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361 Civilizations in the Galaxy?

I promised a quick return to recent work on the Drake Equation, which helps us estimate the number of communicating civilizations in the galaxy, but a BBC story on Duncan Forgan has me back at it even sooner than I had intended. It’s no surprise that the matters encapsulated in Drake’s thinking should be in the news. After all, the era of Fermi and Drake was without firm knowledge of extrasolar worlds, of which we now know over three hundred. For that matter, the concepts of habitable zones around both stars and the galaxy itself had not come to fruition, nor had anyone ever heard of the ‘rare Earth’ hypothesis.

We also work today with knowledge of Charles Lineweaver’s studies of the median age of terrestrial planets in the Milky Way, which point to civilizations around other stars having had as much as two billion years-plus to emerge before our own Earth had even coalesced. Until we know more, I suspect we’ll be adjusting Drake parameters for some time, as Duncan Forgan (University of Edinburgh) does in his new paper.

Hypotheses and Conclusions

Forgan’s simulations, which take current exoplanet findings into account, work with several contrasting hypotheses, for each of which statistical methods were applied. The BBC explains:

The first assumed that it is difficult for life to be formed but easy for it to evolve, and suggested there were 361 intelligent civilisations in the galaxy.

A second scenario assumed life was easily formed but struggled to develop intelligence. Under these conditions, 31,513 other forms of life were estimated to exist.

The final scenario examined the possibility that life could be passed from one planet to another during asteroid collisions – a popular theory for how life arose here on Earth. That approach gave a result of some 37,964 intelligent civilisations in existence.

Forgan’s statistical approach relies on so-called Monte Carlo methods that use repeated random sampling to compute results in complex systems. I note this from the Wikipedia article on these algorithms: “Monte Carlo methods are useful for modeling phenomena with significant uncertainty in inputs, such as the calculation of risk in business.” And nothing would suggest uncertainty in inputs more than the classic Drake Equation, especially in terms of the biological factors relating to how life develops and turns into civilizations.

Problems with Exoplanet Modeling

And, of course, there is still much we don’t know about exoplanets, considering that we have yet to observe a single terrestrial world around another star. But in our simulations we can use existing exoplanet data to provide a distribution of planetary parameters. These can then be applied in sampling, including factors such as the planetary mass function, the distribution of planetary orbital radii, and the metallicity of the host stars.

Note what Forgan has to say about this challenge as he threads his way through this statistical analysis of Drake:

…as with the stellar parameters, a population of planets can be created around the parent stars, with statistical properties matching what can be observed. However, this statistical data is still subject to strong observational bias, and the catalogues are still strongly incomplete. There is insufficient data to reproduce a distribution of terrestrial planets: therefore it is assumed that life evolves around the satellites of the planets simulated here. In essence, this constitutes a lower limit on the number of inhabited planets: the work of Ida and Lin… shows that, as a function of metallicity, habitable terrestrial planets are comparable in frequency (or higher) than currently detectable giant planets. This data is hence still useful for illustrating the efficacy of the Monte Carlo method (at least, until observations of terrestrial exoplanets become statistically viable). All this should be borne in mind when the results of this work are considered.

The ‘Single Biosphere’ Issue

And in biological terms, we are even more up the creek, since we base our thinking on observations of a single biosphere, our own. To keep the number of free parameters to a minimum, Forgan works with “a biological version of the Copernican Principle,” the notion that our Terran biosphere is not special or unique, so that we can think about life on other worlds as sharing many of the same characteristic parameters. The thinking on these matters — and the statistical methods used to explore them — forms the most absorbing section of the paper.

About half of all emerging civilizations destroy themselves under two of Forgan’s three hypotheses, the panspermia approach and what he calls the ‘rare life hypothesis,’ while self-destruction is a bit more likely still in the ‘tortoise and hare hypothesis,’ where life evolves easily but evolution toward intelligence is more difficult. We also have to factor in possible ‘reset’ events that may annihilate a biosphere before a civilization can emerge, and make estimates on the amount of time needed between life’s appearance and the first civilization.

There is so much we simply don’t know. But Forgan is no ideologue. He knows he’s working with outputs that are only as accurate as their inputs will allow. Thus this, on planetary modeling:

Current data on exoplanets, while improving daily, is still insufficient to explore the parameter space in mass and orbital radii, and as such all results here are very much incomplete. Conversely, as observations improve and catalogues attain higher completeness, the efficacy of the Monte Carlo Realisation method improves also. Future studies will also consider planetary parameters which are sampled as to match current planet formation theory, rather than current observations.

Where Statistical Analysis Is Taking Us

How much work remains to be done on the ever complicated Drake Equation? Quite a bit, in the opinion of this researcher. Forgan notes that we need an improved three-dimensional galaxy model that incorporates the evolution of the Milky Way over time and takes into account its components, such as the bulge and the bar. We also will continue to plug in better models for star formation and the spatial distribution of stars. New input from our space-based observatories should gradually strengthen our statistical models as we tune what Frank Drake started into an ever more sophisticated instrument for SETI research.

The paper is Forgan, “A Numerical Testbed for Hypotheses of Extraterrestrial Life and Intelligence,” published online by the International Journal of Astrobiology (January 23, 2009) and available here.

Comments on this entry are closed.

  • andy February 10, 2009, 15:47

    Significant figures anyone?

  • Ron S February 10, 2009, 18:20

    Significant figures? None, of course.

    Here is what I know about this type of analysis:

    First, given Earth’s and our existence, by direct application of the Principle of Indifference (it’s in Wikipedia if you want to look it up) it is straight-forward to calculate the relative probabilities of 2 or 3 or more instances of proto-Earth equivalents to develop life or intelligent life. However it has nothing to say about the absolute probabilities.

    Second, if you (again) apply the Principle of Indifference to our growing catalog of exo-planets, and acknowledge that we have no evidence of life on those planets (not disproof, just no proof), you can estimate the prior probability of life (or intelligent life – it makes no difference). This gives one divided by the total number of planets (including Earth, which is also the ‘one’ in the numerator). If it’s a population of 300, the prior is 1/300. You can go nuts with your extrapolations from there, provided you keep in mind…

    Third, Bayesian analysis provides a means of calculating the probability of a probability. Consider this a measure of confidence in the estimate of the prior. Regrettably the confidence level is ridiculously low due to the paucity of data, though you’ll have to excuse me from attempting the proof. This means that just about any probability other than the estimate of the prior I showed above is equally probable.

    What all this means is that we have more smoke and mirrors. Like I said in the previous thread, it doesn’t matter how you interpret and analyze the formulas when you have no data: it’s still garbage-in, garbage-out. As for Monte Carlo methods, which I have implemented in the distant past, it’s a useful tool when you have nothing better, but it’s awful in comparison to other methodologies. It’s just that it’s difficult to use another technique when you first need to characterize the data, and that data doesn’t exist. Here, even without reading the paper I would claim it’s a red herring.

    I hope I’m not sounding too negative, but I think it’s important to be realistic about we truly know. And it ain’t much. We need data.

  • kurt9 February 10, 2009, 19:56

    Forgan’s paper does not discuss “rare Earth” issue like plate tectonics or biological issues like mitochondria and the emergence of the Eukaryote and complex life.

  • Adam February 11, 2009, 2:19

    Hi Ron S

    As a total statistical noob I’m glad you know what you’re talking about. The most interesting part of the paper was what we do know and how Forgan used the data – the final numbers are largely irrelevant because of the multitude of unknowns still current. As the planet catalogues build up and our remote sensing is refined I do wonder just what we should expect to see out there. If we ignore the hunt for ETIs, can the stats tell us anything useful about finding planets with atmospheres in thermodynamic disequilibrium – methane with oxygen, oxygen and ozone, and so forth? Those sorts of questions will be answerable in a few years as telescopes find terrestrial planets and get some hint of their atmospheres. In 20 years we might have a reasonable level of confidence in answering how many planets can be expected to have life-as-we-know-it. Or so I’m hoping.

  • Ronald February 11, 2009, 7:51

    I agree with Rond S and even found the publication (which I had already seen mentioned someewhere else) rather amusing: the precise numbers suggest a leven of accuracy which simply isn’t there at all.

    The only parameters in Drake, of which we have a reasonable idea now, are those pertaining to the prevalence of sunlike stars and planets. That’s about it.

    As I said before: we can theorize and model as much as we like, but nothing, really nothing, beats observation.

  • James M. Essig February 11, 2009, 15:44

    Hi Folks;

    It is interesting to consider feedback loops that may result from the growing technological sophistication of any ETI civilizations. Once a given galaxy becomes a host to at least one intragalactic travel capable civilization, the potential for ETI life to spread throughout a given galaxy grows like a wild fire.

    The ETI planetary colonies might then develop their own characteristics by natural selection and other evolutionary processes or perhaps by technological and medical enhancement of both their physical and psychogenic properties. The augmentations might not even necessarilly be cybergenic modifications, but could be based purely on applied biological science. In some ways, we already do such on Earth wherein certain sperm and egg donor banks that contain cryogenic eggs and/or sperm only take semen and eggs from persons with measured IQs above say 130, or only take semen and eggs from althletically gifted, or artistically gifted individuals, especially such persons with IQs in the 130 or above range.

    In short, once only one to a few species within a galaxy becomes capable of intragalactic travel, the whole story changes in regards to the number of inhabited planets within a given galaxy and the formation of new species from both natural and artificial processes.



  • Laurent Chauvin February 11, 2009, 18:39

    At least the Monte Carlo name is a real point of notice. Why not simply admit that there is no way to really make a realistic guess of how much life is out there and simply admit that it surely must stink with life everywhere we look. All we have to do is to look at a square meter of earth on our own planet to see that it’s really nothing but a huge block of life. And why would it be any different any other place in the universe. To think that this is the only place is really, well lets think the world is flat and Only Catholics, or Islamics, would be allowed to live by God’s rule.
    The Universe is stinking with life. and most of it isn’t Catholic, or Islamic. And as long as we insist on believing religious views they will most likely keep avoiding us like the plague we are.

  • Administrator February 11, 2009, 19:21

    By the same reasoning, there is no way to assume that any sentient ETIs will be substantially different from us, being the end product of an evolution that tends to favor the predatory. It’s nice to imagine an alien race as a society of philosopher pacifists, but with no data we can’t make this kind of assumption.

  • kurt9 February 12, 2009, 12:55

    I read Nick Lane’s “Power, Sex, and Suicide” about mitochondria and the rise of complex life where he lays out the argument that the evolution of the Eukaryote was such an improbable sequence of events that it likely happened only once in the entire galaxy. Now I am reading the 2004 version of “Rare Earth” where Brownlee and Ward argue that things like plate tectonics and having a large moon are also likely necessary for the development of complex life.

    One thing that Brownlee and Ward does NOT discuss is the possibility that an Earth-sized planet lacking plate tectonics is likely to have periodic global resurfacing events like Venus. No doubt such events would have a very bad effect on whatever life exists when they occur. If this is the case (and I think it is), it makes the Rare Earth scenario even more likely than Brownlee and Ward would have it.

    The Drake equation is a useful statistical tool for calculating the occurrence of Earth-sized worlds around the various stars. It says nothing about the geological characteristics necessary for those planets to host complex life or about the evolutionary biological processes that can give rise to such life.

  • Adam February 13, 2009, 2:27

    Hi kurt9

    I suspect oceans and plate tectonics go hand-in-hand – something is needed in the mantle to bring down the solidus and encourage convection and that something is water. Venus, as many are beginning to suspect, has a very dry mantle at least near the crust, thus no easily convecting asthenosphere.

    The Drake Equation can be expanded to include as many improbabilities as you like, but we know so very little about how complex life evolves.

  • kurt9 February 13, 2009, 13:02

    I think oceans, plate tectonics, and large moons all go together. It’e likely we got plate tectonics as a result of the collision that made the moon. Most of the crust that made up the original Earth is what makes up the moon today. By having a thin ocean crust and thicker continental crusts, this arrangement allowed plate tectonics to happen. A cross section of the Earth shows a crust that looks like the cross-section of a patterned semiconductor wafer. No other planet in the solar system has this feature. As far as we know, all of the other planets consists of simply concentric layers of material (like uniform thin-film layers on a substrate). This is the reason why I think planets with plate tectonics, and a large moon, are quite rare. I also think that plate tectonics (and the large moon creating collision that creates them) is essential for the emergence of complex life.

    I just read the part about the Cambrian explosion. This also strikes me as a rather unlikely event.

    So far, its like we won three million dollar lotteries in a row; 1) plate tectonics, 2) the Eukaryote, and 3) The Cambrian explosion.

    Then there’s intelligence. The evolutionary period from the last mass extinction until we came along is 65 million years. The dinosaur period (mesozoic) was like 150 million years between mass extinctions. Yet, there is no evidence that any intelligent species emerged on Earth during the mesozoic period, even though the animal life then appears to have been as complex as in our own period. This suggests to me that intelligence does not come about very often and that we got lucky again.

    Make that 4 lottery wins in a roll.

    I really do think we are along, at least in our galaxy.

  • tim gueguen February 13, 2009, 16:58

    How much does the question of resources come up in these kind of calculations? It doesn’t matter how clever a species with human level intelligence might be if it can’t get access to the materials needed to create the various technologies that eventually to the evolution of a modern industrialised society. Perhaps the galaxy is filled with hundreds of intelligent species, but they’re stuck at Neolithic stage because their planets don’t have the materials needed to discover the smelting of metal in an accessable fashion.

  • Benjamin February 13, 2009, 20:47


    I’m a little more optimistic about those numbers than you are. Per Bak put the case for self-organised criticality as a model for evolution in “How Nature Works”, which remains one of my favourite popular science books. The predictions of this model are mass extinctions and rapid periods of evolution as an emergent process from the immediate gradualism of an organism and its environment, where the environment is the entire ecosystem which is co-evolving. This feedback leads to a fractal distribution in time for evolutionary changes. Therefore I’d happily expect the Cambrian explosion, at least, to occur again on another planet if this model is correct: it is simply at the very large end of this statistical fluctuation. So the Earth had been buying tickets for this lottery for millions of years.

    As for intelligence, it does seem unlikely to see it emerge on other planets. I would be curious as to what mark it would show in the fossil record if anything in the age of the dinosaurs really had been intelligent; back in 2006 there was a New Scientist article suggesting we ourselves would leave little trace in one hundred thousand years other than our cemeteries and golden grave goods, and the extinctions we caused; what about in 65 million years?

  • Administrator February 13, 2009, 21:22

    Benjamin writes:

    back in 2006 there was a New Scientist article suggesting we ourselves would leave little trace in one hundred thousand years other than our cemeteries and golden grave goods, and the extinctions we caused…

    Interesting. Benjamin, if you get the chance, would you post the full reference to the New Scientist story? I’d like to read it.

  • Benjamin February 14, 2009, 7:56


    http://www.newscientist.com/article/mg19225731.100-imagine-earth-without-people.html should be the link for you. I might single out the quote I’m referring to:

    “All things considered, it will only take a few tens of thousands of years at most before almost every trace of our present dominance has vanished completely. Alien visitors coming to Earth 100,000 years hence will find no obvious signs that an advanced civilisation ever lived here.

    Yet if the aliens had good enough scientific tools they could still find a few hints of our presence. For a start, the fossil record would show a mass extinction centred on the present day, including the sudden disappearance of large mammals across North America at the end of the last ice age. A little digging might also turn up intriguing signs of a long-lost intelligent civilisation, such as dense concentrations of skeletons of a large bipedal ape, clearly deliberately buried, some with gold teeth or grave goods such as jewellery.”

    (Page 4)

    This I note because I was reorganising some of my old books and papers and found this New Scientist in a corner and read it again. Apparently there’s a popular science book dealing with this topic in great detail, but for the life of me I can’t remember the title or author so I’m having a hard time finding it. I should very much like to read it.

  • Benjamin February 14, 2009, 8:03

    I’ll add to that one query: whether landfills of plastic and whether the dense concrete jungle of cities would leave a geological record, owing to the durability of the materials, even if only in a compacted and partially decomposed layer; I’m sure there would be some mineral evidence of artificial composite materials.

  • george scaglione February 14, 2009, 11:43

    hello all. i read some of the above and i surely respect the ideas of others here but i think i agree with the first couple of comments most of all ! 361 civilizations in the galaxy!? my god why not 402 or 801 or 17?? please anybody correct me if i’m wrong but don’t these figures just seem like wild guesses!?? sometimes people are so desperate for answers that they just make them up,imho. but anyhow,respectfully to all….your friend george

  • kurt9 February 14, 2009, 15:34


    You’re right if the previous civilization never made it into space. If they did make it into space, we should be able to find space junk that is recognizably artificial origin even after 65 million years.

  • synthesis February 15, 2009, 17:13

    James M. Essig:

    “once only one to a few species within a galaxy becomes capable of intragalactic travel, the whole story changes in regards to the number of inhabited planets within a given galaxy and the formation of new species from both natural and artificial processes”.

    Accurate conclusion James!

    On February 5th at 21:02 in Centaury Dreams commentaries on ‘New Angles on the Drake Equation I wrote:

    “A point to consider in future improvements of this modelling is not only ‘interstellar panspermia’ but very fundamentally ‘Interstellar Colonization’:

    The intelligence timescale is calculated in [Forgan’s] model by stochastic process: life does evolve on a planet by evolution stages that are randomly sampled. The resetting events (catastrophic life destroying events) are placed uniformly throughout each of the stages. If Nresets > Nstages, then any
    given stage may suffer several reset events. The model compute that if a reset occurs and if that reset results in annihilation, life is exterminated, and the process ends; otherwise the evolutive process decreases.

    If we permit strictly darwinian “Dawkins’Intelligent Designers” (interstellar interveners) in the eons of life’s evolution on colonizated targets then this stochastic ‘designer events’ must be included in [this(these)] model(s): any
    given evolutionary stage may ’suffer’ several ‘designer’ events and the model must compute that if a novel design is seeded and if that design results in life (intelligence)’s improvement then the target planet’s evolutionary path is changed for the better; otherwise the evolutive process in it is left to extremaly slow (classical) local darwinian stages”.

  • Ronald February 17, 2009, 5:15

    @synthesis: if I get you well, you refer to the possibility of ‘seeding’ (dissemination) of life by an advanced civilization, a fascinating concept.

    If this has ever happened it should be reflected in the genetic and other biochemical signature of life on various planets, which will in that case show a striking similarity that cannot be explained exclusively by natural selectively driven evolution. Somewhat similar to the way that we can see that the dingo did not originate in Australia, but at a more fundamental biochemical level.

    If we ever find and are able to investigate planets that show this remarkable similarity, that may raise an interesting future discussion about the mechanisms of evolution, panspermia and even ID (meaning Intelligent Dissemination, pun intended).

  • Maciej February 17, 2009, 11:52


    well, regarding Cambrian explosion – I think that today evolutionists start to agree that it was not such an explosion as it seemed to be. More and more evidence on complex life before Cambrian perion are found – so called Ediacara organisms for instance.

    Leaving it aside I would say that Fermi Paradox is puzzling enough to treat rare-Earth rare-(complex)Life hypothesis seriously. Certainly the arguments you mentioned should not be neglected easily.

  • James M. Essig February 17, 2009, 22:27

    Hi synthesis;

    Thanks for providing the above reference to your post on February 5th at 21:02 in Centaury Dreams.

    I am personally fascinated by the concept of artificial evolutionary processess and how ETI might have used such to augment their species and how we humans are starting to do such, or may be about to do so for our species.

    In a way, we already have some modern scientifically created emplements to alter the human central nervous system and human psychological characterists. One has to simply review the vast numbers of psychotropic pharmacological agents used to treat major mental/emotional disorders, as well as the agents being researched to mitigate developmental disorders such as autism, and the new treatments for Alziemer’s such as the prescription drug under the trade name Aricept.

    Stem cell R&D promises to produce even more profound degrees of medical intervention, and perhaps, genetic engineering, or in the case where genetic engineering will not be palitable to the general public because of faith based ethics, post birth genetic intervention, will permit still more profound medical advancements.

    It will be interesting to somehow know how future human settlements or colonies such as might be developed on the Moon, on Mars, and on words beyond will evolve in their unique biospheric habitats. I supposed that travel between these colonies will act to provide some genetic homogeneity among the future human ciivilization.



  • Jose March 11, 2009, 12:28

    I think it’s reasonable to assume that the number of advanced civilizations in our galaxy with an interest in communicating with us is most probably zero.

    The old assumption, popularized by Carl Sagan, was that the galaxy was probably teeming with such civilizations but they hadn’t contacted us because they haven’t been able to detect us yet. However with recent advances in Astronomy we can safely assume that any advanced civilization within a few thousand light years of earth should be able to take high resolution images of our planet.

    I think it’s reasonable to assume that any highly advanced civilizations in our galaxy know about us assuming that they a)develop advanced technology and b) are curious.

    So it seems probable that if any technologicaly advanced civilizations exsist in our galaxy they’ve chosen not to contact us.

  • Chris Blue June 18, 2009, 18:32

    Im not a mathmatician, just an observer. that being said, it seems a bit absurd to believe that out of 100 billion galaxies, we are alone. sounds arrogant to me. other thing is, we are looking at things from an earth bound point of view. why do you have to have a moon to have a civilization? why do you have plate tetonics? we only know about our planet and we assume all planets are like our own. that seems absurd. we know so very little about our own solar system, our own planet, our own oceans. how do we suppose we know about other planets? we need to think bigger, we need to open our heads a little more.

  • ljk July 31, 2009, 10:39


    Thursday, July 30, 2009

    Fermi Paradox Points to Fewer Than 10 Extraterrestrial Civilizations

    The absence of alien probes visiting the solar system places severe limits on the number of advanced civilizations that could be exploring the galaxy.

    The Fermi paradox focuses on the existence of advanced civilizations elsewhere in the galaxy. If these civilizations are out there–and many analyses suggest the galaxy should be teeming with life–why haven’t we seen evidence of them?

    Today Carlos Cotta and Álvaro Morales from the University of Malaga in Spain add another angle to the discussion. One consideration is the speed at which a sufficiently advanced civilization could colonize the galaxy. Various analyses suggest that using spacecraft that travel at a tenth of the speed of light, a colonization wave could take some 50 million years to sweep the galaxy. Others have calculated that it may be closer to 13 billion years, which may explain why we have yet to spot extraterrestrials.

    Cotta and Morales take a different tack by studying how automated probes sent ahead of the colonization could explore the galaxy. Obviously, this could advance much faster than the colonization wave front. The scenario involves a civilization sending out eight probes, each equipped with smaller subprobes for studying the regions that the host probe visits.

    This is not a new scenario. One previous calculation suggests that in about 300 millions years, those eight probes could explore just 4 percent of the galaxy. The question that Cotta and Morales ask is this: what if several advanced civilizations were exploring the galaxy at the same time? Surely, if enough advanced civilizations were exploring simultaneously, one of their probes would end up visiting the solar system. So that fact we haven’t seen one places a limit on how many civilizations can be out there.

    The numbers that Cotta and Morales come up with depend crucially on the life span of the probes doing the exploring (and obviously on the number of probes each civilization sends out). They say that if each probe has a life span of 50 million years, and if evidence of their solar-system visits lasts about a million years, there can be no more than about 1,000 advanced civilizations out there now. If, instead, these probes can leave longer-lasting evidence of a visit–evidence that remains for 100 million years–then there can be no more than about 10 civilizations out there.

    Of course, we may not have discovered the evidence yet. But if we finally find a black obelisk on the Moon, the paradox will be resolved.

    Ref: http://arxiv.org/abs/0907.0345: A Computational Analysis of Galactic Exploration with Space Probes: Implications for the Fermi Paradox

  • Helmkat December 1, 2009, 18:14

    It may very well turn out the Humans have been under observation by “Stable” civilizations” for a long time, its is also possible the Earth has given birth to more then one sentient species. Stable civilzations would have advanced so far ahead of us that we would have no hope of detecting them unless they wanted to be known. We live in a very old house and the other tenants are behind locked doors.