Galaxy Calc Shows Aliens

What makes a planet a good host for life? That is, what does a planet need for life to originate there and then evolve to something at the human level? Astronomers today say a planet at least needs a star that 1) lasts long enough, 2) has enough heavy elements, and 3) is not too often hit by nearby supernovae or gamma ray bursts. Using such criteria, several astronomers (mentioned below) have tried to calculate “galactic habitable zones,” i.e., galactic distributions of good-for-life planets, in both space and time. Such calculations are far more important than I had realized – they can help say how common are aliens! Let me explain.

Imagine that over the entire past and future history of our galaxy, human-level life would be expected to arise spontaneously on about one hundred planets. At least it would if those planets were not disturbed by outsiders. Imagine also that, once life on a planet reaches a human level, it is likely to quickly (e.g., within a million years) expand to permanently colonize the galaxy. And imagine life rarely crosses between galaxies.

In this case we should expect Earth to be one of the first few habitable planets created, since otherwise Earth would likely have already been colonized by outsiders. In fact, we should expect Earth to sit near the one percentile rank in the galactic time distribution of habitable planets – only ~1% of such planets would form earlier. If instead advanced life would arise on about a thousand planets, Earth should sit at the 0.1 percentile rank. And if life would arise on a thousand planets, but only one in ten such life-full planets would rapidly expand to colonize the galaxy, Earth should again sit near the one percentile rank.

Turning this argument around, if we can calculate the actual time distribution of habitable planets in our galaxy, we can then use Earth’s percentile rank in that time distribution to estimate the number of would-produce-human-level-life planets in our galaxy! Or at least the number of such planets times the chance that such a planet quickly expands to colonize the galaxy. If Earth has a low percentile rank, that suggests a good chance that our galaxy will eventually become colonized, even if Earth destroys itself or chooses not to expand. (An extremely low rank might even suggest we’ll encounter other aliens as we expand across the galaxy.) In contrast, if Earth has a middling rank, that suggests a low chance that anyone else would ever colonize the galaxy – it may be all up to us.

At the moment published estimates for Earth’s time percentile rank vary widely. An ’04 Science paper (built on an ’01 Icarus paper) says:

~30% of stars harboring life in the Galaxy are older than the Sun.

Except this is regarding a distribution cutoff at today. Eyeballing the distribution:

suggests Earth is at ~10-20 percentile of the total distribution. An ’11 Astrobiology paper gives a distribution of habitable planets that exist today and have existed for four billion years:

To my eye, this seems to suggest Earth is even earlier, perhaps 5-15%, as confirmed by this quote:

The [star formation rate] experienced in the last few billion years, coupled with increasing levels of metallicity, suggests that many more planets will be conducive to complex life in the future.

That paper also estimates that 80-90% of habitable planets are closer than Earth to the galactic center.

Finally this cutoff distribution from an ’10 International Journal of Astrobiology paper suggests Earth is very early:

Though that paper gives no explicit Earth date percentile.

Clearly there’s much room for progress here. But progress seems feasible, and quite valuable. With just a bit more astronomy calculation effort, we may be able to learn something substantial about just how common are aliens in spacetime!

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  • Brett

    Why would we expect a civilization to expand rapidly across the galaxy once it reaches human-level intelligence? Interstellar travel is very difficult, and represents a massive investment in energy, technology, and time. To make matters worse, the light-speed lag is going to fragment your civilization over distances beyond several hundred light-years.

    Compared to that, massive space expansion and development of a solar system is a cakewalk.

    • Michael

      People don’t necessarily spread in order to grow an empire. It only takes a handful of folks fleeing persecution to colonize a planet. Interstellar travel will be expensive, but someone will do it eventually.

      A fragmented civilization can still spread people across the galaxy.

    • Konkvistador

      There is the possibility of information trade. Surely there are calculations even a solar system sized computronium arrangement would take years, decades or even centuries to process. Making a transmission time of a few years or decades viable economically.

      Now while trade with a solar system 300 light years distant might be impractical, trade with a 150 light years distant might just be feasible and trade with one distant 100 light years might be very profitable.

      Also note that the time needed to develop a solar system is presumably pretty short relatively speaking, because once you arrive in it you can go exponential locally.

      Solar systems on the edge of the colonizing wave where stars are close together thus potentially have an economic incentive to develop uninhabited solar systems, even if those at the core or those that are more isolated don’t have one.

  • Preferred Anonymous

    @Brett: Why would you NOT expect a civilization to expand rapidly across a galaxy?

    Presumably, if some as-of-yet not know property of physics (which you can’t say positively doesn’t), or even simply a known one with a more efficient method of deployment, exits, which allows one to achieve interstellar travel, then it stands to reason, due to a mixture of probable natural curiosity (case in point they were curious enough in the first place to invent interstellar travel) and simple economics, that such civilizations will be very eager to expand.

    I just think you’re extrapolating a bit too much off our current experience, no, the rockets we have today are nowhere near useful for even getting anywhere within our own solar system, but its like comparing the Internet to the telegraph…the telegraph was ineffective at transmitting voice and video calls, that doesn’t mean the Internet was prohibited from existing due to economical costs. I hold your argument then, is utterly absurd.

    • James Kelly

      It’s not an absurd statement about energy costs and man hours spent on research and devellopment for the simple reason that the current political system is collapsing in on itself; Oil reserves are thining out, and economies are faltering. Also, the progression from telegraph to internet was mostly a social movement. There are no social benefits from deep stellar colonization therefore the likelihood of energy or manpower to be spent on this would also seem unlikely. The commoner had the money and time to dabble on the early internet to boost its popularity but really? Do you think the commoner will be able to invest in colonizing space? No, because that would be the advantage of an industrial entreprenuer. And that is who controls space now that NASA is falling apart. NASA is but a shell of its former self which really allows us to point our fingers in the direction that we are going, and that is not up. Just remember, necessity is the mother of all inventions and if we don’t need to go to space, or can afford to go to space, then we are not going. Besides, all space travel is in the theory stage. Unless you believe in the X-files, you would certainly doubt that anyone on Earth is even testing these theoretical rocket engines. Furthermore, remember cabin fever makes it unlikely that a crew would even get far enough to be killed by new bacterias and diseases for which the colonists will have no vaccine. Does anyone even know how long it would take to safely excellerate to light speed? This isn’t Star Trek. And how would you slow down? It might take years to do either.

      • PA

        *sigh*

        None what you say has anything to do with the problem at hand.

        Forget, for a moment, our technological accomplishments.

        Now, proceed attempting to make an argument.

  • http://timtyler.org/ Tim Tyler

    *If* civilizations expand rapidly across the galaxy, the chances of there being intelligent aliens are pretty low – else they would have been here already. We know that before doing any “habitability” calculations.

    • Brett

      @Preferred Anonymous

      Presumably, if some as-of-yet not know property of physics (which you can’t say positively doesn’t), or even simply a known one with a more efficient method of deployment, exits, which allows one to achieve interstellar travel, then it stands to reason, due to a mixture of probable natural curiosity (case in point they were curious enough in the first place to invent interstellar travel) and simple economics, that such civilizations will be very eager to expand.

      What “simple economics” justify interstellar travel? Don’t say “population growth is driving it” either, since the only civilization we know of so far is actually on a path towards an end in overall population growth.

      And “natural curiosity” is not enough to make an interplanetary space colonization mission, never mind an interstellar one. Look at Earth – we could potentially be putting a lot of money towards off-world colonization, but we don’t (and it’s not from a lack of “natural curiosity”).

      I just think you’re extrapolating a bit too much off our current experience, no, the rockets we have today are nowhere near useful for even getting anywhere within our own solar system, but its like comparing the Internet to the telegraph…the telegraph was ineffective at transmitting voice and video calls, that doesn’t mean the Internet was prohibited from existing due to economical costs. I hold your argument then, is utterly absurd.

      Perhaps, but I think you’re ignoring some real world physical limitations that we’re already starting to bump up against.

      @Tim Tyler

      *If* civilizations expand rapidly across the galaxy, the chances of there being intelligent aliens are pretty low – else they would have been here already. We know that before doing any “habitability” calculations.

      That’s assuming that the aliens choose to stick around in-system and develop. If they just send exploring spacecraft through every so often, then we could quite easily miss them, seeing as how we’ve only been watching the sky with something other than the naked eye for several hundred years.

      • PA

        @Brett Population growth is not the driving factor. Basic resources are much more important. In an economy that never stops “growing”, we require more and more produce as time goes on. You can say this correlates with population growth, which would probably be true, but irrelevant, or you could claim that a stable population only needs a very specific set amount of resources, but this’d be ignoring known economic behaviors (known to humans, that is).

        “Look at Earth – we could potentially be putting a lot of money towards off-world colonization, but we don’t (and it’s not from a lack of “natural curiosity”). ”

        I think we can both agree this has nothing to do curiosity…we simply don’t have the feasible means to colonize another world yet. There are plenty of curious folks focusing on what might or might not be on other worlds, all of whom are operating within their means.

        If natural curiosity has nothing to do with colonization, how do you explain our own “colonization” of Earth? Sure, there’s some economic incentive to new trade routes, but I think I covered that already in terms of basic economics…you can’t deny curiosity was a factor that lead to English colonization of the “New World”. Curiosity, politics, economics, (law in the case of Australian penal colonies). A bit of Imperialism, even.

        “Perhaps, but I think you’re ignoring some real world physical limitations that we’re already starting to bump up against. ”

        Like? You make it sound like we’ve only just discovered the limits of engine fuel.

      • http://timtyler.org/ Tim Tyler

        Re:”That’s assuming that the aliens choose to stick around in-system and develop.”

        A pretty safe assumption: living systems tend to saturate every available niche.

      • Konkvistador

        Don’t say “population growth is driving it” either, since the only civilization we know of so far is actually on a path towards an end in overall population growth.

        That’s just silly. As long as natural and memetic selection operates on humans demographic transitions are a temporary affair.

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  • PA

    @Brett

    …In light of the above, you have still yet to mention why you think alien civilizations wouldn’t expand, given the technology. This whole discussion is a little silly given that we are assuming that said aliens are at the same point technologically, and psychologically, as us.

    Again, of course it’d be dumb to drive around with wood-stoves powering all our transportation, that’d doesn’t preclude the average citizen from driving a car, it simply betrays a lack of knowledge of (more profitable) fuel chemistries.

  • Sigivald

    With just a bit more astronomy calculation effort, we may be able to learn something substantial about just how common are aliens

    Nope.

    We will be able to make slightly more educated completely inaccurate guesses, though.

  • Douglas Knight

    Why so many exclamation points and so much expression of surprise? You’re just saying that you’ve run into a new detail relevant to the Drake Equation and thus to the Great Filter, right?

    Yes, the Great Filter is important and thus every detail is important, but this post feels as if you’ve discovered it for the first time. (I’m also surprised you don’t mention it by name.)

  • arch1

    Robin, I like this time distribution of habitability approach. A few comments:

    1) your description conflates habitable with inhabited throughout (though as long as these are proportional, I don’t think that affects the gist)
    2) Your “20% of total distribution” estimate for the 1st chart seems(?) to imply that the green area doesn’t extend much above the chart’s top boundary (which IMO they should have made parabolic to better represent galactic geometry). That appears at odds with the quote under the 2nd chart.
    3) The analysis relies linearly on your (debatable) assumption that the probability p of human level life quickly permanently colonizing the galaxy is high. If we admit ignorance letting p range over (0..1) we can conclude considerably less.
    4) Even granting your p~1 assumption, I would tweak your summary to say “With just a bit more astronomy calculation effort, we may be able to learn something substantial about just how common are aliens over the galactic lifespan“. As you somewhere imply, the analysis won’t say much about the current number of aliens unless our percentile turns out to be extremely low.

    • http://hanson.gmu.edu Robin Hanson

      I updated the post language to address your points #1,2,4. My language “or at least the number … times” was to address #3.

  • arch1

    “My language “or at least the number … times” was to address #3.”

    And that it does:-) I managed to first misunderstand, then forget your explicit treatment of p<1. So my #3 is wrong; your analysis looks more robust than I had appreciated, and I am now very interested in seeing more criticism to vet it and more data to feed it.

    PS. Parts of this are reminiscent of Jim Gott's approach to computing a 95% confidence interval for the lifetime of (for example) the human race.

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  • Howard A. Landman

    I think it’s dangerous to anthropomorphize hypothetical intelligent aliens. They could be FAR more different from a human being than, say, a dolphin or an ant colony is.

    We have no examples yet of an intelligent species lasting longer than an average unintelligent species, i.e. 10-20M years. Thus it is too early to even say that intelligence is a long-term survival enhancer.

    Freedom from gamma bursts is a strong function of local star density. Being out on the rim, we probably are better off than average in that regard.

    • Dan Browne

      Yup. I did the calculation of roughly back of the envelope how much better off we are somewhere else in the comments.

    • ADawg

      Here’s an interesting question; Suppose that we just concede that intelligent species (defined as using radio technology) will be detectable for an *average* of 10-20M years. If the Milky Way has produced 10,000 of these civilizations over it’s history, what would be the probability of another such civilization currently being detectable by us?

      If you convert Lineweaver’s space-time distribution of habitable planets into a probability distribution for the space time location of intelligent life you could answer this question by randomly sampling 10,000 planets from that distribution.

      The answer is, if you want a reasonable chance of detecting a civilization this way, the AVERAGE civilization needs to remain detectable for timespans measuring tens or hundreds of millions of years. But if that average drops into the 10,000 year range, the odds are that humans are alone in the Milky Way.

      But on the other hand, if you assume that MORE planets could develop simple life which could last billions of years, the chances become very good that there is simple life out there right now.

  • Lord

    I think we have a poor level of knowledge about how rapidly such an expansion can occur. We only have two centuries to extrapolate and technology may have some speed bumps if not fundamental limits we know little of. It may take much more than a million years to colonize a galaxy. This would mean we won’t grow as fast in the future, but I see no reason why this can’t be the case.

  • Abelard Lindsey
    • ADawg

      You mean no alien civilizations? Because there is probably alien bacteria somewhere. Otherwise I agree.

  • Billy Brown

    Funny how almost all the arguments about galactic expansion are based on issues that matter only on a time scale of centuries or less.

    If you ask “Will humans visit Alpha Centauri in the next century?” considerations of economics, sociology and technology are very important. But that’s irrelevant to the issue at hand. On a time scale of even a few thousand years there will be exceptions to every possible poltical or social force that could prevent expansion, and the idea that technology will continue to be a tremendous barrier becomes quite implausible.

    Indeed, on a time scale of millions of years the relevant question would be “Is there some unalterable rule of physics that makes expansion impossible?” Because unless the answer is yes, someone will eventually do it, and then you kick off an evolutionary process in which societies most prone to colonization outgrow their stay-at-home competitors and cover the galaxy.

    • Lord

      Impossible, or very difficult. There is a large range between a million and a billion or billions.

      • ADawg

        And there is a large range between ten thousand and millions. Human civilization is only 10,000 years old. Modern humans are only 100,000 years old. Talking about a technological civilization capable of interstellar travel surviving for time-spans that are many tens of times older than our species is just ludicrous.

    • ADawg

      But really – on a timescale of millions of years there are so many other issues that come up. Evolution for example. Just looking at the Earth, there isn’t a single species that has remained constant over that kind of timescale. So suggesting that the conditions required for a technological civilization to even be possible could exist for that time period is VERY speculative. It certainly has never happened on Earth.

      • Dan Browne

        In general you are right, but there are in fact outliers. The crocodile, shark, coelecanth and sea-turtle are all examples of creatures that have remained the same since at least the dinosaur era.

      • ADawg

        Do the crocodile, shark or sea-turtle’s civilizations require 474 exajoules of energy per year to support? Nope. Not the same thing. It’s *possible* in theory that human beings could remain biologically the same for 65 million years. But it’s extremely unlikely that human civilization would.

        EDIT: In fact it’s impossible since the requirement of our hypothesis (interstellar travel) would require MORE technological change.

      • Dan Browne

        You are right. I’m not speaking to civilizations composed of sea turtles etc just the species that have remained constant over tens or hundreds of millions of years. I think it’s much more plausible that an intelligent species will survive for say 5 or 10 million years (e.g. in a stone age) than a civ surviving for millions of years.

      • IMASBA

        No species remains constant. Even the coelacanth undoubtedly underwent numerous changes to its immune system during its existence, it just happens to look much the same on the outside as it did 300 million years ago (and it’s not like a “species” is a neat discrete category either). I also fail to see how physical changes to an intelligent species would necessarily spell the doom of their civilization. As long as they don’t become less intelligent or die out the civilization should continue to evolve with them.

      • ADawg

        Physical changes to their biology wouldn’t necessarily doom the civilization. However, constant technological advance – required for an interstellar civilization – stresses the social structures which underpin the civilization. At least for humans, it has for the last two centuries. It’s not at all clear that any technological civilization can survive the continuous upheaval for a thousand years – not to mention a million.

        But even if we concede the above point, a technological civilizations require a steady supply of *enormous* amounts of energy. The more advanced the civ, the more energy it requires (ala the Kardashev scale). That energy has to come from somewhere. Can an energy appetite that big be sustained for megayear timeframes?

      • IMASBA

        There won’t be technological advancement lasting millions of years, in a couple of thousand years all that can be learned will be learned. Speciation won’t happen if the population keeps mixing it is possible that over millions of years populations in different solar systems become speciated but that would only happen if physical contact between the solar systems were impossible and in that case there can be no eugenicist movement or interstellar war either and even if one of those things did happen it’s possible one side would simply emerge victorious and continue the civilization.

        The available energy sources (mostly stars) are constant over billions of years. It won’t be the same as with fossil fuels on Earth: an energy source available in year 1 will still be available in year 100 million.

      • ADawg

        Over that period of time, everything will be learned and forgotten again and again. By speciation, I meant the technological cybernetic sort – not the biological kind. But that’s a bigger side discussion on the technological singularity.

        Fusion and Solar energy sources are more or less limitless, yes. But extracting and transmitting that energy also require other natural resources (e.g. precious metals and semi-conductors, rare elements). And even on scales close to today’s energy requirements – those resources are close to exhaustion. And we’ve been using them only a century or so. No. It’s not clearly sustainable.

      • IMASBA

        “Over that period of time, everything will be learned and forgotten again and again.”

        Why? If no extinctions occur knowledge will be preserved.

        “But extracting and transmitting that energy also require other natural resources (e.g. precious metals and semi-conductors, rare elements). And even on scales close to today’s energy requirements – those resources are close to exhaustion. And we’ve been using them only a century or so. No. It’s not clearly sustainable.”

        Metals don’t disappear when you use them, you can recycle them so again, if there are enough of them in year 1 there will be enough of them in year 100 million. Abundant amounts of metals exist on every asteroid, planetoid and rocky planet out there. In the near future there will be a transition from the current state of scarce energy and abundant minerals to one of abundant energy and scarce minerals, that’s when space mining will kick-off.

      • ADawg

        “Why? If no extinctions occur knowledge will be preserved.”

        Really? I think the Romans would disagree.

        “Metals don’t disappear when you use them, you can recycle them so again.”

        Second Law of Thermodynamics? Yes, you can recycle, but each time you do so, some of the source metal will be lost.

        “Abundant amounts of metals exist on every asteroid, planetoid and rocky planet out there”

        True. But we can also make synthetic gasoline. The reason we don’t is because it takes more energy to make gas that way than you get back from burning it. So it’s not economical.

        I’m not saying that interstellar colonization is impossible – just that nothing is a simple as you think. Plus, the Fermi Paradox + Von Neumann’s work together seems to support the notion that interstellar colonization is extremely extremely unlikely. Else…Where is everybody?

  • John Maxwell

    What if civilizations tend to destroy themselves in the process of technological advancement due to some advanced technology that is capable of destroying civilizations and can be developed much more easily than a defense against it?

    http://www.nickbostrom.com/extraterrestrial.pdf

  • Jim Rutt

    Interesting, but LOTS of assumptions about what alioens might do or be capable of doing. An interesting take on the whole Fermi PAradox thing is to be found in Steve Webb’s _If the Universe Is Teeming with Aliens … WHERE IS EVERYBODY?: Fifty Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life_

  • majus

    Is it true that the studies quoted made the assumption that stars in the galaxy orbit at a roughly constant distance from the center? That’s what it looked like to me. Not a good assumption; stars in symmetric, round galaxies have “loop orbits” which range quite a bit in galactocentric distance. Galaxies with some asymmetry (ours has a bar, I believe) host “box orbits”, in which the stars pass through the core. I’m not sure how this will skew the statistical arguments.

  • http://overthemoonscifi.wordpress.com/ Over The Moon

    This is a wonderful counterpoint to Max Tegmark’s disheartening presentation at the Singularity Summit. As the Drake equation is challenged by singularitarians’ trajectories, it’s important to remember this probably isn’t the galaxy’s first rodeo. Aliens could have advanced to the point where they have no need for clunky physical matter, and exist inside simulations. Or, they may be ‘hanging out’ near the galactic core, harnessing energy from the black hole like any good Kardashev type 3. Significantly advanced civilizations probably have no use for exchanging information with pre-singularity species–and we may not have the tools for detecting their footprints.

    -Over The Moon

  • Dan Browne

    There are somewhere on the order of 1 supernovae happening every 50 years. There are approximately two supernovae capable of causing a mass extinction on Earth every billion years. Earth is significantly further away from most of it’s surrounding stars than stars closer in to the center of the galaxy. From that we may surmise that extinction level supernovae occur more frequently close to galactic center due to both proximity and population. How much? From Ohio State dept of astronomy (http://www.astronomy.ohio-state.edu/~ryden/ast162_7/notes31.html) we have this:
    “The estimated number of stars within a parsec from the center is 10 million per cubic parsec. In Earth’s region of space, the density is a mere 0.2.” I make that to be 50 million more stars per unit of volume. If we are able to use that as a metric, then all else being equal we have somewhere between 2 and 100 million mass extinction causing supernovae every billion years increasingly linearly towards the center. If we further hypothesize that life will form on equivalent star systems with an equal probability then there is a 50 million times greater chance of life forming on the stars at the center than out here in the boonies. Thus…. joining the dots in my shaky conjecture, vastly more life forms in the center only to be wiped out every ten million years. If there’s only 100 or 1,000 human-level candidates, how many of them could possibly make it (from e.g. lizard equivalents all the way up to mammal equivalents over a period of several hundred million years) with a mass extinction happening every ten million years knocking them back to rodent scale? It strikes me as a long shot to say the least. That said, I suspect there is very likely viruses and to a lesser extent bacteria/archae equivalent all over the place.

    • ADawg

      I agree. And on a side note, the stellar density near the core would also cause many more orbital disruptions planets those stars may have. Evolution of complex life would be much more difficult under those less stable planetary conditions. But microbial life could still be common.