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Habitable Planets: Working the Odds

Want to play around with some numbers? The process is irresistible, and we do it all the time when plugging values into the Drake equation, trying to find ways to estimate how many other civilizations might be out there. But a question that is a bit less complicated is how many terrestrial planets exist in the habitable zones of their stars? It’s a question recently addressed by Jianpo Guo (National Astronomical Observatories, Kunming, China) and colleagues via simulations. By ‘terrestrial’ world, the researchers refer to planets between one and ten Earth masses, although they note that some scientists would take this figure lower, to perhaps 0.3 Earth masses, which may be enough to retain an atmosphere over long geological timescales and to sustain tectonic activity.

Guo’s team is interested in the distribution of terrestrial planets in our galaxy, and the simulations that grew out of this study create a probability distribution of such planets in habitable zones. The paper is laced with the specifics, but let’s cut to the chase. Guo’s figures show 45.5 billion terrestrial planets in the habitable zones of host stars in our galaxy. The team also worked out the probability for planets in the habitable zones of different types of stars, concluding that M-class dwarfs host 11.5 billion such terrestrial worlds, while K-class stars are the most fecund, with 12.9 billion. G-class stars like our Sun weigh in with 7.6 billion, while F-class stars show 5.5 billion.

Image: M81. Our estimates of the habitable worlds in galaxies like these are widely variable, but they all imply countless chances for life to get its start. Credit: Jonathan Irwin, DSS2.

It’s interesting to weigh these numbers against the year-old estimates of exomoon hunter David Kipping (University College, London). Kipping starts with the galactic distribution of stellar types. He’s assuming about 300 billion stars in the Milky Way (increasingly cited as the best estimate) and noting that 90 percent of these are main sequence and thus stable for long periods of time. He goes on to whittle the number down, eliminating M-dwarfs because of tidal lock and also cutting out short-lived stars higher than F-class. 22.7 percent of main sequence stars in classes F, G and K thus remain.

Citing Michel Mayor’s Geneva team, which found that roughly 30 percent (give or take 10%) of F, G and K-class stars have super-Earth or Neptune-mass planets, Kipping narrows the field yet again:

Using 30% as a fixed value and assuming that very roughly half of this sample correspond to rocky planets and half to Neptune-like gas giants then we may write down that 15% of all F, G and K-type stars have rocky planets around them. It should be noted that this value is very likely an underestimate due to fact planets of Earth mass are currently below the detection threshold.

But how many of these planets would exist in the habitable zone? Kipping was working with 330 exoplanets then discovered, with about thirty in the habitable zone of their host star, and so he suggested a fraction of 10 percent would be a safe estimate based on current knowledge. He then factors in a galactic habitable zone, assuming that one may exist and that any value he obtains will therefore be an underestimate if it does not. This takes the number of stars with habitable planets down to 5 percent, but still leaves him with 50 million habitable-zone exoplanets in the Milky Way. We can contrast that with Alan Boss’s prediction of ten billion habitable exoplanets in our galaxy and, of course, with Guo’s team, whose whopping 45.5 billion is the largest estimate I’ve ever seen.

The weird thing, as Kipping confirmed this morning, is that his 50 million estimate was actually rounded up from 45.5 million, a figure exactly 1000 times less than Guo and team’s number. Our numbers, then, seem to be all over the map, and Kipping also notes Bond and Martin’s 1978 estimate of 10 million habitable exoplanets. But Kipping is the only one I know who takes a shot at the intriguing question of habitable moons. He is, after all, a specialist in detection methods for moons around exoplanets, studying methods that may help us detect large satellites during exoplanet transits. Noting that a large moon could be found around anything from an Earth-class planet to a gas giant, he boosts Mayor’s 30 percent figure to 50 percent, for any kind of planet. And this is interesting:

Let us also assume that the habitable zone for exomoons is extended by around 50% due to the possibility of tidal heating maintaining temperate conditions in traditionally cold-zones. This means that 15% of all planets can host a habitable exomoon.

How many planets have large moons? Kipping notes how little information we have, but using the Solar System as an example, he finds two planets out of eight where a moon has been formed through a capture/impact process, which he believes to be a requirement for a large moon. Assume, then, that 10 percent of planets host a large moon and you wind up with a figure of 25 million habitable exomoons in the Milky Way. But we have to keep these figures in context. Kipping again:

So our calculation suggests that there are roughly half the number of exomoons than exoplanets. One important thing to realize is that these calculations are based on many guesses but many of the assumptions underlying each calculation are the same. Whether the ratio is 0.5, 1, 2 or 5 is not very reliable right now, but what does seem perhaps more persuasive is that if we talk about ‘order of magnitude’ kind of figures, the number of habitable exoplanets and exomoons is ball-park equal.

Kipping’s figures are truly mind-blowing when he turns to the larger universe. A figure of roughly 100 million habitable environments per galaxy can now be turned around for an estimate of habitable worlds in the visible universe. The number works out to 1018, or 10 million trillion. Even allowing the vast play in the numbers between our low-ball and high-end estimates of habitable planets, the universe is likely to be filled with environments conducive to life. My guess is that it’s out there in fantastic abundance. But how much of it has gone on to sentience and, perhaps, technology? That’s the question SETI continues to poke at, and it’s one that’s emphatically still in play.

The Guo paper is “Probability Distribution of Terrestrial Planets in Habitable Zones around Host Stars,” Astrophysics & Space Science Vol. 323, No. 4 (October, 2009). Preprint available. David Kipping’s Web pages are packed with good information. Start with these articles.


Comments on this entry are closed.

  • djlactin March 10, 2010, 11:57

    Huge as these numbers are, they must be low: they apparently use a narrow definition of the ‘habitable zone’ as one which would support water/carbon-based organisms (like us). Such life forms are all we know, but they’re certainly not all we can imagine. Which is certainly less that what nature could engineer!

  • Eniac March 10, 2010, 14:09

    This is brave work, given that we are on the threshold of finding real experimental numbers to match against those predictions…

  • Tim March 10, 2010, 14:23

    Interestingly, the same authors have examined the effects of UV flux and of evolutionary stage on the habitable zone. [ Habitable Zones and UV Habitable Zones around Host Stars, arXiv:1003.1222v1 [astro-ph.SR] 5 Mar 2010 and Habitable Zones of Host Stars During the Post-MS Phase, arXiv:1003.1207v1 [astro-ph.SR] 5 Mar 2010] They point out that the thermal habitable zone is not coincident with the UV habitable zone which suggests that the actual number of “habitable” planets may be much smaller than their 45.5 billion figure. In addition other factors such as orbital eccentricity, X-ray and gamma ray flux, flare activity, and stellar environment (proximity of O and B type stars), etc. will also severely constrain habitability.

  • Eniac March 10, 2010, 15:16


    Given that life most likely originated under water, what role would increased UV play, if any? Does an atmosphere (and ocean) not keep X-rays and gamma rays out? Why would O and B stars be a problem? Aren’t they generally too far away? Last I checked the opinion was that even a supernova of Alpha Centauri would have very minimal impact on Earth.

    A related question: Why is plate tectonics considered important for life?

    A lot of questions, but I come across these as assumptions all the time, without solid reasoning behind them.

  • Antonio March 10, 2010, 16:37

    Un altro fattore, da tener in considerazione per l’abitabilità di un pianeta extrasolare simile al nostro, sarebbe la presenza di una tettonica a zolle, di un’attività geologica(vulcanesimo, per esempio)non particolarmente accentuata, e un campo magnetico abbastanza intenso, che offra una protezione per le eventuali forme viventi, ed un atmosfera non troppo spessa, per evitare un “effetto-serra” catastrofico, simile a quello del pianeta Venere.

    Insomma, vi sono parecchi parametri geofisici da valutare, per far si che un pianeta extrasolare possa essere considerato una nicchia ecologica adeguata…

    In ogni caso, anche tenendo conto di tutte queste restrizioni e valutazioni, il numero di pianeti abitabili, di altre stelle, dovrebbe essere piuttosto elevato.

    Da quì, la domanda: perchè “E.T.” non si mostra a noi, in modo eclatante, se la vita è cosi'(forse)diffusa nell’Universo?

    Magari, noi siamo come uno “zoo”, che “Loro” osservano, senza mostrarsi più di tanto…

    Un saluto a tutti voi, da Antonio.

  • Administrator March 10, 2010, 16:52

    Re Antonio’s message above, Google translate yields this:

    Another factor to take into account for the habitability of an extrasolar planet similar to ours, would be the presence of plate tectonics, geologic activity (volcanism, for example) are not particularly strong, and a magnetic field intense enough that provides protection for any life forms, and an atmosphere not too thick, to avoid a “greenhouse effect” catastrophic, like that of the planet Venus.

    In short, there are many geophysical parameters to be assessed, in order for an extrasolar planet may be considered an ecological niche suitable …

    In any case, even taking into account all these restrictions and assessments, the number of habitable planets, other stars, it should be quite high.

    Hence the question: why “ET” does not appear to us, so striking, if life is so ‘(maybe) widespread in the Universe?

    Maybe we are like a “zoo”, that “they” look without showing too much …

    Greetings to you all, from Anthony.

  • Enzo March 10, 2010, 18:06

    As Italian, I really marvel at how far Google translator has come : the translation is not perfect but, considering its origin, perfectly acceptable.

    Antonio, regarding your question on ET, life might indeed be common, but complex multicellular life can be expected to be rarer (based on the fact that it seems to have taken a few billion years here on Earth since life appearance).
    The other important thing is that, the way I see it, even complex life does not mean intelligence, at least not technological intelligence that could be noticed from space. Evolutionary pressure push towards better survival, but intelligence is only one survival strategy amongst others. Again, here on Earth we have some rather intelligent species like chimps and crows but nowhere near capable of being noticeable from space (except, possibly, for a biosignature in the atmosphere).
    Habitability for intelligence and complex life implies long term stability : Venus was probably habitable, with a water ocean in the early solar system. If that’s the case, it probably had life (even if not indigenous, at least as Earth contamination). Any life is now gone, except, maybe in the strange UV absorber in clouds. Mars, looks deceptively like Earth, but we can’t even find organics in the polar ice, maybe drilling will help.
    I have more hope for Europa, Enceladus and Titan, but only just.

  • Kenneth Harmon March 10, 2010, 18:20


    Lets add some practical to the theoretical and visonary above and further define the question to something that may impact Human Civilization not in the next 1,000-10,000 years, but in the next 200 years. So taking the various data sets above including the statistics on habitable moons, which is very important in its own right, how many habitable planets and moons might there be within 100 Light Years of Sol/Terra?

    Even if we have a fundamental physics breakthrough in the next 200 years that enables Warp drive (and I strongly suspect we will), and/ or we find new ways to communicate, at least as far as the residents of the Sol/Terra system are concerned about a 100 Light Years radius seems to be our likely maximum zone of influence for Centuries to come. Unless an Advanced Alien Civilization somehow stumbled upon our Solar System by chance many Centuries ago it is highly unlikely that any Alien Intelligence would even know we are here unless they live within a little less than 100 Light Years from Sol/Terra since we have only been emitting as a civilization since about 1920. Furthermore, Sol Terra and its neighbors seem to be located in a very remote part of the Milky Way Galaxy, and riding in a local bubble with a few other neighboring stars which in turn are further wrapped in a larger bubble of about 300 Ly radius, thus further masking our presence.

    Now certainly it is worth looking for evidence of habitable planets. habitable moons and even Advaanced Alien Civilizations beyond 100 Ly, but the fact of the matter is it probably won’t impact us very much for Centuries to come even if we find one since it will take us a long time to communicate with them even assuming we want to. The work above is very important, but lets narrow the issue to something that we can really grapple with in an organized fashion over the next 200 years or so.

    Bottom line, it would be very interesting to take the models above and apply them to an area of 100 Light Years from Sol/Terra to see how many habitable planets and moons there may be, especially given the unique “Geographical” features of our local neighborhood.

  • Phil March 10, 2010, 21:52

    Hmmm, one group counts M stars, another group discounts them because of tidal lock. Admittedly thats not going to account for magnitudes of difference but it seems clear we should try and decide whether M’s are ‘in or out’ of the club as a first job of work.

    Im hoping ‘in’, but I’m aesthetically biased – I want to know how life would work on the tidally locked world (or exomoon – which wouldnt be tidally locked!) of an M star – and they will after all, be literally keeping the fires burning many billions of years hence.

    Incidentally, what is the ealiest F star that is being included here? Are all groups agreed that the entire F sequence is fair game? An F0 star is a pretty feirce and short lived critter for multicellular life to develop I would have thought…


  • Tim March 11, 2010, 1:40


    Your questions are very good and do not have simple answers. Being a simple old retired geophysicist – I celebrated the successful end of my graduate studies and acceptance of my thesis (in astrophysics) by watching Neil Armstrong’s little stroll on the moon on a friends colour TV – I am likely not the best person to answer them; but I will try to offer my best anyways.

    Ultimately questions about habitability should really specify: habitable by what? Which brings up the uncomfortable question of: what is life? (Which is similar to “What is the purpose of life?) I rather like Sean Carroll’s take of this question over at Cosmic Variance: “The purpose of life is to hydrogenate carbon dioxide.” (see “Free Energy and the Meaning of Life”: http://blogs.discovermagazine.com/cosmicvariance/2010/03/10/free-energy-and-the-meaning-of-life/) However, leaving this most basic question alone, lets define life as something that we might recognize as life: i.e has some similarity to the only kind of life that we are familiar with. (We only have one sample, after all!) Then “habitable” defines an environment that might permit “our” life form. The question; “How did life start? is different from “Is a given planet, environment, etc. habitable?” Various hypothesis suggest life may have started in watery soups, in or near hydrothermal vents, on Mars, in interstellar molecular clouds, etc. but our question is where can it thrive, or at least, survive. (One can start a fire outside in a thunderstorm, but will it continue to burn?)

    The question about UV radiation: If we are talking about life as we would recognize it we know that UV at sufficient intensity will kill it (think sunburn, melanoma, etc.) or to quote Jianpo Guo from his paper Habitable Zones and UV Habitable Zones around Host Stars, “Ultraviolet radiation is a double-edged sword to life. If it is too strong, the terrestrial biological systems will be damaged. And if it is too weak, the synthesis of many biochemical compounds can not go along.” Here he summarizes a similar statement from “Ultraviolet Radiation Constraints around the Circumstellar Habitable Zones” (Andrea P.Buccino et al) “UV radiation between 200-300 nm becomes energetically very damaging to most of the terrestrial biological systems. On the other hand, UV radiation is usually considered one of the most important energy source on the primitive Earth for the synthesis of many biochemical compounds and, therefore, essential for several biogenesis processes.” arXiv:astro-ph/0512291 v2 18 Apr 2006. Note that even most marine ecosystems rely on photosynthesis by algae resident near the ocean surface as a base of their food system. Thus, a environment with extreme UV flux is likely to be hostile to life. Extreme fluxes of X-rays and or gamma rays, being of even higher energy than UV also have deleterious effects of DNA, proteins, and many other complex molecules, hence on life. It is true, however, that life could retreat into a deep ocean as a refuge from such radiation (and may have if the Permian extinction was caused, as hypothesized by some, by a gamma ray burst.)

    O and B stars could effect habitability in a number of ways. Because most (perhaps all) stars are born in clusters: that is the collapsing star forming nebula is large enough to form more than one star: stars of different masses are formed close together. For example, the Orion Nebula is a nurturing stellar nursery filled with hot young stars and their natal clouds of gas and dust. But for planetary systems, the active star-forming region can present
    a hazardous and inhospitable birthplace. While the formation of dusty protoplanetary disks seems common in Orion, Hubble Space Telescope close-up images dramatically reveal the torturous conditions they must face while trying to grow into full-fledged planetary systems. In each case, a central young star is surrounded by a disk substantially wider than our solar system. The disks likely contain material in the process of planet formation. However, withering ultraviolet radiation (and strong stellar winds) from one of Orion’s nearby hot stars is rapidly destroying the disks. Planet formation must occur quickly here, if at all. Researchers estimate that about 90 percent of Orion’s youngest protoplanetary disks will not survive the next 100,000 years.

    The danger of O and B stars going supernova is also present because these massive stars evolve much more quickly than dwarfs like our sun. Supernova explosions of O and B stars are not necessarily symmetric and the danger they pose to nearby companions will therefore vary, however, while a cluster is young distance between stars is much less that is the case for older clusters that have had time to disperse (dispersal due in part to differential rotation about the galactic centre.) This shorter distance then increases the danger of an explosion to adjacent stars and their planetary systems. In extreme cases (e.g. Eta Carinae) Phil Plait, the Bad Astronomer at http://blogs.discovermagazine.com/badastronomy/2007/06/20/eta-car-tick-tock-tick-tock/ says “When stars like Eta Carinae (at 100 – 150 times the Sun’s mass,) explode, they tend to shoot of beams of energy and matter that, at its distance of 7500 light years, could kill every living thing on Earth. But since it’s pointed away from us, all we’ll get is a spectacular light show.” However, if this sort of event hapens within a young cluster, nearby planets will be in trouble!

    As to your question “Why is plate tectonics considered important for life?” the short (and unsatisfactory) answer is that plate tectonics is an integral part of a planets surface, atmosphere, hydrosphere regulatory system. The earth’s plate tectonics regime has been sustained and prolonged by the role of water in lowering solidus temperatures. Crustal evolution requires the formation of vast volumes of granite [because granite is lighter than basalt it floats on the primordial basaltic “sea”, thus forming islands and continents.], which in turn is dependent upon the subduction of hydrated oceanic crust. In addition, the weathering process sequesters carbon dioxide in the form of limestone, etc. (CaCO3), From http://en.wikipedia.org/wiki/Carbon_cycle, “The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth. It is one of the most important cycles of the earth and allows for the most abundant element to be recycled and reused throughout the biosphere and all of its organisms.” I encourage you to read the rest of this article, it is quite good.

    Plate techtonics is also related to the hydrological cycle. Life as we know it requires liquid water. Physical habitability conditions are therefore often tied to the stability of water although it is speculated that other chemistries of life may exist It is interesting to note that the elements forming the basis of terrestrial life (H, C, and O) are also key elements controlling large scale planetary processes CO2 is the major atmosphere greenhouse gas (together with H2O and CH4) and by interacting with water, crustal rock, and life regulates the temperature of the atmosphere. Water is the major compound controlling mass transport (convective flow and melting) in the rocky planetary interior. Note that it is likely that before oxygen producing (and CO2 consuming) eukaryotes arrived in sufficient number, weathering processes may have sequestered so much CO2 that the planet cooled (due to lack of that good old greenhouse gas) enough to result in a snowball earth. Continued plate tectonics, accompanied by the resulting volcanic activity along subduction zones, released CO2 back into the atmosphere eventually warming the planet and melting the ice.

    Note also, that the apparent lack of plate tectonics on Venus resulted in there being no mechanism to sequester CO2 and so resulted in a runaway greenhouse effect that has definitely been deleterious to its habitability.

    In general, I think “life” should be seen as an integral part of the overall physical/chemical characteristic of some planetary bodies as not as something that “starts” independently.

    But then again, I could be wrong!

  • tesh March 11, 2010, 3:47

    Unlike Keith Harmon, I do not believe that there will be any “strong break throughs” that will enable us to travel out with the boundaries of the Sol system. Not in the next 200 years and probably not in the next 1000 years. We are more likely to send something akin to generation ships to neardy systems, which will not require fancy new physics.

    I believe that life is rare, intelegent life far more rare and the chances of two intelegent life forms co-existing in the same sphere of influence so unlikely as to most probably being impossible. I posted these points before but here are the reasons I think that we will not hear from ETIs:
    1. Any local ETIs (within 50-100 Light Years) that are behind us technologically, by as little as 50-100 years, will not be able to respond to a call from us or indeed call us.
    2. Any local ETIs (within 50-100 Light Years) that are ahead of us, by 50-100 years technologically, will not respond – there is nothing for them to gain. We do not have any particular resources that are not available else where – apart from maybe our real estate. If they want that then we had better hope they don’t call!
    3. The likely hood of a local ETI that is at the same level of development as us is too remote to contemplate. Further, it, like us, will be unable to recognise a signal from us or be able to efficiently send a signal to us – one that they are sure we will recognise.
    4. The univers is a BIG, BIG place and has been around for a LONG, LONG time. So, plenty of time for ETIs to come and go. “First Contact” between ETIs is just as likely to be of the forensic/archaeological kind as likely to involve a real conversation between two ETIs.

  • James M. Essig March 11, 2010, 11:27

    Hi Paul;

    Good God!

    10 million trillion habitable worlds, and this is just within the visible portion of our universe. If only one such world in 10 has a technological civilization, this is a million trillion civilizations.

    My guess and bet is the same as yours in this regards. I cannot help but feel that the universe is teeming with life.

    The number of animal species might be utterly astounding considering that we have about 1 million animal species on Earth including insects, one celled animals, amphibians, reptiles, fish, worms, birds, mammals, and the like. This could well work out to 10 EXP 24 species of animal life.

    Athena, perhaps you could correct me if I am wrong since you are a biology expert and although I am interested in biology, I know relatively very little about the subject even in regards to the taxonomic classifications of Earth’s biosphere.

    The observable univese may be just one relatively tiny portion of our universe, which may be just one universe among innumerable universes.

    Life might be so common in our universe such that perhaps we might need to view the laws of living organisms as just as fundamental to the workings of nature as the cosmos as say stellar dynamics, galaxy formation, and perhaps even cosmic phase changes since the development of life seems to be an ongoing and slowing progressing phase change.

    As I like to joke, if I could only find a cute ET kitten on Pandora to adopt as a pet, that would be well worth the trip.

  • Athena Andreadis March 11, 2010, 12:36

    As Tim so well put it, the issue of “habitable zone” hinges on “habitable for what?” Permit me to post a longish excerpt from my book on how to define life:

    The Mark of Cain

    “It’s life, Captain, but not as we know it.” How often must this phrase echo through high school biology teachers’ heads as they scrutinize this year’s batch of students? So it stands to reason that the first question they chalk up on the board is What is life?

    Let’s break this into small, discrete chunks. What distinguishes organic (i. e. carbon-based) chemistry from biochemistry?

    The answer is, Very little.

    This is why the results of the Viking probe from Mars and the possibility of primitive microbes in a Martian meteorite are so tantalizing and controversial. In the meteorite biologists found remnants of life processes, like the shadowy voices that a scientist heard when he had a stylus trace the grooves of a Sumerian pot. Any organic chemistry that is advanced enough is indistinguishable from biochemistry and, in fact, is its precursor.

    Cells of live organisms have a complicated apparatus, consisting of specialized molecules which execute specific tasks. Proteins are both structural and catalytic components of cells. Nucleic acids are the information archives and couriers. But these molecules, no matter how complex, are still organic compounds.

    So what characterizes the phenomenon of life?

    Living things metabolize; that is, they consume resources and release energy. Yes, but so does a flame. Living organisms grow and reproduce entities like themselves. Certainly, but crystals seeding out of solution do the same. Living matter has a high degree of organization and complexity. Absolutely, but an elegant computer protocol has similar characteristics. Remember, higher functions, such as social organization and self-consciousness, only enter the equation at the level of vertebrates.

    So, what is life?
    There is one vital (pardon the pun) distinction between life and its absence. But let us list the similarities first.

    Live matter is made of the same elementary particles as its inanimate counterpart — namely, fermions (quarks and electrons) which interact via boson exchange (gluons, photons and gauge bosons). Fermions, which cannot occupy the same space, constitute matter. Bosons make up the fields — electromagnetic, nuclear, gravitational.
    Like all other matter, life must obey the universal physical laws. Anyone who touches a high-voltage wire or jumps from a plane without a parachute, will discover that they are not exempt from either electromagnetism or gravity, whether they think that humans are the pinnacle of creation or not.

    Life shares with other complex systems the property of emergence. A system is emergent if it is greater than the sum of its parts and exhibits characteristics that cannot be predicted by its components alone. Live organisms are open systems with negative entropy (disorder). Life is the highest-order example of a “complex adaptive system”, the favorite topic of conversation and research at the Santa Fe Institute and other Centers of Complex Sciences.

    But life is subject to an additional constraint. It must also follow a genetic program. That is, live matter must contain an inner code which ensures that there will be strong continuity of form and function as the organism reproduces. The repercussions of environmental pressures on genetic programs are more commonly known as evolution.

    Life can be based on any chemical premise that will allow individual complexity, species diversity and most importantly, adherence to the genetic program. On Earth, we have one such premise — carbon.

  • Eniac March 11, 2010, 13:35

    Tim, thank you for this excellent summary in answer to my questions. If I could try to recap and perhaps bring up a few more points on the subject of necessity:

    I much appreciate your distinction between habitable and inhabited. Indeed, I think there is an unfortunate connection: Since oxygen is a product of life, any planet inhabitable by us (in the sense that we could live on it with a minimum of life support) would have to be inhabited by life already, although not necessarily complex life.

    It is my understanding that life, when it originated, was not photosynthetic, and thus not likely to have required exposure to sunlight. Also, I understand UV is blocked by water more than visible light, so there would always be a zone where photosynthesis could take place without dangerous levels of UV. Land life may well be inhibited by high UV. Is it possible, though, that UV shining on an oxygen atmosphere will always generate an ozone layer in proportion, sufficiently blocking the radiation, no matter how strong?

    O and B stars: If I understand you correctly, planet formation could be inhibited by another star that is close enough for their heliospheres to merge. Is that really a common occurrence?

    they tend to shoot of beams of energy and matter that, at its distance of 7500 light years, could kill every living thing on Earth.

    This to me this is hard to believe. Are there any calculations backing up such statements or is it pure dramatism? How much energy and matter would remain after spreading for 7500 light years, and how exactly would it kill every living thing on Earth? Anything arriving here will have to either penetrate or sweep away our atmosphere. Either way, the flux will have to be much higher than that which the sun already provides at a distance of 3*10^10 times closer. Applying the square law, you would require a power output around 10^22 times larger than that of the sun to do serious damage at that distance. I don’t think so.

    Plate tectonics:
    This is a complex subject, and that just makes it harder to conclude it is necessary. Let me address some points, understanding that you yourself have found some of the answers unsatisfactory:

    The carbon cycle: If I understand correctly, on an initially lifeless planet, plate tectonics will be a net producer of atmospheric CO2, which is needed for life to form. Perhaps without plate tectonics there would not be enough CO2? This would work, except both Mars and Venus have plenty of CO2, significantly weakening this argument. Could the opposite be true, could there be too much CO2 without plate tectonics? I doubt it, since life thrives on it and will tend to always sequester all it can access. Greenhouse effect? That just moves the habitable zone outwards a little bit, doesn’t it?

    I like the bit about the removal of CO2 cooling the planet (snowball Earth). If you consider this in conjunction with the steady rise in solar output (~30%, I think) there is probably a pretty tight regulatory feedback loop where increased life activity will reduce CO2 levels, which will reduce temperature, which will decrease life activity. This loop may be essential, but I do think it would work with or without plate tectonics. The point about Venus and the Greenhouse effect, again, does not really prove anything, because Venus might be just perfect for life with all this CO2, if it was just far enough away from the sun for the water to condense.

    None of this, of course, proves that plate tectonics is NOT necessary for life, but it leaves us wanting for better evidence that it is, before such a claim is accepted as the basis for further arguments, as it so often is.

  • Eniac March 11, 2010, 13:49

    I dug into the Eta Carinae story a little bit, and it appears that the rumors of our imminent demise appear to be greatly exaggerated. The Bad Astronomer does state: “When stars like Eta Carinae explode, they tend to shoot of beams of energy and matter that, at its distance of 7500 light years, could kill every living thing on Earth.”, but he does not substantiate this assertion at all. Later on, he says the brightness would be 0.0001 times that of the sun, which is comforting, but it presumably does not include the gamma-ray burst. Given, though, that the atmosphere is opaque to gamma-rays, I would tend to concur with what Wikipedia says:

    According to a 2004 study, a GRB at a distance of about a kiloparsec could destroy up to half of Earth’s ozone layer; the direct UV irradiation from the burst combined with additional solar UV radiation passing through the diminished ozone layer could then have potentially significant impacts on the food chain and potentially trigger a mass extinction.[2][74] The authors estimate that one such burst is expected per billion years, and hypothesize that the Ordovician-Silurian extinction event could have been the result of such a burst.

    Note that a kiloparsec is about half the distance to Eta Carinae. Thus it appears that such a “catastrophe” would be similar in nature to the ozone hole or global warming, potentially serious, but mostly press fodder and way short of boiling the oceans or melting the crust, as some have fancifully extrapolated…..

  • Duncan Ivry March 11, 2010, 14:09

    @ Tim, the simple old retired geophysicist ;-)

    Great comment! Very interesting! I appreciate very much, when professionals give us some of their knowledge.

    This I say as an complicated, old, non-tired … er … whatever.

  • NS March 11, 2010, 16:46

    I think it was A.G. Cairns-Smith who defined life as “anything that can be acted on by natural selection”. This strikes me as interesting in that it does not anchor life to any particular chemistry or genetics, but does distinguish it from other “far from equilibrium dissipative systems” [sic?] to which life is often compared.

    Already in over my intellectual head so I’ll stop…

  • Athena Andreadis March 11, 2010, 17:16

    NS, some computer programs are written in such a way that they evolve by natural selection. So Cairn-Smith’s definition is not quite definitive (he has an fascinating life origin theory, by the way). You may notice that in my excerpt I don’t specify any details of the genetic program. The continuity itself is the essential part.

  • Malcolm Ramsay March 11, 2010, 17:29


    “10 million trillion habitable worlds, and this is just within the visible portion of our universe. If only one such world in 10 has a technological civilization, this is a million trillion civilizations.”

    Think on this: even if only one world out of 10 million has a technological civilisation, that’s still a trillion. **That’s ** amazing!

  • Eniac March 11, 2010, 17:33

    Life can be based on any chemical premise that will allow individual complexity, species diversity and most importantly, adherence to the genetic program. On Earth, we have one such premise — carbon.

    I like the “adherence to the genetic program”, or “strong continuity of form and function” in self-reproduction as the defining characteristic of life. Minor quibble is that the premise might not be chemical. Mechanical would be another option, or something more exotic like nuclear. Also, computational, as in the Game of Life.

  • NS March 11, 2010, 18:15

    To Athena Andreadis, thanks for the reply.

    Yes, I did read Cairns-Smith many years ago (probably where I got that quote from memory) and was fascinated by his clay life origin idea. I hadn’t been aware of much follow-up and sort of assumed it was another idea that didn’t pan out, but I just ran across this:


    Just from what I can gather on a quick skim the researchers saw some problems with Cairns-Smith’s hypothesis, but at least point to ways in which it might be researched.

  • Athena Andreadis March 11, 2010, 18:20

    Eniac, see my reply to NS regarding the details (or lack thereof) of the program for self-reproduction.

  • kurt9 March 11, 2010, 18:53

    Maybe eukaryote life is rare. Maybe even simple pro-karyote life is rare.

  • Eniac March 11, 2010, 19:36

    Here’s an interesting example of a non-chemical, self-replicating organism with a genetic program and continuity of form and function:


    I would would not mind calling it life, but others might. If you do, you will have to specifically exclude artificial environments in your definition.

  • Zen Blade March 11, 2010, 21:16

    45 billion planets in habitable zones does not equate to 45 billion habitable planets… And 45 billions sounds kind of high. That comes to more than 1 in 10 stars… which would mean that within 20-30 light years there should be multiple terrestrial planets in habitable zones.

    That being said, I think there are a lot of worlds out there with microbial life… but nothing bigger than that.

  • Eniac March 11, 2010, 23:32

    A lot of people seem to be convinced that microbial life is common but “complex” life rare. I would like to challenge this notion. The questionable implication that microbes are not complex aside, I would submit that evolution is a well-understood and powerful force driving life towards higher and higher complexity, inevitably.

    In my view the largest uncertainty is in the first steps, when organic molecules started aggregating in cooperative relationships that led to the first “proto-organisms”, if you will. The gap between random arrangements of molecules and a fully functioning organism (as we understand it) is large, and our ideas about how it was bridged are very speculative. We are fairly sure that central to the process were auto-catalytic hypercycles, quasispecies, and self-replicating RNA molecules, but all we have, really, is idealized theoretical models. Plus we know practically nothing about what environment and what molecules other than RNA (lipids, peptides, clay, etc.) might have been involved.

    Once evolution takes over (presumably somewhere around the quasispecies stage), the path to ever greater complexity is pretty clear. There are obstacles, chief among them perhaps the accumulation of free oxygen necessary for animal life, but nothing nearly as mysterious as those origins.

    Where does the “primitive common – complex rare” notion come from? My best guess is from the observation that it took a few billion years for complex life to appear on Earth. That time, however, I would submit, is geological in nature, rather than statistical. Specifically, I think the delay may easily be explained by the need to saturate a variety of geological oxygen sinks before free oxygen could accumulate. Climate may also have had an important role, together with the steady increase in solar output. Neither oxygen accumulation nor increased solar output are flukes, both are likely to be present wherever conditions are suitable for photosynthetic microbial life.

    Why should complex life not be inevitable? What reasons am I missing?

  • tesh March 12, 2010, 11:02

    With regards to the post by Eniac ending, “Why should complex life not be inevitable? What reasons am I missing?”, I think I would like to track back and instesd suggest that life may be rare, complex life a little bit tmore rare (just a feeling) but technologically able life, very, very, very rare indeed. I don’t think that once you have life, multi-cellular life is a given and once multi-cellular life is present, self-aware and technologically able life is likewise not a given. The history of our planet seems to suggest that the evolution of technologically able life is a very rare event – to the point that is more likely NOT to evolve. As far as we know, there has only been one technologically able lifeform ever to evolve on Earth. That’s 1 in an almost infinate (infinate is a bit of exaguration but you get the idea…) number of life forms that have ever existed!

  • Impatient Mike March 12, 2010, 12:39

    With only our sample of one to go by we can speculate until the cows come home about extraterrestrial life.
    But at least it’s clever speculation by clever speculators here.
    Need more data. Hopefully we’ll see more data in the near future, 1 to 20
    years. (Kepler,HST(biomarkers very long shot).JWST(biomarkers not so long shot maybe),Allen Array SETI(who knows?).

    Say Eniac,how critical would increasing solar luminosity be to the rise of multicellular life?
    Wouldn’t other factors like the position of land masses and green house gas levels have had a greater influence on temperature and other conditions favorable for life? Ofcourse increasing luminosity is considered a given for all main sequence dwarfs but I thought that would be just another factor for life to adapt to,not a main requirement for complex life.
    Also Eniac,I believe complex life is everywhere,but I sure wish we had more data right now to answer these questions.

  • Zen Blade March 12, 2010, 14:31


    First off, I am a biochemist/molecular biologist (but not an evolutionary biologist). I agree microbial does NOT equal simple. Their ecology can be tremendously complex and their ability to form different structures and to an extent differentiate based upon their neighbors is definitely complex.

    However, based upon the currently established fossil record, and some speculation about evolutionary timelines… It seems likely that the transition from chemical life to biological life was quick. It also seems a virtual certainty that there existed an RNA world. BUT, if the transition to biological cells (which we could all agree upon) was quick… then this may suggest (sample size = 1) that such transitions can be similarly quick elsewhere. However, if we have had single cell organisms for literally billions of years… and perhaps relatively simple “multicellular” organisms as well…, the question becomes what took plants and animals (in particular) so long to get going?

    You are correct, maybe it was ONLY the atmosphere… which would still be a consideration for most other environments. But, that also means then that no large organism developed (and survived and left fossil remains) in that older environment.

    However, it could also be an effect of time… maybe it just takes a really long time to reach a certain level of genetic diversity or parts, which are required for the leap to multicellularism.

    But, it could also be that there is a certain advantage to being small in (genome) size with lots of offspring (generation time) that out competes being a bit LESS small in (genome) size with a few less offspring (generation time).

    Relative fitness in a particular environment and generation times are hugely important. How superior is having two or three or 10 cells (as a single organism) AND all the added genetic materials needed to have those cells work together, AND all of the raw materials to fully support those cells WHEN compared to a single-cell organism. There are many single-cell organisms that form colonies or clusters that work together to form larger structures, and some will even end their own lives or decide not to reproduce when in such structures… so there is clearly communication between these different cells, they know that their neighbor is a relative vs a stranger… and there are even mechanisms in some (all?) of these species that weed out cheater relatives within these communities… *I should point out at this point in time that the evolutionary pressures facing a microbe or generally very different from those facing a large organism. For animals and plants, energy is not such a big concern; we care more about information (genomes). Smaller organisms focus on energy and quicker generation times.

    In many cases it may be a better survival route to be a single cell that can work with 100 other cells (when needed), than to be stuck with the 100 other cells at all times, including when there are few resources, or when their is a predator around… We actual see similar behavior in animals via a different mechanism (herds confer partial protection from predators, for example).

    There are also clear cases (with regards to animals) of apex predators essentially being dead ends. There tend to be fewer of these larger organisms. So, if tragedy befalls a relatively small number of them (untimely death, illness, isolation), the entire species can be wiped out. And then a newer, slightly different version of the Apex predator rises up.

    So, unless a situation persists where being the biggest multicellular organism is the best thing since before apple pie (and probably for a significant period of time)… the development/evolution of such organisms may be a VERY difficult outcome to achieve.

    Eniac, here are some examples of how more “complex” organisms can arise though (you may know this already):
    *symbiosis could be a great start to becoming multicellular; can confer distinct advantages, but you still have dueling genomes. Chloroplasts/mitochondria are but two examples out of many. There are some modern examples of microbial organisms that as part of their life… “plan?” take in a second organism once they reach a certain stage in development.

    *Forming colonies. As I mentioned above, forming larger structures and having at least limited differentiation between the different members of the colony. But, again these organisms are not necessarily multicellular since they can survive when just a single cell. The question here is when would the community of organisms decide that the “time is right” to become one, forever.

    Again, I am not an evolutionary biologist, so I apologize for any mistakes in what I wrote above. But if anything was unclear, let me know, and I will try to better explain myself.

    -Zen Blade

  • Eniac March 12, 2010, 15:38


    Say Eniac,how critical would increasing solar luminosity be to the rise of multicellular life?
    Wouldn’t other factors like the position of land masses and green house gas levels have had a greater influence on temperature and other conditions favorable for life? Ofcourse increasing luminosity is considered a given for all main sequence dwarfs but I thought that would be just another factor for life to adapt to,not a main requirement for complex life.

    I think a 30% change in luminosity will beat out any other factors, except maybe for the greenhouse effect.

    From the few bits and pieces I have read on the subject (not being a geologist at all), my impression is that the Earth started out with a reducing atmosphere thick with CO2 and without oxygen. At the time, the sun was substantially (30%?) weaker, but the greenhouse effect made up for it to provide cozy (warm and carbon rich) conditions in which life could emerge.

    The appearance of photosynthesis led to a steady conversion of CO2 to O2 and carbon fixation, which, after a long geological struggle, led to the high oxygen and low CO2 levels we see today. There were lots of bumps along the way, such as the “Snowball Earth”. That one presumably happened because CO2 was diminished, the greenhouse effect subsided, and the sun was not yet strong enough to keep the Earth from freezing over. Animal life had to wait until a) There was enough oxygen, and b) the sun was sufficiently strong to keep the Earth from freezing without the CO2.

  • amphiox March 12, 2010, 15:39

    “That’s 1 in an almost infinate (infinate is a bit of exaguration but you get the idea…) number of life forms that have ever existed!”

    Or, alternately, 1 out 1 (=100%) of all habitable biospheres ever observed have produced at least 1 technologically capable species. 1/1=100% of all known planets that developed life developed diverse ecosystems consisting of billions (if not trillions of species). 1/1=100% of all known planets that developed life had at least one species evolve oxygenated photosynthesis. 1/1=100% of all known planets that developed life had geological events that allowed the accumulation of a high O2 atmosphere. 1/1=100% of all known planets that developed life developed rich and diverse multicellular life. 1/1=100% of all known planets that developed life had diverse complex environments and ecosystems in which at least some niches favored the evolution of intelligence. 1/1=100% of all known planets with life ultimately produced several lineages with pre-technological intelligence of varying levels of sophistications, and in 1/1=100% of all these known planets with life, at least 1 of these pre-technological intelligences successfully made the transition to technology and became spacefaring.

    Thus, the evidence clearly shows that 100% of all planets that develop life-supporting biospheres that endure for at least 3.5 billion years will produce at least 1 technological species at some point in its history.

    With a sample set of 1, pretty much all and every conceivable probabilstic argument is equally valid or invalid. Without more data, we just can’t say.

  • amphiox March 12, 2010, 15:51


    What we know about evolution on earth (and remember, sample set = 1) is that evolutionary processes seem extremely capable of filling almost every available niche provided by the environment with something or another.

    But, evolution has no foresight, and is constrained by history.

    That is, beyond the intial abiogenesis, B must come from A, always, and to get from A to B a pathway of intermediate steps must exist wherein every step is advantageous compared to the previous one (specifically, niches must exist for each step where that step has an advantage over the previous one. The new step doesn’t have to be superior to the prior step in every aspect, it could even be inferior in every single aspect except for one, but that one lets it colonize a new niche that its predecessor cannot, and so it survives).

    So that is the question. Will the niches that allow for the evolution of B (complex, multicellular life) from A (“simple” single celled life) exist universally in most or all planets that can support life, or are these niches rare and unique among all the possible habitable worlds in the universe?

  • Eniac March 12, 2010, 18:31


    These are all true arguments, but I am not sure they do anything for the “simple common – complex rare” hypothesis (perhaps they are not meant to). There is no one reason why multicellular organisation might be advantageous, there is a large variety of possible reasons. There is thus a large number of different paths from A to B. And multicellular organization can take many different forms, so there also is a large number of B’s in the first place. And you do not need physically separate niches, necessarily, most of the fitness of an organism is inextricably linked to the presence or absence of other organisms, which are evolving themselves, providing a large variety of environments and competition even in physically simple or homogeneous settings. Take the open ocean, which is as homogeneous as it gets, and still teeming with thousands (conservatively) of species of all kinds.

    In short, I have a hard time imagining that ANY evolutionary development would need some sort of very specific and rare kind of niche, or what such a niche would look like.

  • tesh March 13, 2010, 5:35

    “Or, alternately, 1 out 1 (=100%) of all habitable biospheres ever observed have produced at least 1 technologically capable species. ” – amphiox

    Very good! Of course I cannot put forward a counter but I’m still left wondering why it took 3,500,000,000 years for the first technological species to arise? Why not earlier? Why not later? I wonder if we, the n=1 of the Sol system” are slow or fast? I guess we may never know…

  • Ronald March 13, 2010, 10:30

    Fascinating topic, galactic ‘population biology’! One of my favorites.

    I have a few thoughts on this myself, which I will share soon (after a bit of searching and reading), but for now this:

    Tim and others, with all due respect: 7500 ly as a quoted ‘danger distance’ in case of supernovae is highly exaggerated. A typical case of careless quotation, I think.

    As Eniac rightly points out, all (diffuse) radiaten diminishes with the square power of distance.
    The maximum dangerous distance of a ‘normal’ supernova explosion for an earthlike planet with a biosphere, that I have read about (backed up with some calculations) varied from 10 to 30 ly.

    Only hypernovae/GRB’s could be dangerous up to a few thousand ly, if the burst is directed the ‘wrong way’.
    But GRB’s seem tot be extremely rare and distant, even at a cosmic scale, and typical of a specific type of galaxy during a specific period of its existence (mostly the much earlier universe).

    O and B stars are comparatively massive, but still (mostly) main sequence. Maybe some of them can become supernova, but not hypernova and surely not GRB.

  • Ronald March 13, 2010, 10:32

    Hasty typing: ‘radiaten’ should be ‘radiation’, of course.

  • Adam March 13, 2010, 21:55

    Just on the variety of habitable planets and the fate of our own, I personally think Earth is more likely to become a ‘desert planet’ with lakes near its poles and a lower atmospheric pressure. Such a planet will never experience a runaway greenhouse and perhaps allow some life to persist until the end of the Main Sequence. Such planets may well exist elsewhere in the Galaxy and shouldn’t be written off as ‘lifeless’ as we explore from afar.

  • spaceman March 14, 2010, 3:38

    45.5 billion rocky planets in the regions around the galaxy’s stars where water remains a liquid does not seem like an unreasoble estimate. The reason: all of the available observational evidence and the best planet formation simulations indicate that almost all stars should be accompanied by planets down to Mars size. In a recent Astronomy magazine article planet hunter Geoff Marcy estimates that ~90% of all stars have planets.

    Given the robustness and ease of the planet formation process, the distance between planetary systems in our part of the galaxy is probably equal to the average distance between stars, ~5 light years. Afterall, we know of a gas giant only 10 light years away– Epsilon Eridani b. If I had to guess, the nearest planetary system exists around Proxima Centauri even if it only consists of 3-5 mercury to Mars-sized small rocky planets well below the current detection threshold. The nearest planet that has liquid water oceans and at least microbial life akin to Earth ~3 billion years ago may be ~50 light years away. The nearest planet with complex multicellular life may be ~500 light years away given that more things have to “go right” for complex life to arise. Following this 5 X 10 logic logic, the nearest intelligent civilization may be ~5000 light years away . Based on these estimates, there may only be a several dozen technological civilizations within our galaxy right now.

    In sum:

    1. ~5 light years between planetary systems.
    2. ~50 light years between habitable planets with liquid water oceans and at least microbial life
    3. ~500 light years between planets with biospheres comparable in richness of multicellular life as Earth is.
    4. ~5000 light years between planets with technological civilizations.

  • T_U_T March 14, 2010, 6:05

    Adam. to that to happen, plate tectonics and magnetic field would have to cease before the increasing insolation starts the runaway greenhouse effect. That is going to happen in a billion years +-. The plate tectonics nor the core dynamo will be extinguished sooner, therefore there will be a plenty of volatiles to hang in the atmosphere and boil to death any life.

  • kurt9 March 14, 2010, 14:48

    “I personally think Earth is more likely to become a ‘desert planet’ with lakes near its poles and a lower atmospheric pressure.”

    Like Arrakas?

  • Terraformer (given name = Tobias Holbrook, getting my digital paper trail up) March 14, 2010, 15:33

    Perhaps we need to differentiate between Easily Terraformable Planets, Biocompatible Planets, and Habitable planets? Do remember that having a Habitable planet is pretty much dependent on life (to oxygenate the atmosphere, to avoid a runaway greenhouse effect etc), which throws into the mix the probability of life occurring.

    Plus, there’s the question of whether water worlds are conducive to life, given how much of the minerals will be locked away by a layer of exotic ice.

  • Eniac March 14, 2010, 21:51

    Adam. to [for?] that to happen, plate tectonics and magnetic field would have to cease before the increasing insolation starts the runaway greenhouse effect. That is going to happen in a billion years +-. The [neither?] plate tectonics nor the core dynamo will be extinguished sooner, therefore there will be a plenty of volatiles to hang in the atmosphere and boil to death any life.

    Wait a minute. If I understand you correctly, you are saying:

    1) Plate tectonics is why we have volatiles is why we have an atmosphere.

    If that is so, what about Venus? No plate tectonics, plenty of atmosphere?

    2) Increased insolation leads to greenhouse effect leads to extinction of life.

    How exactly would that happen? Increased insolation will lead to higher temperatures, but as long as life still thrives, it would also lead to more carbon fixation, so no CO2 greenhouse effect, until it is so hot that life succumbs, first, and then you get the CO2 greenhouse effect. Are you thinking about water vapor as greenhouse gas at increased temperatures? Something else?

    3) The magnetic field has an effect on climate

    Really? How?

  • Adam March 15, 2010, 5:04

    Hi kurt9

    A bit like Arrakis. A lot like Lowell’s Mars really.

  • Ronald March 15, 2010, 7:48

    Very interesting thread and discussion. But with so many unknowns!

    The variation in estimates of number of terrestrial and habitable planets in our MW galaxy that I have read about is just enormous, from a few tens of millions up to several tens of billions.
    I presume that much of this depends on the definitions of ‘habitable’ etc.

    As several people here already stated, planets and even terrestrial (small, rocky) planets are probably quite common. Even water (in any form) may be rather common.

    However, when it comes to habitable planets, meaning terrestrial planets with liquid water residing long enough in the Continuously Habitable Zone (CHZ) of a star to give rise to any biological (= carbon based) life, I would be a bit more cautious. And, although I have only read their abstract, I think that the Guo team may be overly optimistic. And I tend to go with the Kipping estimate or at least something much closer to that.

    I have done some ballpark guesstimating myself, based on what is known about the galactic ‘population structure’.

    First of all, though controversial, I tend to follow Lineweaver et al. (e.g. ‘The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way’; 2004) and Reid (e.g. ‘Extrasolar Planets: A Galactic Perspective’; 2006) with regard to the Galactic Habitable Zone (GHZ). This not just, and not even primarily, for the absence of life-extinguishing supernovae, but even more so for the abundance of required heavier elements and stellar ages. There seems to be a distinct galactocentric (radius) gradient with regard to metallicity. Which is why the halo is probably too metal-poor and unsuitable and the interior parts too metallic, too active and also unsuitable. And the GHZ is being defined as a torus (annular region) roughly between 6 and 10 kpc (about 20 to 33 ly) around the galactic center, within the galactic disk.

    If we start out with a slightly more modest total galactic population of 200 billion and cautiously assume some 10% of this within the GHZ, this means 20 billion stars of all types in the GHZ.

    I would be much more modest with regard to suitable star types and leave out all M dwarfs, but also any stars before F7 (i.e. most F) and any star beyond K3 (i.e. by far most K). This still includes all G stars. Furthermore, I would then also drop anything that is not main sequence (i.e. keep only V and maybe early IV), all true variable stars and all close binaries (say under 15 AU, of course making an exception for Alpha Centauri, to keep good relationships with the neighbors ;-) ). Finally I would also drop the stars with too low metallicity (below 1/3 of solar) or extremely young stars. Maybe also those with excessively high metallicity (say, beyond 3 times solar) and the very old (which would probably also show up as moving off the main sequence anyway).

    Depending on how strict we are, that would leave us, according to my own and other estimates of the local galactic population, with about 1.5 to 3 % of stars in the GHZ, which means about 300 to 600 million stars which are prime hunting territory.
    If we then assume that some 10% of these prime stars do indeed possess a terrestrial planet in a stable orbit in their CHZ, that leaves us with a final guesstimate of about 30 to 60 million stars with truly habitable planets in our MW galaxy. Very close to Kipping’s (and other) estimate. Maybe double this but probably not a lot more.

    Though this may seem like a high figure, and indeed looks promising with regard to the prospect of habitable (and terraformable) planet hunting, at the same time it does not make me overly optimistic when it comes to ETI and technological civs in our MW.

    I also tend to agree with Eniac about the atmospheric (and geological) conditions (oxygen accumulation) probably being primarily decisive for the rise of complex (multi-cellular, etc.) life on earth rather than pure statistics. Which in turn implies that there may a universal rule with regard to advanced planetary age (i.e. sufficiently old and ripe) and the rise of complex life.

    Though Lineweaver estimates (see above and ‘An Estimate of the Age Distribution of Terrestrial Planets in the Universe: Quantifying Metallicity as a Selection Effect’; 2001) that most earthlike planets in our galaxy are probably older (average almost 2 gy older) than we are, it still means that easily 80 to 90% of habitable planets could have only microbial life.

    Leaving some 3 to 12 million planets with complex life in our galaxy. Considering how rare advanced intelligence is on earth, how long it took to arise, and how totally unnecessary high intelligence is for life in general to survive, the question then becomes which small fraction of these will possess an (advanced, techno-capable) ETI. Most probably a minute fraction.

    Statistically speaking, the distance to the nearest star with a planet harboring complex life must then be at least tens of ly away, the nearest star with a planet harboring an ETI ??? Hundreds, thousands, …? As spaceman also indicated.

    BTW I agree with Terraformer on the need to differentiate between (easily) terraformable, biocompatible and habitable (the last one probably meaning inhabited) planets.

  • amphiox March 15, 2010, 13:03

    “In short, I have a hard time imagining that ANY evolutionary development would need some sort of very specific and rare kind of niche, or what such a niche would look like.”

    It isn’t really a question of rare or specific, but ranges. And the critical moment in terms of the question of multicellular vs unicellular is probably very early on, close to the moment of transition from free living individual cells to the very first multicellular groupings with the very first inklings of specialization. Thus – what are the range of niches in which remaining unicellular outcompetes the earliest first steps towards multicellularity, and what are the ranges of possible niches where the reverse is true, and what is the relative ratios of one to the other, among the range of different planetary environments out there?

    On earth this seems to have required the oxygenation of the atmosphere – both for energy generation and creation of structural support (oxygen is a critical component of both collagen and lignin for example) needed to build big complicated bodies. Before this happened, it is possible that multicellularity was fully genetically possible, but any variants that moved in that direction got outcompeted by their simpler, faster replicating unicellular relatives with lower energy demands.

    So one question would be whether or not oxygen specifically is necessary or if other chemical process can substitute, and if so, how common/likely to arise are these chemical processes?

    And of course there are countless other similar questions that can be asked about other various aspects of what it takes/means to be multicellular.

    The only way we can ever really answer any of them is to find other biospheres, other examples of abiogenesis, and compare them with our own and each other.

  • amphiox March 15, 2010, 13:15

    “but I’m still left wondering why it took 3,500,000,000 years for the first technological species to arise”

    Various possibilies come to mind:
    1. It really is very improbable, and so took a long time to occur on earth and is rare in the universe.

    2. It is quite improbable, but for any given level of average biodiversity, over a long enough time period, the odds of it happening at least once rise to 100%. For earth’s biosphere, that time period was 3.5 billion years. Thus it will be common or almost universal among biospheres of sufficient age, and it will occur sooner in biospheres more diverse than earth’s, but later in biospheres less diverse than earth’s.

    3. It is actually very probable, but requires a certain level of genetic variability that has to be built up, slowly, one mutation at a time, and so will always take several billion years to occur.

    4. It is actually very probable if certain other enabling preconditions are also met (such as oxygen atmosphere or something like that), but these preconditions depend on random events, and it just so happened that on earth these preconditions were not met in its first 3 billion years or so.

    5. It actually didn’t take 3.5 billion years on earth, because we are not the first technological species. We were actually preceded, perhaps by billions of years, by an earlier technological species that left the planet/went extinct, and we have just not yet found evidence of them yet because of the long time and sparcity of the fossil record.

    We can’t really say anything intelligible about any of these scenarios until we actually get a chance to examine another biosphere in detail and compare it earth’s.