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.

Aliens in Binary Star Systems

Can an alien civilization arise in a binary star system? This is not a relevant follow-on question to the principal one: Why haven't aliens visited us recently? It is one that is relevant to the hunt for alien civilizations from Earth, as if they won't come to us, we'll have to go to them. It is important to build some filters to separate out solar systems where aliens might be found, versus ones where they certainly can't have originated. After an alien civilization has mastered interstellar flight, they could go to any solar system they want, which makes the hunt more challenging, but if we are trying to find ones where they could have originated and specifically not where they might have seeded themselves, we can come up with some sharper criteria. So, could there be an alien home world in a binary star system? We don't want to spend precious telescope time on the impossibilities. First off, even if a binary or multiple solar system has a star which is suitable for origination, a G star like our sun, or sometime close to it, an F or a K star, that doesn't mean there aren't difficulties for life origination. When we see a binary, a physical binary of course not just a visual binary, if the companion star, or one of the companion stars in a multiple system, is a large star, we know that the age of the solar system is too young to have aliens, as these stars do not live very long. For example, even a mid-class F star, like an F5, doesn't last long enough for life, at least if evolution is as slow there as it was on Earth. Thus, both stars must be smaller than about a F7. If the other star is a white dwarf, this is also a bad sign, as white dwarfs are the end-stage of stellar evolution. It means that at some time in the past, they went through the red giant stage, then ejected most of their matter and collapsed to a white dwarf. A planet around a binary companion of this process would likely experience severe disruption, and any life that had originated on that planet would be either terminated or put through some severe extinction processes. While somehow life might re-evolve after this if the white dwarf process had concluded billions of years in the past, it would seem more fruitful to look at binary systems which have not endured the end-stage of stellar life. The next requirement is for stable planetary orbits. Three classes of orbits can exist in a binary system. One is where the two stars are close together, and the planet away from the pair of them by many times the inter-star radius. If you were such a the planet, you would see the two stars at once, circling each other. A second class is one where the planets are around one of the stars, and the other star is far distant beyond any planetary radii. The third is everything else. Your imagination can run wild here, with orbits making figure eight loops or some sort of modified oval around both of them. Clearly the discriminating ratio is the inter-star distance divided by the planetary radius, or for complicated orbuts, the mean distance over a long period of time from the planet to either of the two stars. If this ratio is very small, you have type one, very large, type two, mid-sized, type three. So far, it does not seem there has been a Kepler for type three orbits, and so we don't have a nice classification of them, along with the limits for stability. We hardly have the limits of stability for non-binary solar systems, so this is hardly unexpected. Type three orbits are better left ignored for now, although some computations could be done fairly easily to search to see if there are any weird orbits that are stable in this category. Type one orbits have a different problem. With two stars circling each other instead of a single star, a planet will fell much more of a tidal pull. In other words, two close co-orbiting stars will tend to transfer angular momentum out to the planet much quicker than a single star could. Since angular momentum increases with radius, this means the planets would be driven outward and eventually dispersed. Maybe that would be billions of years, but for life to evolve, a planet needs to be in a near constant orbit for these billions of years. The good-for-life situation is that a stars stays quiet and constant for eons and the planet is in a stable orbit. Alternatively, the planet could slowly drift outwards as the star becomes hotter with age; both of these processes happen quite slowly and fortunately go in the right direction. This matching is not something that would likely work with a type one orbit however. This means that we should look for planets hovering close to the star, meaning also that binaries of interest must be long-period binaries, the hardest to detect. In other words, if we already know a star is part of binary star, it is a poor candidate for an origin-of-life source because we can only identify short-period binaries with our current telescopes. Earth's astronomers have not identified many binary star systems yet, compared to the number of nearby stars, but somehow an estimate has been made that a third or half of all stars are in a binary system. Hopefully for the existence of aliens, these are mostly very distant binary systems. To use Earth as an example, we might have a binary companion star, maybe another F class, at 50 thousand AU, nearly a light year out, and it would not have prevented life from evolving here. At five thousand AU, perhaps it would have, and there is some boundary of influence that remains to be calculated, once we actually figure out how life originated, that is. To do a better job at identifying binary star systems in the neighborhood of our sun, we need bigger telescopes. Perhaps a verey large one at an Earth Lagrangian point could be used to develop btter parallax readings on nearby stars to get their distances and proper motions more exactly. One out at a Saturn Lagrangian point would be even better. There is little hope in simply watching far-separated stars to see if they circle on another. The type of orbits we are looking for, where a planet can be safe to originate life, means the two stars circle with orbital periods of the order of a million years. This is the limit of permanent connection. Stars cannot be in binaries at several light years distance from one another, as other passing stars will exert too much influence and destroy the orbital containment. So, distances of a tenth to a half of a light year are what to look for in a binary system where aliens can peacefully live and develop their civilization and hopefully star travel.

Heavy Elements in Galaxies

One question relating to the geological separation of useful mineral ores on exo-planets, something critical for an alien species to develop technology and socially evolve into an alien civilization, is about the distribution of heavy elements around the Milky Way. If a exo-solar system evolves from a gas cloud with very little heavy elements, above neon for example, it might evolve life on a suitable origin planet in that solar system, but the aliens, after becoming intelligent, wouldn't find the metals they need to go from a stone age to a bronze age, and they would never develop an advanced civilization. Thus, in order for us to have visitors from a particular exo-solar system, it has to have formed out of the same set of materials in the gas cloud, approximately, as Earth did, or maybe one which was richer in heavy elements.

These heavy elements are thought to be produced in supernovas, of which there are multiple kinds. Stars are nuclear ovens, gaining energy from nuclear fusion, which produces the elements above helium. Larger stars burn nuclei up to the nucleus with the least energy per nucleon, iron-56, but the kinetic process of showering nucleons into nuclei produces a wide distribution, centered around iron. Other phenomena produce heavy elements, and may produce a different distribution than burning in stellar cores. One example is the merger of two stars, in particular, neutron stars. So there can possibly be multiple sources of heavy elements, but they all involve stellar fusion processes or stellar disruption processes.

There are very few observable supernova in our galaxy, and probably very few stellar mergers as well, down in the number of a few per century. This rate cannot have produced all the heavy elements we see today, so the rate of production must have been much higher in the early galaxy.

Galaxies may form from the condensation of gas clouds of appropriate size, and as they condense, there are fluctuations in density leading to places where individual stars can form. As the enormous, galaxy-sized gas cloud condenses, if the density is relatively large compared to our current location, large stars will form as opposed to small ones. Large stars invariably turn into supernovas, and the largest of them might even totally explode, rather than just the outer layers exploding. The center of the star will be almost all heavy elements, with iron as the center of the distribution of elements, and larger stars may be more likely to have completed more of the fusion, so the central iron-dominated core will be a larger fraction of the total stellar mass.

This means that during the first phase of galactic evolution, long before the disk evolves to carry away the angular momentum of the cloud, the gas will be large homogeneous, or at least homogenous in spheroidal layers. The disk will form from the outermost layers of the galactic gas cloud, and thus we might expect that the disk will be fairly homogeneous with respect to the amount of heavy metals that exist in the disk and spiral arms. Thus, to a very coarse first assessment, solar systems close to ours might be expected to have the same distribution of isotopes and therefore elements. So, unless we want to think of stellar travelers coming from distant parts of the galaxy, the initial fund of elements should be sufficient on origin-type planets to allow any civilization which develops to get past the stone age, and move onward to industrial development and past that, provided that the geological separation processes on their exo-planet were sufficient to allow the useful elements to collect into bubbles within the molten core, and drift out to the crust and condense there into a solid.

The crust of an approximately Earth-sized planet does not have to be stable. Lying just underneath it is a hot molten layer, which may be in motion relative to the crust. Why? Because tidal pulls on the crust and on the molten layer are different, and induce a differential motion. Tide does not affect different materials the same, and a molten layer might move differently underneath a frozen crust. The crust might be flexed, and molten material leak upward, in what is called a basalt flood, if it is large and spread over an area, or a volcano, if the leak is confined to just a crack in the crust.

It would seem that a moon, during its early days of being much closer to the planet, had yet another task to perform that would be useful to an alien species which would arise much, much later. It causes a mixing of materials between the upper part of the below crust layers and the crust layers. If the two of these are each filled with different ores, the upper surface, where alien miners might get to it, would have an even better mixture of elements than there would be on a planet without a large moon initially close into the planet.

Often solid materials are more dense that liquid ones, and thus the crust, if it breaks into fragments, might be denser than the upper part of the layer below it, which might be called the mantle as it is on Earth. Then any cracking of the crust would allow part of it to sink down slighly, providing an opening for mantle materials to move upwards, and cool. There would be a balance between these materials cooling and becoming more dense, and the pressure inherent in the mantle both pressing them upward and condensing them to higher density.

The iron core would be largely elemental, but the condensing minerals would be combinations of metals and anions of various kinds, as there would be plenty of these elements in the initial cloud as well. The proto-planet would have elemental carbon and oxygen, which might combine to form a carbonate with some metal. And so on for all the other types of compounds found in ores. It might even be that the gas cloud, which has some percentage of dust mixed in it, already has some beginning compounds, and these partially remain intact during all the condensation and heating phase of planetary formation.

It would seem that the best way to explore our local galactic neighborhood for planets containing life and also alien civilizations would be to improve our telescopes and other detectors, and look for an Earth sized planet, located in a stable orbit relative to the other planets, and with a large moon locked into a orbit around it. Of course the stable orbit must be in the liquid water zone, have some axial tilt, and not be in too elliptical an orbit, which may be implied by the stability of the orbit, unless there were no large planets in the solar system.

This tangentially raises another interesting question for our exo-planet astronomers: are there any solar systems which have only one planet? Or is this an impossibility due to some feature of the mechanism of planetary formation? We on Earth have detected only one planet in most of the solar systems we have so far discovered, but that is not the same thing. It would be fascinating to find out there were many like this, with one planet only. This revelation would mean that we have less guidance from our home solar system toward understanding what goes on in other ones.

Does the Drake Equation Make Sense?

The Drake Equation was developed in the infancy of the SETI project. The Search for Extra-Terrestrial Intelligence was a US-sponsored project starting over sixty years ago, designed to listen for any kind of electromagnetic broadcasts than another intelligent civilization might be emitting. The equation is simplicity itself, just a product of conditional probabilities. Here it is:

N = R_*^.f_p ^.N_e ^.f_l ^.f_i ^.f_c ^.L

and N is the number of detectable alien civilizations in the galaxy,

R_* is the rate at which stars form in the galaxy,

f_p is the fraction of stars which develop planets,

N_e is the number of planets within a planetary systems which have the right conditions for life,

f_l is the fraction of planets with the right conditions for life which develop life,

f_i is the fraction of planets which develop life up to the level of intelligence,

f_c is the fraction of planets with intelligent life that build systems to radiate electromagnetic waves,

L is the length of time such civilizations persist in their radiation.

There are a number of assumptions made which permit the formula for N to be expressed this way. Let us discuss a few of them.

First, the Milky Way galaxy is chosen as the basis for measuring everything. We only know that life can originate on a spiral arm, far away from the bulge and the black holes which inhabit the center. It seems quite reasonable that life needs billions of years to evolve to the stage where a civilization emerges and starts emitting radiation, and in the bulge, distant stellar encounters happen much more frequently than in the spiral arms. A stellar encounter can create a gravitational pull on a planetary system to disturb it, and a planet which had conditions for life prior to the encounter may be moved either inward or outward relative to its star, where the conditions do not hold. Two solutions might be done for this, either change R_* to only count spiral arm stars or modify all the subsequent probabilities to take into account the different conditions between the spiral arms and the central bulge.

Second, the term detectable can be defined a number of ways. Since radiation dies off as the square of the distance travelled, without absorption, and worse with absorption, does detectable mean detectable with some particular equipment? Imagining a ten kilometer aperture radio dish out beyond Neptune's orbit, and compare that with the original SETI equipment. If one wants to be able to detect a civilization's emissions from the other side of the Milky Way, assuming the central bulge does not intervene, something huge would be required on both ends.

Third, the equation seems to be assuming roughly isotropic radiation, spreading equally in every direction, including the one direction that heads toward Earth. Why would any civilization do that? There might be some transitory period when they were broadcasting for their own planetary uses, but if they wanted to communicate from solar system to solar system, they would develop a narrow beam system that would require only a tiny fraction of the power of an isotropic radiator. But then detectable means that Earth is in the beam of such a system. That would be rather fortuitous.

Fourth, the fraction of stars which develop planets might be, as we now know, approximately one, but developing planets does not mean life can evolve on one of them, or certainly not to the threshold of EM emissions. Stars heavier than our star burn out quickly, and if one included them in the count, they could have planets and one could have the right conditions for life, but these conditions would soon change as the star evolved and died. On the other end of the scale, M dwarfs, the most populous kind of star, doesn't have enough energy output to have a planet with the conditions for life, except if it is close in, and there it would be likely phase-locked, with the same face always directed at the star. There are good objections to assuming life could evolve in such a system.

For the mid-range of stars, where our sun resides, there might be planets, and one or two with the conditions for life, but we know little about the migration of planets, even without the evolution of the parent star. Do smaller planets keep their orbits for billions of years in any planetary system, or does it take billions of years for them to gradually migrate inward or outward? If we change the definition to having a planet of the right size in the liquid water zone for billions of years, the number might drop from about 1.0 to 0.00001. Figuring out long-term stability of orbits should be a fairly simple task for the current state of mathematical astrophysics, but it does not seem to have been done in a comprehensive way that enables on to figure out this term in Drake's equation.

Fifth, exactly what does the “conditions for life” entail? If it is made very loose, the corresponding probability would be high, and the subsequent probability would be less to make up for the looseness. If it is made tight, the inverse happens. At the time the Drake equation was written, mankind did not know how life forms, nor what were the conditions needed for it. Now, sixty years later, the same situation exists. We don't know. It is appalling that so little work is done on the origination for life. One particular question is that, are there some conditions in which life forms over a period of time, something large compared to human lifetimes but small compared to solar lifetimes, like a million years? Or is the situation completely opposite, life only forms if some event happens, and the probability of the event might be very, very small.

Just suppose, as hypothesized in this blog, a mild collision with a large planetoid, which becomes a satellite, is necessary for life. The collision would have occurred in the early part of the solar system's existence. Earth-like planets which did not have that collision in their history might be similar in many conditions to ones which did, but, if the hypothesis is correct, only the latter could have life. There are certainly other events in the history of a planet which might affect the origination of life, such as the chemical composition of the crust, volcanic heating, and asteroidal bombardment.

To be generous, life originated three or four billion years ago, and we do not know the conditions of the Earth's surface, so we are limited in imagining how life could originate. The delay in life origination work might be caused by the delay in planet origination work. Neither is in a good state. There is no reason to think that current conditions on the Earth could lead to an origination of life, assuming all consequences of life were removed. One of the conditions discussed in this blog is the existence of organic oceans on the surface, able to nurture membrane formation as well as complex protein formation. Where might they come from? The mild collision hypothesis is a possibility for this.

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.

Ice Ages and Alien Civilizations

While it is true that the location of non-habitable planets doesn't matter much in a solar system where a possible planet for an alien civilization exists, there is a significant exception: Climate effects. When a planet co-exists in a solar system with other planets, they exchange angular momentum, which translates into a change in the semi-major axis of rotation. This means that in a stable, resonant planetary system, planets will slowly drift in and away from the star, not very far, but far enough to possibly trigger climate change. And climate change caused by this oscillation in average radius may be almost negligible, or it may be near a threshold where an instability in the climate system on the planet flips from one condition to another.

The obvious one is Ice Ages, but it is not the only one. It is, however, an easy example to discuss. Ice Ages happen because the absorbed solar energy is less because the average rotational radius from the star, averaged over a hundred thousand years or so, is a bit larger than it had been before the Ice Age started, plus there are important feedback effects. Energy absorption depends on the amount of energy emitted and its spectrum, the reflection and absorption in the atmosphere of the planet, and the fraction absorbed by the surface. 

Feedback effects come from the average albedo of the planet, which measures the fraction of energy reflected. When the planet gets a little colder, ice forms, which has a high reflective coefficient for visible light, where much of the energy in a mid-level star exists. A little ice means less absorption, and a colder planet, and more ice – starting the feedback loop. The final state of this loop is the peak of an Ice Age. This final point might be a saturation point, where almost all the available water is frozen, or where the convection of heat away from the warmer equator is insufficient to cool the equatorial regions down to freezing. There might also be a timing question, where the time necessary to freeze the planet is of the same order as the time for orbital change.

Consider a second example. The second feedback effect is much more difficult to detect eons later and has been given no name at all by Earth scientists, and may not be recognized as such. It is when the albedo of a planet is increased not by snow and ice, but by cloud cover. Clouds also reflect solar energy more than rock or other terrain, and once they begin to cool a planet, a “Cloud Age” might occur. Clouds occur at altitudes where there is sufficient moisture, but insufficient heat. High clouds do not lead to rain as low clouds do, but they do reflect well. Unfortunately for science, clouds leave little geological record.

Atmospheres with different compositions may have different types of clouds, and these may also have albedo effects. Atmospheric compositions on exo-planet has not been thoroughly examined, but it seems likely that some constituents might be present in an atmosphere and still have life supported.

Quite naturally, the opposite to an Ice Age might occur when the planet gets shunted in a bit closer to the star, all the ice having been long ago melted and all the high-altitude clouds dissipated. Life starts dying because of thermal effects, and when animal life is migratory but vegetation is not. If vegetation dies off over large areas, one area, a desert forms and less carbon dioxide is recycled back to oxygen. With more carbon dioxide in the atmosphere, heat is trapped near the surface, raising the surface temperature more, killing off more vegetation. Good numbers for this effect might well show it is much less powerful that the other two.

The effects that an Ice Age, and possibly a 'Cloud Age' and a 'Heat Age', to coin a term, would have on an emerging alien civilization are evident. These phenomena would lead to a large reduction in the surface area able to support vegetative life. If a civilization was still dependent on agriculture for its nutrition, or if there was a pre-civilization, perhaps at the pre-city stage, it could be doomed. For the Ice or Cloud Ages, if there was some small area near the equator still not completely blocked off, the numbers of survivors would be limited, and civilization would be stymied. Ice ages on Earth have lasted millions of years, and no civilization could be expected to survive this. Heat Ages would force population to the poles.

But the time scales are not commensurate. Civilizations can be estimated to last of the order of a million years, meaning that they could come into existence during the interregnum of a climate extreme survive, flourish, come to visit Earth if they wish and find it possible, and then disappear before the next extreme starts. These epochs are controlled by the interactions of planets, shuffling angular momentum between one another and slightly shifting their orbital radii. A question which might be answered in short order is: In which type of configuration of planets would there be periods this long between these catastrophic ages, and in which type of configurations, if any, would they be too short to permit a civilization to develop? Knowing how to translate the duration between climate extremes to planetary configurations would immediately tell us which configurations are worth examining in great detail on our hunt for alien life.

These types of calculations can be done now, but there is no clear understanding yet of what to look for. This means that large numbers of scenarios would have to be computed, until a pattern emerged, and the ensemble of possibilities could be winnowed down. Maybe the key variables involve the existence of two gas giants in some particular relationship, orbit-wise, and once this is established, other planetary resonances correspondingly exist and can hold planets. If one of these resonance bands is of the right thermal characteristics, and a potential host of other variables are within some ranges, we might be able to say that some particular planet, out of the millions which will be detected over the next century or two, are 'life-possibles', and deserve a great deal of specialized attention. 

Rain and Life

When we are searching exoplanets for life, one of the premier signals is supposed to be the presence of oxygen. Oxygen typically is chemically combined unless it is renewed, and vegetative life provides this renewal. As all schoolchildren know, photosynthesis involves chlorophyll acting as a catalyst to break carbon dioxide into oxygen, which is released, and carbon which is utilized. But photosynthesis can be done in the oceans by near-surface plants, and while life in oceans would be tremendously interesting, life on land would be even more tremendously interesting. It is very hard to see how intelligent civilizations could evolve underwater, but on land, there is the possibility.

Thus, oxygen might be a great signature for life, vegetative specifically, but not so certain an indicator for alien civilizations. What might be used in addition?

It is certainly worth asking the question. Just consider that a hundred years from now we find ourselves in a galaxy with thousands of planets with life, but all of it wet. Now consider instead of that situation, we are in a galaxy with even hundreds of planets with alien civilizations. Orders of magnitude more interesting.

So, what is beyond oxygen as a signature of life on land? Trivially, there must be dry land, and that would have some reflective signature. Rock doesn't look like water when reflecting sunlight. But just having rock doesn't imply that there is anything living on it. There needs to be some preconditions before life can crawl out of the oceans and take up habitation on dry land. One is rain.

Life needs water. It doesn't need to be immersed in it, but it needs to have it to drink. Water evaporates, and water on land needs to be renewed. That means rain. Water evaporates from the oceans, drifts over the land, condenses into droplets and falls to the ground. There aren't too many other possible mechanisms. One could consider tidal flooding, which might produce wet areas, but if the ocean has dissolved some minerals from the rock, like salt, it might not be drinkable. If there was a lot of volcanic action, possibly someone could come up with a process that, on some suitable planet, might pump water, distill it in the volcanic heat, let it condense elsewhere, and expel it into a river. This complicated a mechanism doesn't seem likely, at least during the later life of the planet. So it is rain that is the mandatory precondition for an alien civilization and for animal life on land as well.

Detecting water vapor in the atmosphere might be the first surrogate for detecting rain. On Earth, water vapor is at a much lower concentration than oxygen, and therefore more difficult to detect; but it is not impossible to foresee that that would be a further step in astronomical capability, once oxygen was detectable. Carbon dioxide might be detectable first, or some other compound or element, such as argon, but eventually water vapor would succumb to astrophysical technology. These gases would likely first be detected for a transiting planet, where the light of the parent star shines through the atmosphere and gets spectrally absorbed. Later they might be detected from reflected light.

Rain would have to be either a local phenomenon, evaporating over most of the ocean area, and then precipitating on some region, or else a seasonal phenomenon, evaporating during one season of the year, and precipitating during another. Wet ground has little difference in albedo than dry ground, however, so even if telescopes grew sufficiently in aperture to see different parts of the exoplanet, seeing wet areas would be quite difficult. But if there was sufficient temperature range on the planet, and there was snow, then a significant albedo change might be detected. This would be even easier in the seasonal case, where evaporation occurred all summer and then all winter, snow happened. A planet with an elliptic orbit might produce this situation.

Another option is clouds. Rain and clouds are not the same thing, but there must be clouds to produce rain. Searching for clouds might be considerably easier that searching for rain. Clouds do change the global albedo, and monitoring for these changes would be an indicator of cloudiness, and by implication, rain.

Another variable which affects the detectability of atmospheric gases is the thickness of the atmosphere. Earth has a very thin atmosphere, about 10 km thick compared to 6000 km radius of the planet. This is to be compared to Venus, with approximately the same radius, but an atmospheric thickness of 250 km. Probably the components of Venus' atmosphere would be much easier to detect on a transiting planet. Clearly one must compare the loss of signal due to the longer transmission path with the larger cross-section of the atmosphere and the lengthened time for absorption. For a reflective signal, the loss of albedo may make the comparison go the opposite way, with thicker atmospheres being more difficult to break down into components.

One interesting question is, if there was an exoplanet with an atmosphere as thick as Venus' atmosphere but of the same composition as Earth's, would rain be possible? One might also make an assumption that the rotation rate was identical to Earth's as well. Similarly the average temperature would be assumed to be the same as Earth's. The vapor pressure of water is the same no matter where it is, so the amount of water in the atmosphere would be the same as Earth's, maybe a half percent on Earth on the average, with more of it at lower altitudes. On the exo-planet it would be a half a hundredth of a percent. Rain forms when the temperature of the atmosphere drops sufficiently that liquid water can form. Would the thermal inertia of a large atmosphere, with 100 times the atmospheric mass of Earth, prevent this temperature drop? Temperature drop comes from heating or cooling of the atmosphere, and a portion of the lower atmosphere gets a certain amount of heating from the solar energy passing through it, which is largely identical and a certain amount from the reflected heat from the planet. On the exoplanet, much more solar energy would be absorbed by the atmosphere, leaving the surface much darker and receiving less energy. Solar energy incident on the atmosphere would be largely identical for different longitudes, meaning much less opportunity for the temperature change that is required for rain. This possibly means that finding an exoplanet with a thick atmosphere would imply no rain, and no land lifeforms, and no alien civilization.

One hypothesis about the reason for the thinness of Earth's atmosphere is that atmospheric mass is almost unaffected by the aging of the planet, and once thin, it stays thin. If the formation of the Earth was mediated by the impact of a protoplanet, which led to the formation of our large moon, and the legacy atmospheric hypothesis is true, looking for a large moon might be the fastest way to find land life and the possibility of a civilization.

Hunting for Life in the Milky Way

In earlier posts, there was some discussion of what would be the goals of an advanced alien civilization, assuming they had come together to choose one and then to work on it. In a different blog, some more thinking on this matter indicated that the most reasonable and likely goal of the civilization is that of life itself, which is best broken down into five separate goals, survival, reproduction, adaptation, evolution and dispersion. These goals imply many choices that the alien civilization would make, in order to further their alignment with these life goals. The last goal is dispersion, and that means first expansion all over the planet, then the solar system, and then outwards into the galaxy. Just think for a minute what this implies: all or most advanced alien civilizations are going to attempt colonization and seeding, both of which support the dispersion of life.

Neither of these tasks are easy, as has been noted in all the posts in this blog on these topics. Colonization means setting up some replica of the alien civilization on an exoplanet ,while seeding means starting out life with the alien version of DNA on some planet which doesn’t have the prerequisites to become an origin planet on its own, but can support life, being in the habitable zone plus all the other conditions. Current theory here on Earth indicate the cells that did that were cyanobacteria.

Looking for seeded planets would likely be the same as looking for origin planets. Seeding might put photosynthetic organisms into a planetary ocean, and then, after a few hundreds of millions of years, an oxygen atmosphere might exist, which is a tremendous benefit for evolution, allowing life to expand beyond chemotrophs and cyanobacteria cells to a food chain. The oxygen in a seeded planet’s atmosphere would look the same as in an origin planet’s atmosphere.

So, assuming most of the alien civilizations do both seeding of potential life-supporting planets and colonization of others, which are not able to support life, but which provide the resources necessary for the alien civilization to sustain itself for a long time. Which ones should be looked for?

Seeding a planet gets over the hump of life origination, which might be tremendously difficult, rare and improbable, at least according to one theory, mine. Is evolution fairly certain after that, or are there more highly improbable-to-overcome barriers along the way to intelligence? Suppose there aren’t. Suppose evolution is as easy as rolling down a hill. However, it takes a long time. Earth is our only example here, and life took about two and a half billion years after the atmosphere changed to partially oxygen to evolve to intelligence. This means that any alien civilization which evolved in the last two to three billion years has not had enough time yet for their first example of seeding to have led to a new alien civilization, evolved from cyanobacteria to tool-using creatures of one form or another.

This raises the obvious question of when, in the history of the galaxy, was life likely to originate? There are stars around which are ten or so billion years old. Could these have had planets soon after they formed, and perhaps one which met the prerequisites for life to originate? Unfortunately, current astronomical tools do not allow us to figure out much of the history of the galaxy. It likely started out as a gas blob, and condensed irregularly, with stars forming all over it, but more in the denser region in the center. Gas was more dense that now, as by now much has been consumed in star formation, and that implies that stars which formed early would be larger. Large stars live short lives, and end in a supernova explosion. There would not have been the neat division of the galaxy into the disk and the central bulge, so star motion would have been more random and Boltzmann-like. Neither of these two things bode well for planets. Supernovas going off near a planetary system sterilize it, but may also disturb planetary orbits, causing them to be ejected or rarely crash into the star. The passage of a nearby star does the same thing, pulling planets out of their orbit, leading to a planet-planet interaction where the smaller ones get ejected. So, while little definitive is known, it would seem likely that planetary formation in a system which perseveres long enough to originate and evolve life is more likely in the later stages of the galaxy and out in the disk. Finding planets to seed would also be more likely in these conditions.

Putting this together means that seeded planets might be around, but life on them is too young to have evolved into an advanced civilization. There really is a double time here. For life to originate and produce an alien civilization, capable of star travel, might take four billion years since the planet formed into a habitable world, with the right temperatures and everything else needed for life. Then if that civilization seeds another planet, we have another three billion years or so to wait. That is seven total, and seven billion years ago, there might have been so much turmoil in the galaxy that life couldn’t originate and evolve. So, planets which have been seeded might be common, and even many which have had the few hundred million years to produce an oxygen atmosphere. But if we are hunting for alien civilizations, seeded planets are not worth the effort. That leaves origin planets and colonized planets.

The previous post, on frozen worlds, indicates that colonized worlds, if the aliens choose worlds which are the easiest to colonize and which will sustain them for a long time, might look absolutely different from origin worlds. It also indicated, because of the very different time scales involved, that there could be very many of them all over the Milky Way, or at least out in the disk. The idea was simple: if there are sufficient resources on the planet, fusible and fissionable elements, plus all the other minerals necessary to supply the civilization with its raw materials, buried in the ground, they can simply build their civilization under the surface, on a frozen world and maybe some not so frozen. The ratio between colonized worlds and origin worlds might be a thousand to one. There would also be much larger numbers of previously colonized worlds where the alien civilization has used up the minerals and life on the planet was no longer sustainable for them.

How do you detect a mine shaft and a starship landing zone? Maybe there would have to be some surface transportation, if they needed to have mines in multiple locations. It might be possible, with a kilometer sized telescope, to see large oceans on an exoplanet in our vicinity, but even one ten times larger than that could not detect something as small as tens of meters or even a kilometer in size. Looking for a tiny heat source is conceivable, but unlikely as the resolution at deep infrared wavelengths is so much less than in the visible. If the concepts trotted out here and in the last post are viable, it means that alien civilizations are not detectable, and they would have no interest in coming to Earth, either for seeding as we are way past that, or colonizing as there are too many potential difficulties. It wouldn’t align with their goal of dispersing life at all to visit Earth. So the only thing we have any hope of doing is detecting an origin world, but if there is only a few of them,they might be on the other side of the Milky Way or in a different spiral arm. Perhaps a double hope of there being easy ways to originate life and our detecting oxygen in exo-planet atmospheres is the only possible salvation for the quest to find aliens.