Detecting Alien Civilizations

Aliens haven't visited us as far as we can tell. They also haven't sent us messages that we could recognize. So, we have to peer out into space and look for them. Finding a planet which has oxygen in its atmosphere is regarded as a signature of life, as oxygen likes to bind to the exposed surface material and wouldn't exist in the atmosphere if it is not being replenished by life processes. At least that's how Earth works, and other planets may use this design as well. But oxygen or not, this says nothing about detecting aliens themselves. If they have an advanced civilization, they may be beaming messages in space, but we haven't been invited to join the network, and don't have a clue as to how to fill out the application. So we need to look for them, and then perhaps we might send a signal that says we want to chat. At least we would know where to send the signal. Detecting alien civilizations on a planet is difficult because they likely would not create any signatures on the planet which would be visible at lightyears distances, unless we built some very large telescopes. Even then, seeing some city on the planet's surface is unlikely. Perhaps if they traveled in space they might be detected. Consider the background of the signatures we could look for. If there was a planet like Earth, with life and even worse, weather and geological features and water features and more, all these would make the detection of life with low-resolution telescopes difficult. By low resolution, we do not mean little things like Palomar, but instead telescopes which have only ten to a hundred pixels resolution across the diameter of the exo-planet. That means, we would be seeing, at the best, only things which could stand out at those resolutions. What might they be? Suppose there was a very large city somewhere on the planet. This might be a few kilometers across, compared to the size of the planet, which might be several thousand. This is not going to be visible unless there is some spectral assistance. For example, if one pole of the planet was very cold, at the time we observed it, and the city was warm, we might see one pixel bright in the far infrared, surrounded by black (in infrared) pixels. This would be a good option, except infrared is absorbed by any atmosphere we might expect on a Earth-like planet. Maybe they have a thin atmosphere, very warm cities, and very cold polar areas, and then we might see the city. There is a much better chance to see some warm city on a satellite without atmosphere. If they had, on one of their planets, a moon with no atmosphere, but plenty of minerals and other things that were useful for the aliens, and they built some surface habitation there, it would be easier to see. The habitation would certainly be smaller, but the moon might be, for at least part of its orbit, much colder and not only that, more uniform in temperature. Thus, the detetability of a far infrared signal might be easier, even if the habitation was smaller than a city on the origin planet. So, an alien civilization with interplanetary capability might be easier to detect. There does not even need to be the assumption that the origin planet is in the same solar system. No matter how they get to the cold, cold satellite, the detectability calculation is the same. If, for example, their origin planet was on one star of a binary system, and the satellite they were visiting and colonizing was on the other, they would be detectable. And it certainly does not have to be a satellite. Any small world with no or a thin atmosphere would be just as good for detection. It might be that the future of alien space travel from this particular planet was very practical. Since there might not be any planet similar to their home planet within many light years, they might have decided they were going to go to many of the solar systems near them, within say ten light years, and set up colonies wherever they could be self-supporting. This could mean some good fraction of the solar systems around them will have some colony there. Perhaps a good fraction of these colonies would be detectable. How many colonies might there be? Suppose the universe is generous, and it is possible to set up a self-sustaining colony on a wide variety of smaller planets. Because we don't have any good knowledge of this number, none at all actually, because no one seems to have worked on it, let's assume it is 10%. So, if the average density of solar systems around their origin planet is about one in every 10 light year cube, the average alien civilization should have a colonizable solar system within about 9 or 10 light years. If their ship travels at 1% of the speed of light, it should take them about 1000 years of travel, plus some preparation time, to move to their first colony. If the universe is even more generous, and a self-sustaining colony can build their own starship in a thousand years from the foundation, they can start their second round of travel at 2000 years and arrive at the next planet at 3000 years. If they do two at a time, this means by 3000 years they have seven planets. In 2N-1 thousand years, they have 2 to the Nth – 1 planets. This works out to a million planets in about forty thousand years and a billion in less than sixty. These numbers are not realistic, but just are shown here to explain that covering the galaxy with alien colonies doesn't take that long. They could go much, much slower if they chose, and use up fifty million years colonizing the galaxy. Or whatever. If we want to go looking for alien civilizations, so that we can contact them or sell them our planet or just wish them well, it seems there is a fundamental division in how we choose to do it. The deciding question is: Is star travel possible, for an advanced alien civilization with a solar system full of resources and plenty of time to do anything necessary? If the answer is yes, it seems rather foolish to concentrate on looking for their home world. We want to know where could they have a self-sustaining colony, because there could be a billion of those and only one home world. Bad, bad odds. If the answer is no, then we might first ask: why are we doing this? Every civilization is all isolated in their home solar system, and what possible use could it be to find some other set of prisoners? Commiseration? But if someone could come up with a non-nonsensical, seriously rational and utilitarian, answer, for looking for somebody else's home world, we need to do some fundamental research which seems to be virtually ignored. If you want to find the home world of some aliens, you need to figure out what characteristics of the planet and its star are necessary, and what other conditions there are, such as having a satellite, low eccentricity, large gas giants in the same solar system, axial tilt and so on. A simple temperature of water condition is foolishly simple. We need to find the conditions both for life to originate and then, completely separately, for an intelligent civilization to evolve. That's what this blog is all about, but much more could and should be done.

Nearby Black Holes

Currently, it is very hard for Earth astronomers to detect black holes. Black holes are neutron stars which have enough mass to generate a Schwarzschild sphere around them. Neutron stars are stars which have a density like that of an atomic nucleus, except there are simply neutrons there instead of a mixture of neutrons and protons. Neutron stars are not black, meaning some light can get out of them, but for larger ones, it is not much. Consider a neutron star just a little lighter than a black hole. Light emitted at the surface will fall back to the surface unless it is going directly up. In this vertical case, it gets reddened an extreme amount, making it hard to be collected. A slightly less mass neutron star would have a wider cone of light which could escape from the surface, but still it would be strongly reddened and therefore hard to detect. If a neutron star is adding mass, by infall for example, its emission cone gets narrower and narrower, and the photons that do escape get redder and redder. The limit is reached when the cone goes to zero, and then even vertical photons fall back to the surface of the neutron sphere. The highest point a photon can get is called the Schwarzschild sphere of a black hole. Neutron stars are terribly difficult to directly detect for another reason. Any photon which is created even a few neutron radii below the surface is likely to be absorbed before it gets to the surface, so not only does light-bending make them invisible, so does the lack of emission sources anywhere but in the thinnest layer of the surface. Exceptions are those neutron stars which have intense magnetic fields and emit radiation at the poles, and others which rotate rapidly and radiate pulses due to some interaction of the magnetic field and surrounding matter. How many of these mostly undetectable black holes and neutron stars might there be? The only mechanism found so far for generating them is the burn-out of large stars, ranging from 10 to 25 solar masses for neutron stars and more for black holes. A simple table of such stars, showing their lifetimes divided into the age of the galaxy can produce an estimate. One can assume that the number density of these large stars has been the same during the life of the galaxy, or something else that would be higher, as there was earlier more gas to form large stars. This gives a number of the order of a billion neutron stars might exist now, but since they are almost undetectable, the estimate could be far off. Black holes form either from the collapse of even larger stars, or from a neutron star which collects more mass. How many of them exist in the Milky Way? If most neutron stars wind up as black holes, the number could be something like a billion. If the production of large stars in the Milky Way when it was younger was more intense, there might be ten times that. To get some casual estimates, this number can be compared with the number of stars in the Milky Way, but regrettably, that number is quite uncertain as well. Perhaps there are a hundred billion. If the density of neutron stars and black holes together is a tenth that of stars, and the density ratio holds in our part of the galaxy, it means that there might be a black hole or neutron star something like five to ten light years from many solar systems. In some cases, one might be closer than the nearest star. Neutron stars have about the same mass as the sun, and black holes start at perhaps twice the mass of the sun. This means that if one were nearby to a solar system where there lived an advanced civilization, it could be fairly close, perhaps closer than a half lightyear, and still be hardly detectable. If we consider the Earth as an example, if there was a three solar mass black hole at 30000 Astronomical Units out from the sun, it would not affect the solar system much at all, and therefore not be indirectly detectable. Gravitational pull from the black hole would be of the order of a few billionths of that of the sun on the Earth, and not much more on the outer planets. This radius is out in the Oort Belt, whose existence is somewhat controversial, as nothing in the Oort Belt has ever been detected. Its existence is surmised as the source of long-period comets which come hurtling in toward the sun from time to time. A black hole out there could serve as the instigator of the comets as much as having a hidden planet there or just having one icy blob interact with another to change the comet's orbit to an extremely elliptic one that passes near the sun. What would it mean to an alien civilization to have a neutron star or black hole a half-light year from its sun? These objects would certainly be detectable with huge telescopes for the civilization, just as they will be from Earth as soon as we start building them. There are really two different situations here. One is that if the black hole (or neutron star) has planets, it would be a very convenient location for an initial starship to head to. But can a black hole (or neutron star) have planets? Large stars are just as likely or even more likely to have planets than ordinary-sized stars, so just before the star starts its supernova process, the planets will be there. They might be the size of Earth and rocky, or gas giants, or icy mid-sized planets or any other combination. When a supernova goes off, a tremendous amount of mass and energy is emited from the star, and it comes crashing into the planet. What happens? In the first stage of the process, for a rocky planet, the side of the planet facing the star turns incandescent, increasing the pressure almost instantaneously, which starts to blast mass away from itself, towards the star. This process, explosive ablation, builds a barrier between the planet and the supernova so that the ablated material absorbs some of the radiated energy. If some gets through, the ablation process gets more intense, and larger quantities are blown into the barrier. This is a feedback effect, and if the planet is big enough, it might stop itself from being totally vaporized, so that when the supernova explosion process ends, what is left can reform into a planet. It will be in a more elliptic orbit, but that might circularize over some millions of orbits. A gas giant or an icy semi-giant will also have an equivalent process to explosive ablation, but the atmosphere will be torn off and if there is a core, it might be exposed. Exactly what is left depends on the strength of the supernova, the mass of the planet, its initial radius, and a whole lot of very interesting physics. At least some possibility of a planet surviving a supernova exists. Alternatively, a black hole could capture a rogue planet that came near enough to it. Too near, and the black hole would eat it, too far and the planet would continue on past, but at some intermediate range of closest distance, it could get captured. Since the estimate of rogue planets in the Milky Way exceeds the number of stars, this is not terribly unlikely. Thus, if the alien civilization was quite fortunate, it might have a black star or neutron star reasonably nearby and there might also be a solar system of sorts there as well. It seems beyond doubt to assume they would make that their first destination after they had explored their own solar system's planets, and any solar system on a binary companion to their own star. This would be a learning experience and might eliminate the need for a very chancy shot at a solar system a hundred or two lightyears away. The other situation is where there are no planets, and then the alien civilization would have to build a observatory to orbit the black hole, which is a large undertaking. They might prefer to go to the nearest attractive solar system.

Mineral Planets

Let's use the term mineral planet for planets that an alien species could turn into a sustainable habitat. These are a far cry from an origin planet, which is one which could give birth to life by evolving its own first cells. It is a far cry from a seedable planet, which is one which could not evolve its own starting cells, but which could take a seed of some sort of cells and have them multiply and eventually evolve into something interesting, like an alien civilization. Instead, a mineral planet is one where an advanced civilization could establish mines and habitats, on the surface or below it, and thereby produce enough resources, energy and minerals, to sustain an alien colony without any continuing support from the home planet. It has to persist for a long period. 

There may be very few origin planets in the galaxy, and somewhat more seedable planets, and maybe a huge number of mineral planets. One implication of such a lopsided ratio would be that mineral planets can be stepping stones for an alien civilization to cross the galaxy. Note that some or all alien civilizations may adopt the goal of seeding as many seedable planets as they can, following a philosophy that life is its own goal, and that just like planet-bound species try to disperse as much as they can, alien civilizations try to spread life as much as they can. Traveling 300 light years from a civilization's origin planet to the nearest seedable planet might be simply too much to do, and so finding a network of mineral planets in the general direction of that seedable planet would allow them to gradually work their way over to it, and when close enough, to accomplish the seeding effort with more payload and duration in orbit that they could have if they had to travel 300 light years.

Reliability might play a role here. If a speed of 1% of the speed of light is used as a guess of the maximum speed the civilization might attain with its colony ships, this means 1000 years of reliability is necessary to go to the nearest mineral planet, but 30000 years would be necessary for the closest seedable planet. If the probability of enough equipment lasting 1000 years can get raised to 98%, a risk the civilization might be willing to take, the same equipment has a probability of the same quorum still working after 30000 years of travel of only 55%.

Monitoring a seedable planet is also easier from 10 light years away than 300. It might be that seeding a planet is necessarily a very chancy situation, and multiple visits are the only way to accomplish it and verify that it has been accomplished in such a way that a billion years of evolution or two can follow without total extinction. Maybe seeding can only be handled by landing a small colony on the planet, and staying there for a long period. This could also be accommodated better from a nearby solar system than from a distant one.

Is there anything which can be credibly said about the prevalence of mineral planets? The formation of stars seems to leave a disk of matter revolving around it, which can turn into planets. This is a matter of the disposition of angular momentum, and how hard it is to collect it all in a central body. Everywhere we look we see planets, and our ability to find them is not so great right now, so there are probably many more per cubic light year than we have discovered in our locality. If there are several planets on the average per star, how likely is it that at least one of them is a mineral planet?

To be a mineral planet, the planet has to be mineable and habitable. Planets too close to the star are too hot on the surface to establish a colony, and the temperature below the surface would be higher than the average temperature at any latitude. The orientation of the planet would indicate the spread of temperatures over the planet, from pole to equator, and indicate if there was any latitude above which a colony ship could land and stay without thermal damage. Phase-locked planets provide a different criteria, but if the north pole of a non-rotating planet is designated as the closest-to-star point, then again, there may a latitude beyond which the ship could land.

Too much atmosphere would interfere with colonization, and planets might be excluded on this basis. Since smaller planets cannot long hold onto the atmospheres they have at formation, size is an indicator of this problem. The maximum size depends on the distance from the star, as it is easier to hold onto an atmosphere if the planet is far from the star and the atmosphere is very cold. Cold gases evaporate much more slowly.

Another question to be asked is the radiation level. If the star is a very active one, the colony ship would not even be able to come in close to it, unless some sort of shielding was build into the hull. Perhaps advanced engineering could figure out a way to get a mine dug, and alien colonists down into the mine without receiving too much radiation. Once under the surface, all the radiation is absorbed before reaching them. This is an interesting project to be considered.

With all these factors eliminating candidates, how much might be left? Our surveys of exo-planets are too limiting to calculate this number, but it might be that 90% are no good, meaning one in three stars, of middle class, has a candidate. There is more to being a mineral planet that simply being mineable with a surface not too lethal. There has to be the right mix of minerals.

An alien body has certain needs for elements, and alien technology has a different set of requirements for elements. Together they comprise the shopping list of elements, or rather minerals from which the needed minerals can be extracted. Some small molecules might also be extracted, principally water and carbon dioxide, maybe some others. The distribution of elements on a planet is a result of the original composition of the gas cloud, which comes from the effect of nearby supernovas in the cloud's history. Then there is the condensation question and the diffusion question, with minerals forming as elements and condensing into dust, and then being filtered by the solar wind and light output from the star over millions of years. After that, when the planet forms, geology plays a role in determining which minerals are at the surface.

The only planet we have any experience with is Earth, and it can provide us with a model problem. Suppose there was a planet in a state just like modern Earth but without any atmosphere, without any fossil fuels, no life, and of course no people, meaning no mining. Could an alien colony ship find the right minerals, in accessible form, so that it could produce a sustainable colony here? Perhaps U-235 is the key. We can mine uranium ore, refine it, enrich it, build a reactor, and extract more energy than was needed to construct the reactor and keep it fueled. Alien reactors should be even more efficient in the use of fissionable and fertile fuel than ours are, as we have had only a few decades of experience with fission power. Perhaps the guess of one solar system in three having a mineral planet is not too far from the truth. 

Choosing Colony Planets

An alien civilization with a philosophy of life requiring it to spread and disperse life throughout the galaxy, as far as it can, would need to be very circumspect about where to send a seedship. This adventure would require a great amount of effort from the civilization, and perhaps a good fraction of the resources available to it. On Earth, we have not even begun to figure out how this might be done, but we can assure ourselves this is not going to be easy for any civilization. As little would be left to chance as possible.

If we ask ourselves about the possibility of our civilization encountering another one, knowing where they would likely be is a critical question. They start on their origin planet, but then where do they go? We have learned over the last decade or two that there are huge numbers of exo-planets in the galaxy, but of these, which ones might be even initial candidates for an alien civilization's colonies? What are the characteristics of a possible colony planet? If we know that, then we can concentrate our search for alien life, or rather alien civilization, on that class of planet, and spend less on others.

What we cannot assume is that they will only go to other planets which have already originated life. If their philosophy and reason for continuing their existence is to spread life throughout the galaxy, an origin planet would be low on their list – it already has life and there is no need to go there and seed it. That would be superfluous. Instead, they would want to go where there is no life and is not likely to be if the planet is left to itself. Not just any planet would do. There are certainly some detailed criteria for a reasonably nearby planet, within a hundred or two light years, to even be considered as a possibility.

Many planets might only support the alien civilization itself, and not some ecology of plants, animals, microbes or whatever on it.  Their mandate is to spread life, but if the only way that can be done is to establish a colony, then that is the solution to the lack of planets which might be harbors for primitive life.  The alien civilization can set up colonies in many places, but needs to discriminate as to what distinguishes one possibility from another, as far as spreading life goes.  

One dominant aspect of the choice is sustainability. Sustainability means, for some particular alien species or collection of them, the ability to live for a very long period, measured in lifetimes, on the resources and energy located near and accessible to them. It includes the idea that the population should be able to grow up to some minimum value and still live there for that long period. It is about resources existing on the planet, near the surface, but also about being able to extract enough materials to make an energy source that produces much more energy than all the energy needed to collect and process the materials used in the energy generation and distribution process. There must be enough surplus energy for the other half of the problem, providing all the components, such as habitat and food, needed to sustain the new alien population.

The colony starts out with only the equipment that can be carried on the seedship. The development of the colony would consist of several preliminary stages before the uniform growth stage expands the colony up to the desired population. The first stage involves the landing of whatever is necessary to initiate power production with a minimally sized power reactor, create a habitat, and locate and start to mine and process all necessary mineral deposits. A central manufacturing complex would need to be created that can produce, from the ores found, all the specialized items needed for all the operations of the civilization.

Control of this process is not so critical. Can this be done in an automated fashion, or is it necessary to spend time in orbit, gestating the first generation of aliens, before sending them down with the initial lander? Whether the seeding operation is under AI control or under alien control, much the same steps have to be done. The principal difference is that habitats need to be made for the alien landing party, or some additional manufacturing facilities need to be made to enable expansion of the AI capability.

The question of sustainability is not easy to calculate in advance. Yet this is what an alien civilization must do before attempting to spread its population to a new planet or satellite in a distant solar system. They must make an estimate of whether or not a colony could survive on an exoplanet before taking the extreme expense of sending a seedship there The question is not just can the colony survive for a while, but survive and build a large civilization on the planet. The ultimate question involves the possibility that an alien colony, on a colony planet, could create a civilization large enough to send out its own seedship. If the colony planet was a dead-end, without enough resources for the civilization to grow large enough for the project of going out yet further to another colony planet, it should not be chosen.

The calculation depends on what goes along with the seedship. How much energy does it carry to support the transition from nothingness on the planet to a viable colony? Before this is used up, a seedship must arrange for native energy sources on the planet. This might seem to mean a uranium mine, but the uranium metal is actually a small part of what is needed to build an energy-producing fission reactor. Some parts for the first reactor might come from the ship, and this means that sustainability in energy is going to be developed in stages. The energy from the first reactor would need to be deployed toward a variety of tasks.

Total sustainability means that all mandatory mineral resources are present in the planet's crust, easily accessible, and with not too large a cost in transporting them from their mining site to the central location where the colony's initial population will be centered. For an alien civilization attempting to create a very credible and accurate estimate of this, which they would need before a launch, they would have to first collect all possible information about the planet and the solar system it is located in. The only way to do this from their planet is to build huge telescopes, and it also means asymptotic technology as far as planet formation goes, i.e. having geology completely understood, from the time of the gas cloud through all the changes that go on with the crust of the planet. They would need to be able to tell from the spectrum of the star what the gas cloud that created it contained, as for different elements and the relative concentrations of each.

At this point in Earth science, we have not attempted to make any such calculations, and so we don't really know if it is possible, or how accurate it might be. The accuracy is likely a function of the age of the star, as mixing will take place over its history, and the origination elements will also be burned up as they go deeper into the star's core. Light comes from the star's corona, where elements will linger the longest and where transmutation would be slowest.

Data is also available from the spectrum of the planet itself, where it is simply the reflected spectrum of the parent star. This might tell what was in the atmosphere, and what the large areas of the surface have, to a degree. Reflection spectroscopy is necessarily more difficult that emission spectroscopy, but if the telescope is large enough to portray the planet across many pixels, then some information might be gained from each one. Planets rotate, but that should not interfere with the data collection, once the images start coming in. A telescope of large size, perhaps ten kilometers or more in aperature, would be needed.

Information about the nature of the gas cloud that formed the target exo-planet might be gained also by looking at the other planets of the solar system containing it. A gas cloud which undergoes the great transformation from a rotating self-gravitating glob of dust and gas into a proto-star and spinning disk would also have undergone much diffusion, and this implies that at the radius where the target exo-planet condenses there would be some distribution of elements, but at the radii where other planets condense, there would be a different one, and knowing each of them helps the alien scientists to determine the larger picture of the composition and evolution of the cloud.

One kink in this process is that planets don't necessarily stay at the radius where they condense. The effect of the largest planet or the largest few planets might, in some instances, drive a smaller planet to a different orbital radius. The largest planets might also mutually share angular momentum, and drift to different radii as well. Can this be determined from telescopic observations so that the data from all of the planets can be put to use? A good question, and one we on Earth have not begun to fathom.

None of the scientific steps needed seem to be impossible, even from the viewpoint we have now, with our very limited science. An alien civilization a few hundred years further in science than ours should be able to accomplish them, as far as it is possible. Once this mass of data has been collected, perhaps over a century of observation, the estimation of which visible planet would be best to seed can be done.

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. 

Colonizing Frozen Worlds

An alien civilization which has mastered the art and science of traveling between solar systems might have done something else which will surprise us. They might have decided that worlds like their origin world are fine for the origination of life and evolution and technology development, but there are better choices for an established, advanced, expanding civilization. They might like frozen worlds.

Back here on Earth, we are all excited about the developments in the detection of planets around other stars than ours, and are contemplating how we might search for life on them. The great hope is that oxygen in the atmosphere will be the clue. Oxygen is a reactive element, and would combine with exposed rock, removing it from the atmosphere. It is plant life which renews it, by taking in carbon dioxide and releasing oxygen. Before we had plant life on Earth, we had a different atmosphere. Nitrogen is not very reactive, and carbon dioxide even less; they were there. There may have been other constituents, but no matter. Oxygen was not one of them. If life all died out on Earth, the oxygen in the atmosphere would disappear. So atmospheric oxygen has been chosen as the signature of life.

This means that our search for oxygen in the atmosphere of all those exo-planets is directly solely at finding origin planets. To understand what this means, let’s consider an example. Civilization Z originated on planet A101, after a billion years or two of evolution. They used up all the resources, but being a very intelligent civilization, they developed star travel before that happened, so they could travel to other solar systems and use up the resources there, before traveling on. Maybe they had communication between the different colony worlds, and maybe not. Perhaps they always did more than one new world from each colony, after about a million years on each one. Then, to do the very simple math, after one million years there would be two colonies, plus the origin planet which can no longer support a civilization, lacking resources. After two million years there would be four colonies, plus two old colonies which had died out and old A101. After three million years, eight colonies, plus six old colony worlds, now without any civilization, and A101. Just remember, in the calendar of the galaxy, three million years is very, very short. Maybe they would run into some expansion problems at some time, as exponential growth gets large very quickly. For the sake of the example, suppose that at the time we start looking for life in the galaxy there are five thousand colony planets with life, meaning civilization Z, two or three thousand ex-colony planets, and that old origin planet.

So, if some brilliant astronomer wants to find life in the galaxy at this point, there might be five thousand planets with life and a civilization to boot, and one origin planet. Where should he look for life? Origin planets?

To try and figure out what planets might be serving as colonies for civilization Z, ask: "What are they going to need?" Resources, and principal among them, energy. If the planet being considered is not a rogue planet, floating free in interstellar space, there will be a star to orbit around, which is giving off energy in the form of photons. These might be collected. Otherwise, there is uranium and thorium to fission and deuterium and other light nuclei to fuse.

Perhaps there are two stages of resource needs. One relates to the initial time on the planet, after the alien colony ship arrives and lands. Since fission reactors are relatively simple to build, compared with fission ones, at least as far as we know, they might seek planets with lots of uranium, and uranium that is not too old so there is still lots of U-235. Old uranium has only the U-238 left, which is much harder to fission. As uranium ages, the fraction of U-235 goes down. The planet from which civilization Z is expanding might start a hundred thousand years before they need to migrate, and send out some exploration ships. A ten thousand year voyage, and they can start reporting back on what the target planet is made of. So, in this particular scenario of colonization, there is plenty of time to carefully plan their next planetary colonization.

To have uranium, which is thought to be produced in supernova explosions, there would have to have been a few in the previous billion years prior to the formation of the star they are considering. Then there would be uranium, young enough to be useful. Figuring that out might not be too difficult, by looking at the contents of the gas clouds around the solar system in question.

Do they want a larger planet, with an atmosphere, or a smaller planet, maybe Mars-sized, with almost none. This might depend on the details of how planets form crusts and how mineral deposits accumulate in the crust. We’re not too sure of these details now, but if Mars has good mineral deposits, then Mars-like planets might be just what they want. Low gravity means not so much propulsion needed to get out of the gravitational hole. Little atmosphere means no winds to worry about.

Our knowledge of exo-planets is fairly sparse at the current time, but it might be such smaller planets are typically cold. If the star is smaller, but there was a lot of residual angular momentum in the cloud it formed from, there might be many smaller planets, completely frozen, but with excellent mineral resources. Could an advanced alien civilization cope with extreme cold? Can they master insulation? Very likely. Thus, perhaps frozen smaller rocky planets are their preference. If so, even a rogue planet might be just fine. There may be huge numbers of them roaming the galaxy, largely invisible to us.

There is another follow-on conclusion from this possibility. Earth would be of no interest to an alien civilization which was colonizing all the mineral-rich, frozen, small planets in the Milky Way. Earth is too big, with too much atmosphere, too large with too much gravity, and also has the minor inconvenience of already having life on it. Perhaps we should think through the alienology of colonization a bit more to see if this option is a dominant one.