If there are lots of planets that can originate life and support its evolution into a technological civilization, and they all use up the planet’s or solar system’s resources and decline in ten thousand years, give or take a factor of ten, what’s left? A galaxy full of used-up planets, without any civilization there. To be clear, it isn’t the soil or the clean water that gets used up, it is the buried mineral resources. It is possible for soil to regenerate over the period of a century or so, and most civilizations wouldn’t be using it once they developed the technology to synthesize great food of limitless variety. Water which flows around in the usual hydrological cycle cleans itself as well, and the civilization will be doing water recycling itself within the cities so that would not be a problem.
The problem is the lithium will be gone, the iron will be gone, the copper will be gone, the rare earths will be gone, the phosphorus will be gone and lots more. Resource extraction will continue as long as there is extractable ore, but that will be used up eventually. Perhaps one planet somehow has more beryllium in it, from some details of what types of supernovas went off in the general vicinity of the gas cloud which compacted into the star system. Perhaps another has lots more nickel that usual, due to some other cause. These details would indicate just how the civilization would descend into scarcity and then exhaustion, meaning which substitutions would be done last, as living standards declined and then sank below the self-sustainment point.
Some materials, perhaps only the rarest or most valuable ones, might be mined on asteroids, moons or even small planets and used to relieve a bit of the home planet’s scarcity issues. But after some time, the cost of the shipping would increase, and eventually put an end to this source of supply. What happens next? The alien civilization is forced to give up the technology it invented, depending on the resources it could obtain, when those resources gradually gave out. No technology means almost no civilization. The decline might be slow as living standards get lower and lower, and the population declines so that more advantage can be taken of natural resources. Natural resources, such as soil and fresh water, were not needed at the height of technological civilization, but they remained available after it was over.
Power from fusion plants could have powered the civilization for millennia, but as the resources needed to construct new generations of power plants became scarcer, the option for the civilization having power drops drastically. Perhaps with remaining resources some other energy sources could be harvested. But eventually, the population returns to something independent of resources, a pre-industrial era reborn after the technological era has run its course.
Once the civilization returns to the pre-industrial level, the game is finished. Evolution will soon start affecting the population again, as some mutations will occur and the high technology needed to repair them is gone. If the alien civilization made the choice to go with solely artificial gestation, they become extinct a generation or so after the power goes off for the last time. If instead there were residual parts of the alien population which preserved biological reproduction, their species could continue, albeit at the mercy of natural forces.
How long would an agricultural civilization continue to exist and inhabit their planet? It is not clear at this time what the major factors would be that would affect their longevity. One factor might be that intelligence would gradually drop as mutations affected the gene kit that contributes to advanced intelligence. Some level of intelligence would be selected by nature, but it would be appropriate to an agricultural society.
The whole planet is used up, but not everything in the planet. What is used up specifically are the resources in the top kilometer or so of crust of the planet. With the passage of geological time, the crust is partially replaced with materials from deeper areas. One source of this is volcanic eruption, including individual volcanoes and also basalt floods, which, although fewer, involve more material. The other source is the upwelling and downwelling of continental material. It is well known that, at least on Earth, continents move. They also collide, which thrusts some continental material up to higher altitudes. Continental drift happens over hundreds of millions of years, so that the crustal material might be recycled over a period of the order of a billion years.
Material deeper in the crust is exposed through weathering, which over geological time, is very extensive. It is also possible that rifts open in the crust, exposing more material. It could be assumed that alien scientists would be able to find all good mineral resources on their own planet before the end of the age of technology, but mining deeper than a kilometer is very expensive, and might waste more resources that it provides.
So, a whole planet is recycled in geological times, of the order of a billion years and it is a great stretch of imagination to assume that the former alien civilization is still farming the land waiting for the resources to come back. What it does mean, however, is that after a billion years another alien civilization might arise, and do the same thing all over again.
This means that if we are investigating whether the absence of alien visitors is caused by them all dying out in a short time, measured in millennia, and the numbers of places they could visit pales in comparison to the number of planets that might be interesting, then it might be necessary to throw in another factor of two or three to take into account that a great planet that originated life once might do it again and again. A larger factor is not possible because it takes geological times to recycle a whole planet, not simply a million years or so. It also cannot be much more because the star is going to change its output over the same time scale, billions of years, and this means the planet will be out of the habitable zone before too long. The planet does not change its orbit, of course, but the star heating up will push the limits of the habitable zone outwards, and soon the inner edge will pass the orbital radius of the planet which once gave rise to intelligent life. No more chances for this planet.
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