Thursday, August 20, 2020

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.

Sunday, August 16, 2020

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.

Friday, August 14, 2020

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.