mean inhabited. There could well be large populations of aliens, established in a prosperous civilization, living under the surface of a world we do not classify as habitable. What it does have is deuterium, and that is the aliens’ definition of habitable.
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