Colonizing Half-Hot Planets

A half-hot planet is one which is in a close orbit to its star, is tidally locked, and is small enough to not have an atmosphere. Without an atmosphere, the only way heat can come from the side facing the star to the other side is by conduction through the body of the planet, which is bound to be slow. This would allow the side facing away from the star to radiate away a lot of its heat, and be cold. Thus the planet would be half hot and half cold. 

A recent post suggested aliens might migrate to a frozen world, one distant from its star, where the temperature is well below that of the outer edge of the so-called “habitable zone”, which is a poor name for the zone where water can be a liquid. The idea is that with enough technology, the alien colonists do not need solar photons to support their civilization, but can instead mine uranium and low-atomic-number elements useful for fusion. If the planet has enough of those, and the costs of mining it are small compared to the energy it would produce, including all the processing and everything else connected with power generation, the colonists can simply live under the surface in a comfortable environment, while they mined from one place or another all the minerals needed to support a good living standard.

These frozen planets don't have to be planets. A frozen moon would work just as well. As long as the planet doesn't create a terrible environment around itself, from radiation or something else, a moon would do just nicely.

Another thing to consider is that they don't have to be frozen at all. They can be habitable zone planets or moons, but too small to maintain an atmosphere. They cannot be too hot, as the temperature under the surface would be above tolerable temperatures, and this means there would be refrigeration needed for the living conditions, and perhaps also for all the mines. For too hot a planet, this would certainly mean it was unusable. Where exactly would be the average temperature that would make them intolerable is not so easy to determine, but it couldn't be too high.

There is one exception to that: half-hot worlds. In our solar system, we almost have one of these gems: Mercury. Mercury is phase-locked, but not 1:1, but 3:2. Mercury does not keep one face toward the sun at all times, but gradually rotates. If it were phase-locked at 1:1, like the moon is to the Earth, it might be a candidate.

One nice, somewhat speculative, thing is that the dust cloud which forms a solar system might have some differences in the mineral content of different planets, and even some basic trend. It could be that heavier atoms are more populous, relatively, on inner planets. It is not hard to imaging that dust collects like or similar molecules, and some dust grains collect more uranium and thorium than others, and then drift inwards, relative to lighter ones, such as calcium and sodium. When planets get around to condensing, this would mean that there would be more fissionable elements on inner planets, and in fact the most on the innermost planets, including the ones so close that they get phase-locked at 1:1.

In order to make this story complete, the planet would have to be large enough to stay molten after formation, so that the iron-like elements could sink to the center, leaving everything else to condense elsewhere, such as near the surface at a depth suitable for mining. Now the stage is set for an alien starship to land on the cold side, and begin to mine, both for minerals and for living spaces. Lots of other constraints might pop up, such as there being few quakes, strong enough rock to support mining, and so on. There would certainly be multiple more constraints, and it might be interesting to try and think up a list someday, but the main point is that phase-locked, 1:1 only, planets might be excellent places for an alien civilization to spread to.

These planets give off no signature of life, and except for some other alien civilization who was visiting or inhabiting the same solar system, the colonizers would be undetectable. An orbiting ship sent by the original inhabitants of the solar system might see piles of spoil from the mining, or the relic of an old starship, provided it had very good optics.

Now we have an interesting situation at hand. If the idea of living without the use of solar photons works, and mineral wealth alone is enough to make a planet colonizable, there could be lots of alien colonies, perhaps at a density of more than one per ten solar systems. All of them would be undetectable, no matter how hard a second alien civilization in a nearby solar system tried. The only way to find them would be to go to the solar system where they were, and spend a good amount of time scanning the surfaces very carefully, covering every large moon and every small planet not in the too-hot zone but including all the phase-locked ones.

If a colonizer didn't want to be detected, it might be possible to disguise the few local signatures of their presence, so that even this visiting starship would never know they were there. This would involve spreading out the spoils instead of leaving it in an artificial pile, dismantling the starship they arrived in and bringing the pieces underground, and building nothing on the surface outside of a few sensors. There would be wheel tracks from the vehicles used to explore the surface and look for new mining sites, and for transporting the processed minerals back to the home mine, but balloon tires might make this hard to see as well.

The upshot of all this is that the Milky Way might have a huge number of inhabited planets, and we will never know about them unless they choose to inform us. Instead of having only a very few origin planets, which are planets able to originate life and support it while it evolves to having an intelligent creature on it, there might be underground alien colonies almost anywhere there is a suitable planet. These planets and moons probably number in the billions. The age of the galaxy is of the order of 10 or so billion years, so exponential growth might have happened, and aliens are everywhere, just invisible to us.

Vulnerabilities of Population Reduction

There are the obvious ones, which relate to medium scale disasters that could annihilate a single arcology, and if an alien civilization concentrated its population in one, because they were so reduced in population that's all that was needed, it would mean the end of them. There should not be any surprises left in their solar system, meaning they know where all the asteroids are and their orbits, they know where all the subterranean faults are, they understand the risk of tsunamis and don't take that risk, and the same for anything else that happens on their planet, like hurricanes. With no surprises left, and a choice about where to site their single arcology, is there really any vulnerability?

Obviously, if they didn't know these things, it would be premature to reduce population to that level, so for the sake of the argument, figure they do know them and there is no geology left undone. On the psychology side, is there any risk in the slightest degree from one of them becoming psychopathic, and attempting to sabotage an essential system? Again, they are long past asymptotic technology, which includes psychology, so this is not really possible. Furthermore, the genetics grand transformation has given them all good genes and they also understand how to raise youngsters to be stable contributors to the society. They have to go way back to find in their ancient history a time when there was war and dissent, as every alien is rational and logical, and politics is a solid science now, so no reputable alien could raise objections to the way things are done. Technology simply brings calmness to everything it touches. Thus, an alien who had a passing thought about being a saboteur would simply recall that there are no political systems better than the one they have, maybe for tens of thousands of years, and as far as there still exists the abstract concept of justice, they have it.

There would be robots to fill all appropriate roles, and intellos, the biological equivalent of a robot, filling ones where they would be more efficient. Someone eons ago would have figured out how to keep everyone busy and interested, in who knows what, so there would not be any bored malcontents. Technology simply solves problems, one after another, until there are no more. If an alien civilization gets to this state, they can stay in it for as many thousands of years as they want, providing resources are sufficient and their star doesn't get nasty.

It would be hard for them to think of any vulnerabilities they have, or might have in the next millennia, as they have solved those problems already, except for one.

Aliens.

Not the aliens themselves, but aliens of a different sort from a different solar system. Aliens 1 and aliens 2, for convenience. If aliens 2 began traveling in space before they had reached asymptotic technology, or made the deliberate choice to avoid the calming effect it has, they might be going to another solar system with an open mind about annihilating whatever was living there and taking over the planet. This assumes there are two planets with life on them at some reasonably close distance, which could be unlikely or likely – we don't quite know that yet. So aliens 1 might be aware that there are other solar systems nearby them with planets which could have given rise to life, and they were old enough to have evolved a civilization.

One question we haven't resolved yet is could one alien civilization detect another, and how many light years away could this be done. If the answer to the question is that it would be too impractical to do this, or the engineering of the sensors to scan all the solar systems around them cannot be done, meaning there is some limit to what can be detected that we on Earth haven't figured out, and the aliens 1 have figured it out and there is no way around the limit, then they have an indeterminate risk. No telescope, no matter how big, can see finely enough to pinpont the signature of an advanced civilization, and certainly not enough to tell what stage it is in.

This means that the High Council of Alien World 1 can be sitting around thinking of how low a population they want to design for, and they have no way of determining if another alien civilization, aliens 2, is nearby and if they are going to be totally peaceful, or if they mastered space flight before mastering their own psychology, politics, economics, and a few other topics. And they have only their own history to guide them. They got super peaceful and would certainly not try and take over another civilization's world, but their ancient, ancient history says they weren't always this way. And they see a way that star travel could have been invented early on, if there was some motivation to steer technology development that way.

Naturally, they can come up with a master list of every way some alien 2 civilization could try and displace them from their own planet, and from this list, look for ways where having a low population, concentrated in one arcology at the extreme, would make them more vulnerable. Their first conclusion would be that it is very difficult to undertake such a offensive mission, and probably no alien civilization would want to spend that much resources on doing it. Then they might consult the alien 1 who was the most interested in ancient history, and ask him if any faction in ancient alien world 1 had ever chosen to spend some large fraction of their resouces on attacking another faction in a different region. If their history is anything like Earth's, the answer is: most of them did.

Perhaps there is something inevitable in the evolution of thinking beings that forces them through a period of time in which military adventures dominate their history. Or perhaps only a few worlds have such an period. But, if aliens 1 decide that somewhere in the near parts of the galaxy, aliens 2 are building some armada pointing in their direction, then they have a completely new basis for deciding on how much population they want to have. 

Population Reduction in Alien Civilizations

After an alien civilization gets to the point of asymptotic technology, that is, science is over and done with, they have a number of choices to make. One of them is how much population they wish to maintain. The choice is directly related to how long their resources will last, as for a given level of recycling, twice as many aliens use twice as much resources per time period. 

One aspect of this question relates to the process for reducing population. The direct and immediate solution is to simply gestate fewer aliens and allow the population to shrink at whatever level they could choose. It could be as drastic as going from a billion to a million over a few centuries or generations of alien life. The choice of what target to use is related to their view of themselves and their role in the universe. Is it to simply go extinct, or do they plan to go to some other solar system, in one of the many ways possible? If they choose to travel, there is a minimum population necessary to build the ship or ships. If they just are content to go extinct, an unlikely alternative, they could do it quickly with lots of aliens, or slowly, with only a few. Neither is very pleasant, as resource shortages do not make for high living standards.

This question is interesting, but also interesting is the process for getting the population down. There is one question that stands out: what about all the infrastructure? They don't need all the infrastructure that a larger population needs, and they probably don't want to spend the additional energy and resources to maintain it if it is only there for a ghost population.

Consider first fungible architecture. By this time, resource pressure, or at least the knowledge that it will be happening in the future, has mandated that the aliens will live in large arcologies, where recycling is pretty much total. One arcology might be almost identical with another, so there is no reason to keep the second one going if the first one can handle the population post reduction. In an advanced alien civilization, recycling will be part of everyone's life, and everything will be recyclable, even the entire arcology. So over a period of time, the second arcology might be taken apart, and fed into the recycling system of the first one, supposing they are not too far apart so transportation costs are not a significant factor. This adds to the longevity of the resource base.

In order to make an arcology recyclable, it would have to be divisable into parts, so there would not need to be any crowding of a double population into an arcology. Time for this could be stretched out, as by this time the civilization will have figured out it might have a million years on their home planet, so there is no rush to do anything in a short time. If gestation cycles are a hundred years or so, spending a few of them combining arcologies will not affect much over the long term. Aliens in the superfluous arcology could be given the choice of moving to the remaining one, or staying in the part of it which was not yet taken down. By this time, the arcologies would be self sufficient, with their own power plants, industrial sources of nutrition, air filtration, internal transportation, and everything else necessary to have a comfortable life for the alien population. There could well be some residual agriculture, for specialty products or for the amusement of aliens who wanted to be involved with farming for a period of time, and these would simply be reduced according to the population level drop.

What about the non-fungible parts of the infrastructure? Would there be any monuments, historic places, unique but antique buildings? There might have been, but the lifetime of any of these might only be a few thousand years at the highest, and after that, deterioration unless it was periodically rebuilt. Suppose they had preserved something from their earliest eras, before technology was greatly developed, and this was a part of their culture and something they used to maintain their heritage. With a drastic population reduction, as in the example above of a billion to a million, or something proportionate to this, a few heritage sites might be maintained, but not a large number.

It might not be appreciated that heritage could be a very important part of the alien civilization. Heritage is the reason they decide, over and over again in each generation, that they think their civilization is worth preserving and should not be allowed to become extinct. Each generation would have the possibility to reverse the decisions of previous generations, and, for example, stop work on a starship and instead use the capital to change their activities, to, perhaps, have more jet aircraft and spend much more time flying over the landscape and visiting unique sites on their planet, in person. Without heritage sites, the pressure to keep on track with previous generations might grow less, and reach a tipping point.

So, perhaps there is another factor which comes into play when an alien civilization is thinking about its target population for the long, long term. Having sufficient population to build starships may be one, but another might be having enough to maintain a critical number of very important heritage sites.

Almost everything else would have been conquered by technology. There might not need to be a minimum number to maintain the automated operations which provide energy and the standard of living to the population. This particular factor is not clear, but it could very well be that star travel and heritage sites are the only things which feature in the choice of population numbers.

What this means to us is that if these two problems of minimum population result in numbers fairly small, there might only be one arcology on an alien planet. When we have our huge telescopes, able to focus in on planets in different solar systems, looking for one with a thousand dots of light, probably infrared only, this may not be the signature of an alien civilization that we can find. There might be one dot of light, where they all live, and the rest of the planet has been returned to nature. This would be the situation for almost all of the million years of existence of the alien civilization, whereas the huge populations might be only a thousand or less, meaning a tenth of a percent chance of seeing them during this phase. This is one more signature of an alien civilization that needs to be carefully thought through.