It's quite easy to write about colonization of one solar system by aliens from another solar system, but just how possible is it? This question has many aspects, such as how to build a starship to move from solar system to solar system, and would aliens be motivated to do it, and so on. But one which is quite important and has not received too much attention is this: would there be any available planets to colonize?
This question is exactly about how many potential planets there would be, and how many competing alien civilizations looking for one. If there are lots of civilizations and few potential planets, it means that no matter how good the starship design is, it isn't going to happen. That's what the vacuuming is all about. It seems likely there aren't going to be any planets to go to, in the large.
Let's toss some numbers on the table. Suppose there are about 10 to the eighth stars in the galaxy. Suppose about ten percent are in the disk, which is where life can originate, mostly. Suppose about ten percent of the stars are G class or K class, which is where life might originate. Higher classes burn out too early and lower classes are too weak. Suppose about ten percent of these G and K stars have a rocky planet in the liquid water zone, and about one percent of them have a planetesimal impact of the right kind to form the conditions for the origination life. Yes, we are sticking to the organic oceans theory for the origination of life. This means that there are about a thousand planets which originate life.
Let's assume that one of them is the first to give rise to an alien civilization. They live their life, figure out everything good about how to maintain their civilization, mine their whole solar system as far as is economical to do so, and eventually start running out of materials. Let's give them a million years, which is really stretching the limits of materials in one solar system as consumed by a robust alien civilization. They need to move elsewhere if they want to preserve their civilization, and for the purposes of this calculation, we assume they want to.
They use some big telescopes to monitor other planets, and find one, which we have called alpha-habitable, to go to. Alpha-habitable means it has life on it, and this is by far the best type of planet to colonize. They go there, and start all over again. But a million years later they are facing the same problem. So they move again. And again. After a thousand moves they have used up the alpha-habitable planets in the galaxy. If they took about four billion years to evolve to intelligent aliens, this means after another billion years, they have to go to planets that were formed later, and became alpha-habitable during the time they were running around colonizing the original ones. This only gives them about another hundred million years, and the third generation is only ten million, and soon they are at a point where there is simply nothing left.
They have vacuumed the galaxy for every alpha-habitable planet. They have no where to go. What are they going to do for the next five billion years?
While they were busy colonizing and living and exploring and such, some of the other alpha-habitable planets were busy evolving, and some of them would come up with alien civilizations as well. The first one in the galaxy was busy taking care of the alpha-habitable planets by themselves, but the second one, assuming they also think their civilization is worth preserving, is going to get into the running for finding and colonizing alpha-habitable planets. With two of them going at it, they have only a half billion years supply. Evolution doesn't stop on other planets as well. There is going to be a third, and a fourth, and a fifth, and so on. Now the supply of alpha-habitable planets is beginning to look pretty small and maybe they are exhausted as early as a hundred million years after the first colonization in the galaxy, or even sooner.
One of the first realizations in this blog was that all alien civilizations would develop the same technology, and it wouldn't be too long before they had done it all. This means that all the alien civilizations can build the same giant telescopes, and the same computers to analyze the data, and the same propulsion systems to drive a starship, and everything else. They are all facing the same fate and the same prospects. If they want to colonize, they are going to have to do something different.
What are the possibilities? One is to live on beta-habitable planets, which are planets without life, like Mars. Unpleasant, and perhaps impossible to do in a self-sustaining manner. We on Earth like to fantasize about having a Mars colony, but that would be done using resources from Earth. The idea that Mars inhabitants would obtain everything they need from Mars in order to survive for millennia without resupply from Earth has not been analyzed. We are exploring the planet, but we have not done an ore survey, or figured out how to operate in hermetically sealed caverns, or much else. It could be that too much energy is needed, as compared with how much could be obtained there. Or there could be some materials that simply aren't common there, but are necessary for modern technology. Some planets might be beta-habitable in the Milky Way, and alien civilizations would likely be able to find some, but then they have to make a choice about whether that kind of life is worth having.
Another choice is to try and become space nomads, and not bother to try and establish life on the surface or near-surface of any planets, but just to live in space and exploit planets for resources. This might be possible, or not, depending on energy and materials tradeoffs.
There is a simpler and clearer way to consider the original colonization idea. That is to compare the probability that an alpha-habitable planet develops an alien civilization with the ratio of the lifetime of the civilization on one solar system to the time between successive alpha-habitable planets arising. If half of the alpha-planets develop an alien civilization that wants to preserve itself by colonization, and the civilization expires on one solar system faster than a new alpha-habitable planet arises, it is out of luck. Once the initial supply is consumed, there is not enough generation to allow colonization to continue. This comparison of two key numbers may dominate what we observe in our galaxy as far as life goes.
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