Wednesday, August 31, 2016

Life Origination on Super-Earths

Super-Earths, meaning planets with somewhat more mass than Earth but not enough to be a gas giant, have been found frequently in the exo-planet searches that Earth’s astronomers are doing. Planets as small as Earth are hard to see, but this may change soon. There seems to be a great interest in finding life on exo-planets, or even on planets or satellites in our solar system, so asking the question about life on super-earths might be interesting.

When you ask, why do we care about life on other planets, various answers come back. One is that this will help us understand the origin of life. Unfortunately, if this was really a motivation, we would be spending lots of money in Earth laboratories trying to test hypotheses about the origin of life, but we are not. The astronomy budget is huge compared to the laboratory origination of life budget. So the real reason must be different.

Another reason is that we want to meet other intelligent life-forms. That doesn’t make too much sense either, as we are not looking for other civilizations so much as for life itself on some planet. Why do we care if there is non-intelligent life on some satellite or exo-planet? It seems to be similar to the reason people like to travel to other places on Earth and see what’s there. An instinct for learning about the universe, or a desire to find new places to visit that might be more interesting that a sphere of rock could be part of the answer. It is hard to imagine building a starship to go visit some amoebas a hundred light years out, however. My guess is that it is solely the entertainment value.

Be that as it may, let’s discuss finding life on a super-earth. As the gravity goes up, a few important things happen. One, we go to a different point on the equation of state curve for water. The melting point changes little, but the boiling point at 10 times the atmospheric pressure of earth is almost another hundred degrees C higher. This means the liquid water zone (LWZ) for a super-earth is wider than for an earth.

Using the hypothetical origin of life theory espoused in this blog, the organic oceans concept, there would have to be organic oceans for a period in the history of the planet to form the first cells. There are some organic compounds which are fragile in higher pressures, but most are not. Similarly, higher temperatures, made possible by a wider LWZ, decompose some, but not too many. Thus, the organic ocean zone, where an organic ocean could exist for long periods without evaporating or decomposing, might also be wider for a super-earth.

The energy generated by gravitational collapse stays with the planet, and keeps the rocky core hot. On Earth, this is not sufficient to heat up the surface and boil off the oceans, but there is a hypothesis that on larger planets, such as the cores of gas giants, it does. A super-earth is somewhere on the continuum of collapse-generated heating between the earth and the gas giants, so it may have enough to keep the two oceans from forming. The structure of Jupiter is still largely unknown, because its composition is only guessed to be mostly hydrogen, and the equation of state of hydrogen at extremely high pressures is not certain as well. The current probe circling Jupiter, Juno, was sent in part to shed some light on Jupiter’s core. Estimates of the top of the liquid layer on Jupiter are around ten thousand degrees C, which indicate that even a super-earth might have a lot of thermal flux coming our of its center.

This implies that super-earths with much more mass than earth may be too hot for an ocean of either kind. This in turn spells no life.

Perhaps the biggest hurdle for smaller super-earths is the formation of massive amounts of organics. They do not inhabit the gas cloud that forms a solar system in sufficient abundance to make oceans on a super-earth. Lightning does form them, and if the super-earth was like a gas giant planet, with great amounts of lightning, there might be enough if they simply accumulated for long periods. Observations to date do not bode well for this. Currently, Jupiter has about as much lightning as Earth, so the amount of organic production would be insufficient on a super-earth, assuming it also has about as much lightning as earth does, and that amount was the same billions of years ago.

Impacts of planetesimals also create the heat and shock waves that make organics out of the atmosphere’s CO2 and H2O, but they wouldn’t have made enough for earth, and likely not enough for a super-earth. If the distant solar system had a much larger number of planetesimals, this could change, or if for some reason the super-earth was located closer to the belt or belts where planetesimals form, there could be more. Perhaps a factor of ten seems reasonable, and then there is the larger size of the super-earth to add some tens of percent more. Still fairly sparse.

The organic ocean hypothesis for the origin of life on earth involves a monumental impact from a large planetesimal, planet-sized, coming from a Lagrangian point on the same orbit. This tears up the crust, rips off much of the atmosphere, and begins a long period of volcanism while the newly mixed core material goes through a resettling and re-separation of materials, reformation of the crust and emergence of any crustal material driven deep into the core by the impact. Thus, if something of this magnitude happened to the proto-super-earth, organics would be present in abundance. Thus, if we want to find a super-earth with life, we need an impact similar to one that an earth-sized exo-planet would need.

This requirement eliminates another obstacle: too much atmosphere for photosynthesis. If the shock of impact depletes the atmosphere down to something where visible photons can eventually penetrate down to the liquid ocean, evolution can produce something like chlorophyll, which means the atmosphere can become oxygenated. This doesn’t mean that intelligent life follows, as a super-earth might maintain enough liquid water to completely cover the surface. There is a chance that intelligent life might evolve underwater, but it would be limited and civilization such as we define it would not emerge. Thus, no star travel from a super-earth.

Tuesday, August 30, 2016

Interstellar Hybrids

Suppose it just happened that there were two solar systems, within a hundred or a few hundred light years of one another, and on each of these there was the origination of life, and on each of them life evolved to photon-users, then to land creatures, then to intelligent tool-using creatures, and finally to the founders of an alien civilization. Suppose even further, now a quite unlikely coincidence, that these civilizations existed at the same time. And now, suppose even further that one of these civilizations decided to do some space travel and left their planet and headed for the other one.

This is also unlikely, as the principal reason that an advanced alien civilization would do space exploring would be to find a new home for their civilization, and if there was already an alien civilization on a nice planet nearby, they would not go there. They would not be interested in sharing what was left of the resources of that solar system, nor in going to long-distance war to evict the natives from it. But in the interests of covering all possibilities, they go. Perhaps they have such a large quantity of resources in their solar system that they don’t foresee scarcity, and they want to go meet the other species.

They recognize this is a thousand-year voyage, and so they expend the time needed to expand on their interplanetary mining ships to make a starship, one capable of making it to the other star system with all its parts functioning. They also figure out something we do not know, besides how to build starships, how to make sure there are living aliens on the ship when it arrives. They do hibernation of a select crew, or they make an AI capable of growing aliens from preserved DNA code, or, heavens forbid, an intergenerational ship. The details don’t matter – they just get there and the receiving civilization doesn’t blast them out of orbit into small bits of space debris, but they welcome them.

The distant aliens come down to the planet and meet the natives, and what do they find? They look alike. This is interstellar convergence at its finest. Interstellar convergence means that if evolution has enough time and area to work over, it will produce the optimal choices for all evolutionary steps, and it is no small leap to figure out that for intelligent, tool-using creatures, there is a kind of optimum. If the target planet were different from the departure planet, say, one was all water and the other all desert, these considerations are absurd and some creatures suited to the environment would have evolved there. But we’re assuming the two planets are pretty much alike. Same mass, same star type, same age, same distance from the star, same orbital parameters, same metallicity, same condensation of stabilizing outer gas giants, same moon and same history of impact, and so on and so forth. It should be obvious now how unlikely this would all be, but it is a thought experiment. You are allowed to do this in a thought experiment.

Now let’s go all Hollywood and assume that one of the arriving aliens, named H, takes a liking to one of the native aliens, named S. And H and S finally, after all kinds of social obstacles and whatnot, pair up and produce a hybrid. NOT.

Let’s get down to the basics. We know lions and tigers can produce hybrids. They are impressive huge creatures, but sterile. Lions and tigers both are members of the cat family, and haven’t had enough time for their genetic code to diverge much. So they can do this. Instead of having two animals that are very related in ancestry, the two aliens are completely different somethings. It isn’t species, it isn’t genus, it isn’t order, it isn’t phylum, it isn’t kingdom, it’s something bigger taxonomists haven’t thought about naming yet. A completely different world of life. Let’s call it bios, for the greek word for life. The two planets have two different bioses.

Let’s push interstellar convergence even farther, and look at the details. Do the two bioses each use DNA? OK, let’s assume that DNA has some combination of fortunate ability to attach to phosphate and carbon bond combinations that don’t take too much torsion to rotate for folding and unfolding, and so on, and nothing else beats it or even comes very close. The two DNA’s are true to the name and use desoxyribose sugar on the power end of their nucleotides Let’s also assume the same mechanism for reproduction of DNA, using mRNA and tRNA, works in both places, because it’s the most efficient method of gathering together amino acids and cobbling them together into a DNA strand or into a protein. Let’s assume all the internal organs are the same. Wow.

Now comes two hurdles. Mutations are random, and are selected by fitness competitions. What about chromosomes? They form randomly. Is there any reason to think that the particular combination of genes on the chromosomes one alien has wound up on the same chromosomes that the other one has? There doesn’t seem to be any fitness for most of these accidental cuttings and recombinings. They are simply genetic errors that slipped through the proof-reading process. So the genes on the incoming aliens are very likely to have the same genes in different places. This means no hybrids. Is there conceivably any way that random genetic errors could wind up identical on two planets? Not likely.

That was the easy one. Look at the coding of DNA. Triplets of selections from four bases form each codon and these are related to one of the structural amino acids used in the bios. Let’s assume that interstellar convergence picks out the same twenty or so structural amino acids in exactly the same way for both bioses and the ones used for the bases are identical also. Maybe there are slight differences in the utility of amino acids that allow the entire set of them to be identical, and interstellar convergence actually scored big again here. Hard to believe, but wait.

There is a map of these triplets, of which there are sixty four, into twenty-odd amino acids. Again, this is randomness. There is nothing in the triplet that relates to a particular amino acid. When evolution was picking the mapping out, whatever started first, stuck. How likely is it that the codings would be identical?

The coding here on Earth is somewhat blotchy. More than one of the triplets can code for a particular amino acid, and there are stop and start triplets, too. Some triplets code for nothing at all. Let’s be conservative and just figure the number of combinations of triplets that would match on a one-for-one basis with twenty amino acids and two auxiliary codes, twenty-two in all. It is obviously 64!/(64-22)!, or about 10 to the thirty eight.

So, there is a one in ten to the thirty-eight chance of this happening, and if it is necessary to match up some of the otherwise unused combinations, less. There would simply be no hybrids. The mismatch of coding would lead to failure of any hybrid cell. Even an artificially fertilized cell would die quickly. This means that any science fiction needs to not hypothesize hybrids, no matter how interesting it would make the plot, if it is to be at all realistic. Most science fiction does not have any hope or interest in being realistic, of course, but perhaps as science eduction pervades humanity, this will change.

Sunday, August 21, 2016

Sol-like Solar Systems

Before we on Earth started discovering exo-planets, the idea was common that our solar system was typical, and other solar systems, once we discovered them, would look like ours. Specifically, all we knew about at that time were the eight planets and Pluto, plus some asteroids and comets. What we expected to see was a spread of planets similar to ours, with a couple of big gas giants out at six to ten AU, and some little ones inside their orbit and some medium sized ones outside it.

Then there started to be exo-planets discovered and, surprisingly, they were mostly of large planets orbiting inside the orbit of Mercury, maybe at only 0.1 or 0.2 AU. The assumption was turned on its head, and there was some talk about our solar system being the unique or unusual one. Is it? Suppose there were just hordes of sol-like solar systems out in the galaxy, even in the corner we inhabit. Are they visible or invisible?

Everyone knows that the wobble technique of finding exo-planets, where a star's velocity is spectroscopically measured to great accuracy, and if the variation corresponds to what the gravity of a planet would do, a candidate is declared. Wobble works best with larger planets and smaller stars, so the relative motion of the star is enhanced. It also works best with planets in-close to the star, as the frequency of motion is greater and easier to detect, and it is larger as well. If you look at the list of exo-planets which have been detected by wobble, there is no Jupiter-sized or less planets detected at Jupiter’s orbit radius, about 6 AU. There are some bigger ones out there, but no Jupiters. So Jupiter is invisible to current wobble capability. There are no planets of Earth mass or less at Earth’s radius or more. In other words, wobble can’t detect Earth or Jupiter. Too bad.

The other prominent technique is transit. There the total output power of the star is monitored, and when it diminished by a small but constant amount, from a planet passing between the star and the telescope, a possible candidate is found. The smaller the planet, the harder it is to see the signal in the noise. Also, the further out the planet is, the smaller the diminishment will be. Add to this the difficulty in finding single occurrences, as opposed to multiple passes. And on top of this, there is the third dimension. A planet which is in a larger orbit has a lesser chance of happening to be between the observed star and the telescope observing it, because the effect of orbital inclination is larger for larger orbital radii. Passage time is also shorter. Everything works against seeing planets at Jupiter’s radius and small ones at Earth’s radius. And yes, in the list of exo-planets discovered by transit, there is no Earth-size or smaller at Earth radius or larger, and there is no Jupiter-size or smaller at Jupiter radius or larger. Which means that Earth and Jupiter are invisible to this method at the current state of technology as well.

Another technique which is gaining prominence is direct observation. The planet’s own light, typically the infrared portion of it, is detected. If the orbital radius is huge, the problem of blocking by the light of the parent star isn’t so bad, but if it is not, but small like Earth’s or moderate like Jupiter’s, the light of the star must be obscured, unless it is a faint star. The first time this was accomplished was for planets around a white dwarf, which is not bright at all. So, for stars like our own, nothing the brightness of Jupiter or less at Jupiter’s radius or smaller, and nothing the brightness of Earth or less at Earth’s radius or smaller, have been detected. In other words, even with direct imaging, Earth and Jupiter are invisible.

There are no other techniques that have been perfected yet for finding Earth-like planets around Sol-like stars, or Jupiter-like planets around the same. If Jupiter is not detectable, neither would be Saturn, and Uranus and Neptune may also have the same problem. Our solar system is invisible to current technology. Perhaps for close-in, neighboring stars, there might be an exception, or perhaps with longer data collection times, this barrier might be crossed. Maybe the next generation of technology is just about to crack through these limits and see solar systems like ours. But until that happens, and until it becomes popular enough to detect our type of solar system in numbers corresponding to other types, our type of solar system is virtually invisible and the hastily-concocted comments about our solar system being the exception are simply untenable.

If you told me there were no ants on your picnic table, but I knew you left your glasses home and couldn’t see some no matter how hard you tried, I would wonder about your credibility in general. However, that doesn’t apply to astronomers who have just accomplished some absolutely astonishing technological developments, both in instruments and in data processing, and have started to see exo-planets after centuries of wondering about them. The credibility of the astronomy community has never been higher, and coverage of astronomical discoveries is much greater now that it has been in the distant past. Yes, the existence of science fiction since the time of HG Wells has certainly helped, but the drama of discovery is in itself something that interests large numbers of Earth residents. Yes, having astronauts and cosmonauts and taikonauts, plus a space station, plus for really old people the visits to the moon, add to the excitement that now envelops astronomy. All together, there is a great demand for news about new Earths and so on. Perhaps this excuses the failure of science commentators to mention that these unusual solar systems are the detectable ones, and ones like ours may be by far the most common, but they are invisible. As far as it goes for figuring out the prevalence of alien civilizations and their propensity for visiting us sometime soon, it is misleading. If there are special conditions that obtained here on Earth for originating life, it is simply not possible to extrapolate out to other solar systems in general until this gap in our observational technology is rectified. Kepler has seen a couple of thousand stars with planets out of the hundred thousand or so they observed, and these two thousand are pretty distinct from Sol-like. But there could be twenty thousand Sol-like systems, and they are just not yet seen. So, another barrier to figuring out if extraterrestrials are nearby exists, waiting to be ruptured and surpassed.

Wednesday, August 17, 2016

Counting Alien Civilizations

If there are about 100 billion stars in the galaxy, with 10% or 10 billion in the disk, and 10% of those of the right spectral type, or 1 billion, and 1% of those have a rocky planet in the Liquid Water Zone, with the right gravity, composition, magnetic field, axial tilt, eccentricity, and rotation, that leaves about 10 million stars. If 0.1% of them have a moon that came from a Lagrangian point and made a glancing strike on the proto-planet, resulting in masses of organics necessary for life to originate, that is 10 thousand stars, and if 10% of them evolve intelligent creatures, that is 1 thousand alien civilizations.

The disk area of the galaxy is about 100 thousand light years around, 10 thousand light years wide and a thousand light years deep, meaning it has a volume of a trillion cubic light years. Each alien civilization would be in a volume about a billion cubic light years, which is a cube a thousand light years on a side. So, if these calculations are the right order of magnitude, alien civilizations should be about a thousand light years apart.

Let’s throw a little time into the mix. The galaxy is about ten billion years old, so that means about 10 stars are born every year. If an alien civilization is bent on preserving itself, and it recycles to the extreme, uses resources wisely, and does everything else right, it might last a million years on its home planet before resource exhaustion forces it to find a new solar system. According to the calculation above, there is about a one in ten million chance that a new solar will grow life and have an easy to colonize planet. That means in the million years that the alien civilization survives on its original solar system, or a colony world, there is about one planet that arises somewhere in the disk that is suitable for them to move to. It could be up to a hundred thousand light years away. Finding it is going to be hard and getting there also hard. They would have to live in their ships for a million years, as long as they inhabited the planet they used to call home. Obviously, only a small number would make the trip, or it would be fully automated and ready to do the colonization work itself when it arrived.

These simple calculations indicate that alien civilizations are likely to simply die in place. A lucky one might find itself near a colonizable planet, but few would get lucky twice. On the other hand, there are plenty of planets and satellites with mineral resources, so a type of nomad existence, where some aliens travel from solar system to solar system harvesting resources so they can continue their voyage, is possible. Possible is hardly a mandate, because life aboard such a ship might not be sufficiently appealing to make the choice of building one popular. The alien civilization might simply decide that there is no good reason to build such a ship, and create modified aliens to live on it. Easier just to calmly go extinct.

Another way of looking at the numbers is to say that every million years a new alien civilization arises, on the average. They last a million years and disappear. This means that at any time in the galaxy, there is only about one alien civilization. Maybe there are two or none, but not a hundred. This means SETI has no change of finding anyone. It means the valiant astronomers who search for Earth-like planets are going to find some, only none with life. It means that no alien ship is going to appear over a capital city on Earth and demand to speak to our leader. It means that no aliens are going to try and take over our planet.

It means the Milky Way is a lonely, lonely place.

The weakness in this calculation is in that part about the origination of life. The theory of life origination espoused in this blog was used, and it requires some unusual events to occur. If some other life origination theory were used, which had a ten times greater probability of origination of life, all the resulting numbers would go up by ten. This means that there might be ten alien civilizations running around the galaxy at any one time, going from solar system to solar system every million years, and continuing to exist for long periods, except for one thing. The distance from a home solar system to the nearest colonizable one only changes by about two, meaning it would be fifty thousand light years away. This is almost as bad. If this serves as an insurmountable barrier, it means that at any time in the galaxy, there might be about ten alien civilizations, all living and then going extinct. Still SETI is not going to work, astronomers are not going to find Earth-like worlds with life, and no one is going to come by to visit us.

Even if the probability of life origination goes up by a hundred from the original numbers, the distance only drops to about twenty thousand light years. If a ship gets up to 0.1 c, this is a two hundred thousand year voyage, and if the ship can only get up to 0.01 c, it is a two million year voyage. There is no time for an alien civilization to send out a probe ship to a prospective planet, even if one could be found.

Despite the huge number of stars in our galaxy, there are too many obstacles here to have life popping up in a nearby star. Nothing so far has spoken about the length of time that life itself would last on a planet. Life is naturally recyclable, and continues to evolve to match conditions, so only a peril of some nature would extinguish it. One of course is the growth of the star to a size that either takes the planet out of the LWZ, or does something worse to it. These factors reduce an already small number.

It does seem sad that we aren’t likely to find any new friends.

Sunday, August 14, 2016

Existential Nihilism in Alien Civilizations

Wouldn’t it be humorous if philosophy were the reason why aliens aren’t buzzing around our planet and waving to us? Philosophy affects almost nothing on Earth, in an explicit way. No one goes around thinking about ‘what would Kant think about this?’ or ‘how does dialecticism affect my decision to buy a new car?’ It may be affecting us on a deeper and more subtle level, but we don’t seem to recognize it as doing so. Aliens on the verge of traveling from star to star may be more intelligent than we are, with a better grasp of technology and every aspect of science, but it still seems like a stretch to hypothesize that philosophy affects the biggest decision their civilization makes.

Existential nihilism, in case you have forgotten, is the branch of philosophy that expounds that life has no meaning. That is the essence of it, and different philosophers, mostly in the nineteenth and twentieth centuries in Europe, twisted and turned that idea so that it seemed to have other implications, such as freedom being the highest goal, or there being no morality, or destruction is mandatory, or some such. These are reflection of their own inspiration, but all existential nihilism is remains very simple to express and simple to understand.

Alien civilizations, perhaps many, many millennia ago, went through the same development process for philosophy that we did, as science is science, no matter who’s doing it. But what we know about existential nihilism is largely the product of a lack of scientific knowledge, rather than science finally getting into philosophy and solving the problems there. In the nineteenth and twentieth centuries, we knew very little about how the human brain operates. Now, more becomes clear, and this information is useful in trying to anticipate how philosophy might ground alien civilizations.

Since the universe provides us with no goals, in other words, does not give any meaning to our lives, we have to choose a meaning, or a set of goals, for ourselves. We do this using the information we have, processing that information as a human being with a large neural network inside our skulls. The information we have is what we received during our training, including all of it, but mostly from the training received in pre-rational childhood. The neural net works by making associations, in countless layers, and possibly even via some feedback loops. So, the meaning of life, as we make it ourselves, is just what bubbles out of the stew of our training. Nothing more, nothing less. In an alien civilization, where they know exactly how to train young aliens to have meaningful lives, the aliens have meaningful lives. Science did it for them, and they did science.

But the training a young alien receives will certainly involve the ability to question assumptions, to think critically, to evaluate assumptions, and all the other rational tricks that we know and they will know. So, while the aliens, upon reaching adulthood, will know how to behave and what choices to make, without having to think too deeply upon them, as their training will have been done excellently. That does not mean they will not question these choices and behavior norms. Assuming they are all intelligent, they all will ask these questions, and elaborate on them. Intelligent people enjoy this sort of thing here, and supposedly intelligent aliens would also enjoy it. That is part of the process of training of course, in that whatever is seriously trained into an alien brain goes in either by association with a positive emotion, or with a negative one, like fear. Presumably some technique like critical thinking will go in by association with other happiness, and so doing critical thinking will be something they enjoy doing. A biological neural network can’t be programmed otherwise.

So the question on the mind of every young adult alien, in every generation, is, should we expend resources on preserving our species, meaning on star travel, and all the associated nuisances of moving an entire civilization to another star system with a really nice planet? Or should we just taper down our numbers and quietly go extinct when we run out of resources? This latter situation might entail returning to earlier levels of civilization, ones where the energy inputs can be satisfied by biological collection of photons from their star. Or that might be too dreadful, and they just decide to turn off their gestation machines and stop making new aliens. Either way, the alien civilization is not going to come to Earth.

So, when existential nihilism hits each alien smack dab in the face, and says, life has no meaning and no goal, and you have been programmed in an excellent way so that you know how to behave and how to interact within our civilization, but that programming, i.e., training, is just something that continues because of some legacy. It became traditional at the dawn of the alien civilization to teach this to everyone while very young. So, that means that it doesn’t have to be followed, as it is simply an arbitrary choice made on the basis of some long-deceased ancestors’ motivations, which were also random and arbitrary. If all the alien young people ask this same question and figure out the same answer, that continuing their civilization is a totally arbitrary act, then they might just decide to spend their resources differently, and perhaps enjoy their lives a bit more.

Perhaps philosophy can affect a civilization. Obviously, it presupposes that the aliens are intelligent and learn to think deeply and confidently. If the alien civilization has fallen prey to Malthusian idiocracy, then there is virtually no one to figure this out and to raise these questions, and no one who understands enough about how the brain works to put together the big picture needed to appreciate existential nihilism. If the alien society bifurcates, into the smarties and the remainder, the smarties will suffer from the twist of philosophy that renders their training questionable and rejectable, intellectually if not emotionally. These smarties are the ones making the decision on whether to go into interstellar space, provided the resources exist to do it, so we can say that any alien civilization that can go into space will have to come to grips with philosophy, and figure out a way to get through it. Or not. Perhaps the simplest solution, deciding to enjoy life rather than struggle with a new planet, would be completely justifiable based on existential nihilism.

Friday, August 12, 2016

Are Black Holes Good Neighbors?

Let’s talk about normal-sized black holes, with mass a few times that of the sun. We don’t have any of these close by that we know about. So, their existence has to remain slightly tentative. They are fairly simple to imagine. Suppose you have a neutron star, which is simply enough matter to overcome the Coulomb repulsion of nuclei and push them together, making solid neutronic matter. Light can escape from a neutron star, if it is small enough. But as you add more and more neutrons to it, the gravitational attraction at the surface becomes greater and greater, until it holds in light. Then it is a just barely legitimate black hole.

Exactly just how many neutrons this takes is known pretty well, as we know the mass and size of neutrons, and we can assume they are only slightly compressible. A few solar masses is about the right number. You can add more neutrons, but it doesn’t become more invisible. It’s already mostly invisible. If there is anything near it, and falling into it, this stuff might emit some radiation. But if the black hole has been around for a while, and has gobbled up most of the gas that was near, it would be just black, but not in the sense of a blackbody. Blackbodies have nice radiation spectra, but a black hole has none.

So how would you know if there was one nearby, say 40 light years away? You only get neutron stars, which is where we started, by a supernova collapse of a burnt out star, and the supernova would have gotten rid of all the planets and asteroids and anything else in orbit around the star. So there wouldn’t even be any planets to detect. Of course, a rogue planet might get caught by the black hole, but that’s a unique situation and not too likely. Black holes are remarkably small, of the size of Earth, so they are not going to be occluding stars and showing up like a thick globule of gas would. Since they are not near any other star, the fraction of light diminished by a transit would be too small to see.

If the black hole had a binary companion, the orbit of the companion would give away the presence of the black hole, but we have that supernova problem here, meaning the binary companion couldn’t be too close. So, the black hole is simply not detectable except in a few special circumstances. This means it is not clear why there would be any effect at all, such as on the decisions of an alien civilization to perform star travel. They would have no interest in going to one of them, unless there is some way to get energy out of it that we don’t see at all, but there is also no likelihood that it would disturb their space travel, unless they were so unlucky as to try to fly right by it. Getting close to it would probably reveal some tiny amount of mass still falling in and emitting radiation as it did, so a slight change in course would avert any close encounter. Thus, having one of them nearby an alien civilization wouldn’t have any effect at all.

Even if they existed in substantial numbers, they seem inconsequential co-inhabitants of the galaxy. There is the thing called ‘missing mass’ for many galaxies, meaning their rotation curves do not jibe with their observed mass, meaning some is missing from the tabulation. This is often thought of as something exotic, but a bit of it could be undiscovered black holes.

Black holes the same mass as ordinary stars wouldn’t migrate to any other place in the galaxy, but they would just stay in the same place as the ordinary stars inhabit. There could be some in the disk, near us, not doing anything. There could be some in the bulge, just acting like a star but with a tiny radius.

What about other black holes? Can there be small ones? The equations of general relativity allow this to happen, and it has caused some stir, but general relativity is currently uncoupled with QCD, and so the existence of small black holes is a mystery. Perhaps it’s not so much of a mystery if you assume something about the compressibility of neutronic matter or matter in general. The point is, what would you make one of these small black holes out of? Known particles are too similar to neutrons in the ratio of gravitational force to density, and they can’t help make a small black hole. Somehow matter has to be compressed to get the Schwarzschild radius smaller than the radius of the mass itself. Only gravity can do that, and it is so weak you need star mass to do that. So there is not much of a prospect for small black holes.

How about large ones? If you assume that neutrons are simply not compressible and there are no phase changes with increasing pressure, then you can make the black hole as large as you want. If neutrons are compressible somewhat, it just makes it easier. If, on the other hand, there is a phase change for neutronic matter at some threshold pressure, into something else, and that something else was not a carrier of the gravitational force, adding neutrons to an existing black hole would be self-defeating. This does make the wild assumption that gravitation has some weird properties, but nothing is known about it, so, let’s assume it.

In this hypothesis, at the core, where the phase change happens, you would be reducing gravitational attraction, even as you increase it on the outermost shell of neutronic matter. If this goes on, then some other properties of neutronic matter come into play, as you have another example, albeit an unusual one, of a kind of Rayleigh-Taylor instability, and the internal core of what-have-you matter might burst out, perhaps in polar jets or something else. In other words, you might destroy a black hole by trying to make it bigger. If this upper limit on mass of black holes exists, then the question is what size is it? And then we have all the observations of large black holes at the centers of galaxies, both near and far. But since black holes are not visible per se, what these observations are is of matter being swallowed up by the black hole. If there was a cluster of the normal sized ones, in kinetic equilibrium such as a globular cluster is, would the effects be similar? So, probably there are large black holes, but not totally certainly. Either way, there doesn’t seem to be any effect on the willingness of an alien civilization to do star travel, as long as they are wise enough to avoid black holes.

Monday, August 8, 2016

The End of the Milky Way

An alien civilization that is motivated to stay in existence might be able to do so by migrating from solar system to solar system. With a small population and the mandatory high-percentage recycling, they could hope to get on the order of a million years from each solar system they colonize, and then move on. There are so many solar systems in the Milky Way, and more being born every gigayear, that they could expect to keep going for billions of years, provided nothing else gets them.

They would have asymptotic technology from the very first solar system, their home world and origin planet, and so could avoid parking near any imminent supernovas, and could arrange to stay in quiet parts of the galaxy. By doing mapping of nearby stars, they could find any nearby solar systems that are moving on a collision course on one they were contemplating as a future home, and avoid anything that would be disrupted by a stellar encounter. They would stay in the disk of the Milky Way, avoiding the ambient radiation present in the bulge and bar. So, if they chose to do so, they could merrily await the end of the Milky Way. Just when is that, anyway?

Not much thought has be given to the fate of galaxies. They don’t participate in the Hubble expansion of the universe, which is simply groups of galaxies parting from each other. Even the local group, Andromeda, Triangulum, and all the dwarfs like the two Magellanic Clouds, doesn’t participate in that expansion, but keeps on orbiting around one another at roughly the same distances, in a kind of ball of galaxies. It’s not even sure that the nearest neighbor Local Group of galaxies is going to be able to escape the clutches of our Local Group. But that collision is truly a long time out. Let’s try and think out what might happen in a shorter time scale.

One known event is the collision of galaxies, which is understood because we have figured out how to measure the proper motion of them, and can do some simulations given that as input data. Small dwarf galaxies are crashing into the Milky Way all the time, but their minimal size doesn’t disrupt it, except locally. The time scales of galactic encounters is measured in the scale of gigayears, a fraction of a gigayear to several gigayears. It would be so utterly simply for an alien civilization to figure out where, for example, the Canis Dwarf Galaxy is going to disrupt some region of the Milky Way, and avoid that area. If they travel something like a thousand light years from colonizable world to colonizable world, which we call alpha-habitable worlds, in a gigayear they would have crossed the Milky Way many times over.

The big collision, expected between Andromeda and the Milky Way, is forecast to happen about four gigayears out, and all alien civilizations would be able to predict it. At this point in time on Earth, we don’t know what will happen, except that there will be major disruption. Andromeda has a bit more mass than the Milky Way, according to our current estimates, so the gravitational pull of it on stars would be comparable to the gravitational pull that keeps stars in orbit around the Milky Way center. The disk may simply be torn into pieces and dispersed, thence to form some dwarf galaxies or even some rogue stars, ones without a galaxy to call home. The majority of the stars might coalesce into a giant bulge, except without a disk around it, it’s called an elliptical galaxy.

This collision will happen slowly, so if an alien civilization is jumping from solar system to solar system every million years or so, they can probably figure out where to jump to avoid being cast into the center of the bulge, where radiation is intense and stellar encounters run rampant, or being thrown into the great dark void to be a lonely rogue solar system. The latter is a problem, not because life would be unpleasant on a planet of a rogue star, but because there’s nowhere nearby to go to. They would be able to migrate out, perhaps on an earlier than normal schedule, if that was to happen. A nice dwarf galaxy would be a good place to go to, provided the Milky Way was being ripped apart into many of these. A large dwarf galaxy might have a billion stars, which means that there would be good hunting grounds for a lot more iterations of the alien civilization’s colonization attempts.

On a colonized planet, occupied by an alien civilization with asymptotic technology, they would have computational resources far in excess of what we can imagine now here on Earth, and would be able to actually predict the motion of the stars in both the colliding galaxies. Finding which stars were going to form a dwarf would be fairly easy with this much computing power, and so many years to do the calculations, thousands upon thousands at a minimum. No problem.

Another fate might be the important one. There is a black hole at the center of major galaxies, as far as we know, and they slowly eat the stars surrounding it. Consumption is not rapid, as angular momentum keeps stars orbiting it. Obviously, a stellar encounter can mix up the orbits of different stars, allowing one of them to be swept into the black hole. This would be noticable because of the X-ray emission that would result, so we know that, for the last few years, this hasn’t been regularly happening. But in the aftermath of a galactic collision, the rate of consumption could rise, for both of the two or more black holes involved in the collision. It would be unlikely that the two of them would come close enough to interact with one another, but if they did collide, there might be an explosion even able to extinguish life in the galaxies involved. Little is known about the history and future of the black holes in our galaxy, so that at least is a source of some finality to the history of a migrating alien civilization.

Just as scarcity drives the alien civilization from solar system to solar system, the galaxy can suffer from it. It runs on hydrogen gas, which is used to make stars, for example, in the spiral arms. As that gas is consumed, it is only partially replaced by hydrogen that is emitted during the lifetime of a star. It gradually disappears, leaving no raw material to make new stars. How many gigayears in the future the Milky Way has before it becomes bereft of gas and starts to turn off is not known, but there is some limit. Once it does so, the brightest stars will disappear first, then the mid-level stars, leaving only the red dwarfs, which last extremely long. What would an enterprising alien civilization do at this point? Maybe call it quits.

This harks back to the question of cosmological philosophy. If all the members of an alien civilization know their culture, lifestyle, history, and existence will be eradicated from the universe, irretrievably, at some time in the future, maybe ten gigayears, what effect does that have on their motivation to endure until the very last moment? Do they think, much, much earlier than that about voluntary extinction? Do they even think about it before they make their first colonization effort, or only after ten or a hundred of them? Is this an inevitable byproduct of asymptotic technology?

Saturday, August 6, 2016

The Non-Fusion Option for Alien Civilizations

If planet-bound fusion proves to be impossible, every alien civilization has the same problem: how to power their civilization. Unless that is solved, the civilization will not be able to rise up to asymptotic technology or determine how to best travel from solar system to solar system. What are their alternatives?

The options we on Earth have thought of might not be a complete set, but they do provide an excellent starting point. There is an extraction of energy from solar photons striking the Earth, either by transforming them into heat or electricity directly, or by gathering up the energy as it diffuses into other modes of energy storage, such as in rain by capturing it at some elevation and combining it with gravity to make electricity, or in fluid motion, either the atmosphere or the ocean. Combustible materials, such as fossil fuels or vegetation are another way solar photons are accumulated. Precipitated methane may exist on the planet, with origin unknown. Flying some sort of direct photon capture in space near their home planet or in solar orbit might be an option as well, depending on how far they get with technology before they need to do this.

Gravitational energy is another option, either via the thermal energy stored and still present from the condensation of the planet itself or via the tidal interactions of a large satellite. No one on Earth seems to think much about the gravity between the sun and the planets and planetesimals, but there is energy there. Disturbing planetary orbits might be bad for the stability of their home planet in the very long-term, but perhaps something which further circularized the orbits of some gas giants might be something they would consider.

We on Earth had a coincidence of the invention of electricity and a mass expansion in the use of fossil fuels, and this seems to have resulted in the movement of electricity around the planet from source point to use point. Perhaps a difference in timing might have resulted in Earth using hydrogen as a medium for transporting energy, and alien planets might find this method rather than having metallic wires stretching all over. Hydrogen can be easily generated from water, and combusting it does not produce anything other than water, so there are some good reasons alien planets might use it.

The other possible source of energy is uranium or thorium fission, which may produce part of the heat in the earth, mixed in with the residual heat from the condensation of the planet. Fission can also be done in a specific site, by concentrating these elements, and the assumption that fusion is not feasible, for the purposes of discussion, has nothing to do with the various possibilities that exist for fission. We on Earth are still in the throes of early design of fission plants, having only several decades of work on the subject, but an alien world with centuries of experience would certainly master those aspects which trouble us. These include safety and security of the plants, recycling the fuel efficiently, being able to cycle the plant on and off to cope with changes of demands, and costs, in terms of energy returned for energy invested in the plant.

In order to power an alien civilization, even partially, with nuclear power, there has to be enough uranium and thorium available to them. Is it obvious that all solo planets would have this, or is it possible that some alien worlds would be devoid of much of these two elements? Some elementary astrophysics answers this.

We make the assumption that the beginning of the known universe, back in the time of the formation of galaxies, had the whole mass in the form of hydrogen and helium, and it was the formation of the first generation of starts, which astronomers like to call population III because they were the first to come into existence, that produced the heavier elements. Heavy stars have short lifetimes, so the age of the universe can incorporate many generations of heavy stars, each starting with more heavy elements and each producing even more when they detonate as supernovas. Of course, all stars produce heavier elements, as that is the nature of fusion which powers them, but stars which don’t explode don’t spread out what they have produced nearly as prolifically as supernovas.

Simple nuclear physics tells us that iron and elements around it have the most stable nuclei, measured in terms of energy per nucleon, so if the universe ran down from hydrogen, the least in energy per nucleon, to the state of maximum energy per nucleon, it would be making iron only. Perhaps this will happen tens of billions of years from now, in processes not yet initiated in the universe and not envisioned, but for now and in the time before now, elements heavier than iron are produced in abundance by stars.

This might be thought to be because in a stellar interior of a heavy star, nuclei are all in equilibrium distribution, and heavier elements, being higher energy states, would be populated in some sort of Boltzmann distribution. Perhaps that does exist somewhere, but it is much easier to make heavier-than-iron elements at somewhat lower temperatures, because of the one-way street that exists with nuclei. In a star, with plenty of protons and alpha particles flying around at high energy, iron nuclei get hit with some, and they form a different element, a heavier one. These elements form as isotopes which are stable, or else they emit alpha or beta radiation and become one. Stable isotopes simply last for a long time, and in fact, collect another proton or alpha particle, pushing their atomic number higher. The long chain of stable isotopes that stretches from iron up to uranium is responsible for the accumulation of heavier elements. Then, when the heavy star goes supernova, all this good stuff gets blasted out into space for later generations of stars and planets to pick up. The supernova process may even do some more pumping up of atomic number with all the hot particles involved in it.

So, the generation of elements beyond iron is a certainty, as the stars which generate iron also generate them, admittedly in lesser numbers as the atomic number goes up. The actual numbers depend on the cross-section of the individual isotopic nuclei for the particle energy distribution that exists in the star, which of course depends on the depth. Isotopes change in numbers depending on the input and output to them, which depends on some particular cross-sections for neighboring isotopes. That detail is not very relevant to simply figuring out if alien worlds would have uranium and thorium – they would. We have another case of interstellar convergence, as the laws of physics are the same all over the galaxy. There is an aging question, however.

There might be an additional question of whether the uranium on other planets is buried more deeply than on our planet, or even more shallowly, and would it be in ores of lower or higher concentration. This is a geophysics question. The presence of an element on the crust, in ores of different concentrations, is a question of something like relative solubility. Since elements and simple compounds like to bind with elements and simple compounds of the same type or the same size and, for compounds, shape, ores form, and they form on Earth the same way they would form on other planets of the same size and location in a solar system. Density also plays a dominant role. Iron, nickel, and other elements which like to dissolve in them form heavier density solids or liquids, and sink downward. There is a gradation of density starting with the center of a planet. Thus, what is left on the surface should be largely the same on planets which are similar in size and composition.

We do not know how close an alien planet would have to be to Earth to have this phenomena occur, where the ores here match the ores there. Is there some threshold at 1.2 Earth mass, or at 0.85? Extraterrestrial geology has not yet mastered these questions, as we are so new to the exo-planet game. However, it would be surprising if there was not some reasonably wide range around Earth mass where planets were similar in ore availability. Since life is likely to originate on such planets, we have a coincidence that favors aliens as much as it favored us.

Friday, August 5, 2016

Best is Not So Good in Evolution

Evolution is perhaps the most detailed step on the process of bringing alien civilizations into existence. It consists of millions of simple steps, ruled by a selection process, that eventually produces organisms that meet some fitness criteria, which changes with time. Most simple steps are fatal, most of the rest of degrading of fitness, most of the remaining ones are indifferent to fitness, and a few are improvements. But they are tied together.

Consider the genetic machinery inside the cell itself. There are three pieces, according to one simple way of categorizing them. They are the ones which support life in the cell, meaning ones which take in foodstuffs and modify them, extracting energy or repair materials and removing waste chemicals. There are ones which build the cell, including the process of making a copy of every part and moving them so that reproduction can take place. These involve reading the genetic code and doing what it says, mainly building various proteins and a few other organic molecules. Then there are ones which reproduce the genetic code itself, so that a reproduction of the cell can have a genetic code copy for itself.

For the first category, there are, in most cases, multiple copies inside the cell, so that damaged parts can be ignored or repaired without much influence on the survival and reproduction chances of the cell. In the second category, errors in reproduction might be swamped by the numbers of things which are produced. For this second category, there might be layers of cellular machinery. One layer might work with the genetic code itself, making the second layer of machinery. The second layer might make a third layer, or actually produce things in the cell itself. Errors in the second or later layers are more likely to be inconsequential. If the genetic code is only read once, then that reading could have a fatal or otherwise serious effect on the cell, but if the genetic code is read over and over to make more copies of the same stuff, redundancy saves the day.

What actually does the reproduction in a cell? Consider the cell wall, which might be simple or very complex, depending on which stage of evolution you are considering or which cell at a particular stage. Either way, there is internal, second or later layers of cellular machinery which repair the existing cell wall, and likely build more as needed. When something signals a time to reproduce, this same machinery can start expanding the cell wall, increasing its area, which can be used for both the original cell and the copy cell. Here again, errors don’t matter much if the layer-generating machinery is sufficiently redundant. With cell molecules numbering upwards of millions, there should be significant redundancy in the machinery which produces them.

Thus, cellular reproduction, as envisioned by this example, doesn’t take place with the genetic code making much new proteins, but instead, the production machinery is already there, and some signaling is what is needed. A previous layer, one that makes the primary production cellular machinery, might have to make more of it. Thus, the genetic code doesn’t seem to be much needed in the reproduction of a cell. Does it produce the signaling that is used to build a dividing wall, or does it somehow keep track that all necessary parts have been produced and neatly divided so the dual dividing wall can be constructed? Or has it generated a set of timing proteins which do the measuring and signaling? Perhaps that is the only set of molecules that has to be produced directly by the genetic code in the original cell. If not, there is not likely to be a large number of things that have to be produced in this reproduction process. It certainly does something connected to reproduction, else cells without a genetic nucleus could survive and reproduce on their own.

If it is not the whole genetic code that is involved in reproduction or maintenance, once the first layer of cellular machinery is in place, this means reproduction can take place with significant errors in the genetic code caused by mutation, and they will not manifest their effects until later on, when some primary cellular machinery has to be produced, and changes happen.

The third part of the genetic code reproduction consists of copying the code itself so that each one of the resultant cells can have its own copy. This copying process has its own mechanisms, and can make its own errors. Suppose there was redundancy or some other forms of error-checking and correcting, and for the purposes of illustration, consider a cell which had developed this to a high degree. It does not make genetic code errors. This means that there is no fatalities caused by this source. A reduction in fatalities means a higher fitness, so this should survive and out-compete cells identical except for this error-correction scheme. So, why isn’t this present in Earth cells?

Because there is something which might be called meta-competition. Cells which mutate at some small rate lose in the fitness competition, but when one of the mutations scores big and wins the fitness competition by a significant amount, that benefit outweighs the benefit of having error-correction working strongly. It’s a gamble that cells win by playing the mutation lottery. Lots of small losses occur when there is no benefit or some degradation to fitness occurs, but one big win erases them. If the mutation rate is too high, the losses mount up too high and the cell with a high mutation rate loses, but if it is too low, too few chances exist for the next stage of evolution to happen.

This means that there is some optimal mutation rate, which might depend on circumstances such as the survival to reproduction probability of cells. The same thing should happen, not just in single celled organisms, but in anything which exists and might compete in the mutation lottery.

This process should be the same on all planets which experience evolution. It is just one more example of interstellar convergence.

Thursday, August 4, 2016

GMO Aliens

One difficulty in understanding alien civilizations that have passed beyond asymptotic technology is that they face a worse difficulty. They simply have no guidelines in designing their own species. Perhaps you would think there are some, and we can't see them because of some fog that has descended over the question, and the fog will clear once we understand better asymptotic technology.

Unfortunately, it is more basic than that. This is simply one more deduction from the fact that the universe doesn't provide us with an instruction manual, or even any clues as to what to do. Any alien civilization that is racing through the genetic grand transition will soon realize they can do much more than just fix genetic diseases, more than improve various attributes such as health and intelligence, more than even bestow excellence in everything on each and every new alien. They can rewrite the book.

If you are buying an appliance, you can think about what you want it to do. You can think about how you plan to use it. You can think about how it might improve your life or save you time or save you money or make some activity easier or better. You can see how the appearance fits in with things around it, if it is a major appliance. You can see how easy it is to transport if it is a minor appliance. There are many other things you can inquire about before you choose which appliance to get. That is because you have some goals, or can deduce some from what your activities are.

With a new generation of aliens, what is the 'you' that has goals for them? In the beginning, it is very easy to do this, as all members of the older generation would agree that genetic diseases should be eliminated, because they project their desires onto the young generation, who is sure to agree that these diseases should go. There might be less universality about other changes, but some might reach nearly a hundred percent consensus. “We should make the immune systems in the new generation as robust as we can.”

Probably there would be a majority opting to provide the new generation with improved intelligence, attractiveness, athletic capability, strength, longevity, better vision and hearing and other senses, and some more. These are also simply projections of what the older generation would have liked to have had themselves, but they were originated before genetic manipulation became possible. The older generation would simply assume the younger generation would want the same things.

There would have to be some determination as to exactly what these fine attributes meant, such as deciding what intelligence really is and if it is somewhat vague, how to define it well enough for the genetic engineers to make up some DNA (or whatever aliens use) to accomplish it. Each of these attributes needs to be defined, as genetic engineering cannot work to ambiguous designs. But in the optimistic world, these are already understood well, or get done at about the same time that genetics becomes an ordinary engineering specialty.

And as in all engineering, there are trade-offs, as perhaps athletic capability requires some brain regions to be devoted to it, which detracts from the amount of processing available for intelligence tasks. Trade-offs abound in any complex engineering task, and it shouldn't be any different with engineering a new generation of aliens. But, in the same optimistic world, somebody or some group manages to figure these out, and some choices are made. Choices are made by aliens in some capacity, likely whatever passes for governance on their planet, and they are made with the assistance of all the artificial intelligence that can be put together. The robotics revolution will be largely over by this time, so there should be no shortage of encyclopedic knowledge or the ability to access and process it, nor of any calculation capability. The figuring out of how to implement decisions is not the problem, it's the making of the decisions.

At this point, the new generation of aliens is recognizable as the same species as the old generation, as they match the best of the old generation, except all of the new generation does. This is where the road gets muddy and the going gets hard.

Take nutrition, or ingesting foodstuffs in general. What to do about this? Some aliens in the old generation might think the new generation should have the nose of a wine connoisseur or the alien equivalent of someone who appreciates greatly beverages, and the mouth of a gourmet, again, the alien equivalent of someone who appreciates greatly the taste and feel of foods. The opposing group of aliens might think that the GMO design should make the new generation greatly happy with simple equivalents, so they would enjoy eating readily available foods and beverages. Another group of aliens might think the GMO engineers should eliminate most taste and the rest of the food-related senses, as there is no need to waste the new generation's time on these irrelevancies, but use these areas in the brain for other purposes. So there might be three or even more groups of old generation aliens, with their own preferences, all in opposition about choosing what qualities to give to the new generation. This is simply one example. Beyond the basics that every alien wishes for, there is nothing but options and no means to decide between them. It would be possible for the GMO engineers to do any of these, but they get no direction as there is no consensus.

One solution the aliens might adopt is to divide the new generation into blocks, and make each different block mimic the choices of a different group of old generation aliens. So, in the food example, some new generation aliens would be gourmets, some would adore simple foods, some would care not a whit about foods. This solves nothing, but just kicks the can down the road. When this new generation asks about what to design into the succeeding generation, they might start by asking why they were given the qualities they had. There is no answer based on any principles, only, that the old generation got their preferences reproduced in the new generation. And would the new generation want to continue them? Assuming they have top-level intelligence, this would hardly be a sufficient reason to make the next generation the same. What would they do? There does not seem to be any answer.

Everything is possible and nothing is required. Try to imagine yourself in such a predicament and gauge the impact of it. It is a strange feeling to not be able to make any sensible decision, where there are no reasons but any random choice will work. This example may help us to understand how alien society is put together. Their choice for star travel is just one of a large number of choices that they must make with no rational basis for it. What would they do?

Monday, August 1, 2016

Planets and Satellite Formation

There are seven natural satellites in our solar system larger than 2000 km in diameter. Where did they congeal? They could be from the vicinity of the planet itself, at a Lagrangian point in the planet's orbit, or in orbit around the sun. All three are reasonable points of condensation. It might be possible that one planet's satellite was lost to that planet and later captured by another, but it seems at first glance the probability of that would be very small.

One of the details of the organic oceans hypothesis for the origination of life is that Earth's moon was formed at a Lagrangian point and was later captured. The original moon may not have been the same size as the current moon, as there was an impact and mass exchange between the proto-Earth and the proto-moon, resulting in the current mass ratio. If some or all of the other large satellites were formed in this manner, perhaps something could be learned about the process by examining all of them, rather than only the case of Earth. The name Theia is used for the proto-moon.

Capturing a satellite that formed co-orbitally with a planet should be much easier than capturing one from a solar orbit. Solar orbits are largely formed in stable locations, and in Earth's solar system, with two giant planets setting up the resonances, getting out of these orbits might require some unique phenomenon. Lagrangian points are also points of resonance, but stability there might be adversely affected by the presence of the other planets. Jupiter is about four to six AU away from a point in Earth's orbit and its mass is three hundred times that of Earth, meaning the gravitational effect of Jupiter is greater than that of Earth on an object at one of Earth's Lagrangian points. This is, of course, irrelevant, as the mass of Jupiter at the time of formation of Theia may have been much less, as it could have been still in the process of forming, and much of its mass could still have been spread around its current orbit. A ring of mass does have a gravitational effect, but not as much as a concentrated planet. There would have had to be some concentration of mass, in order to set up the resonant locations where the other planets would condense their rings of mass and then the planets themselves, but it would not have had to have been the whole mass by that time. The gradual accretion of matter by the proto-Jupiter may have been one of the factors that caused the migration of Theia into impact with the proto-Earth.

What about the other six of the largest moons? They all orbit planets with deep, dense gas atmospheres. Impact is quite different for a large moon impacting a planet with a thick atmosphere. Instead of having to hit the core and disrupt the crust or mantle or whatever the equivalent is on these gas giants, it could come through the atmosphere at a slow speed, and be captured. The speed would be very slow as planets in the same orbit have the same orbital speed, so the only difference in speed would be that caused by the freefall into the planet's gravitational well. That certainly is enough to make the contact spectacular, but it would only take a small reduction in speed via atmospheric friction to reduce the relative velocity of the satellite down to a highly eccentric but captured planetary orbit. After this interaction, more entries into the atmosphere would further reduce the speed and relative angular momentum, allowing eventual circularization of the orbit, aided by tidal interactions. Thus, capture by a gas giant of a Lagrangian-origin moon seems quite feasible.

This also means that the precise depth of maximum entry of the moon on its first encounter is not critical, but almost anything deep enough to draw off some angular momentum would be enough for a capture. This is in contrast to the Earth, which may have had an atmosphere much larger than today's, perhaps a few times more than Venus has today, but this would be nowhere near enough to cause much drag on the approaching Theia. There would have to be an impact, glancing at the least, to do that.

A glancing impact would tend to equalize rotation rates. This would likely mean Theia would slow down, and to maintain the total amount of angular momentum, it would seem that it would have to leave the joint blob of mass with more than it brought in. So the moon is particularly large.

This means that on other solar systems, ones which have not been disrupted by a stellar encounter, there should be numbers of large satellites on gas giants, but few or none on rocky inner planets. This theory therefore reduces the number of solar systems with origin planets. The reasoning goes like this. Organic oceans are necessary for the early origination of life, and a Theia-like impact is the most prolific means of making them. Some might be made by asteroid impact, but the amount would be less, meaning less chance for life to originate. The atmosphere would also be much thicker, making it harder for life to evolve, and much harder to replace the carbon dioxide with oxygen. Less photons would get through the atmosphere and there is much more carbon dioxide to begin with. The moon also may have some beneficial effects on later stages of evolution.

So, if we go to look for life-containing solar systems, using a giant telescope able to see not only planets, but satellites, it will be a clear indication of a promising planet for life if it is located on the inner portion of the liquid water zone, and it has a very large moon. There should also be a minimal atmosphere, such as we have here on Earth. If it does turn out that this is rare, as it may be, then life itself is rare and originates very seldom in the galaxy. This might be the most striking answer to the puzzle of why there are no alien visitors here.