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Habitable Zones in Other Galaxies

We often speak of habitable zones around stars, most commonly referring to the zone in which a planet could retain liquid water on its surface. But the last ten years has also seen the growth of a much broader idea, the galactic habitable zone (GHZ). A new paper by Falguni Suthar and Christopher McKay (NASA Ames) digs into galactic habitable zones as they apply to elliptical galaxies, which are generally made up of older stars and marked by little star formation. Ellipticals have little gas and dust as compared to spirals like the Milky Way and are often found to have a large population of globular clusters. Are they also likely to have abundant planets?

The answer is yes, based on the authors’ comparison of metallicity — in nearby stars and stars with known planets — to star clusters in two elliptical galaxies. While many factors have been considered that could affect a galactic habitable zone, Suthar and McKay focus tightly on metallicity, with planet formation dependent upon the presence of elements heavier than hydrogen and helium. Results from Kepler have backed the notion that metallicity and planets go together, although it’s a correlation that has so far been established only for large planets like the gas giants that are the easiest for us to detect. We can’t be sure that the correlation goes all the way down to planets the size of the Earth, but the idea seems logical.

After all, we think Jupiter-class planets formed by the accretion of gases around a rocky core, and the formation of that core may be similar to the formation of Earth-sized planets. The authors think the correlation between stellar metallicity and planets will extend to smaller worlds, while recent exoplanet discoveries help us explore the relationship and extend our thinking to elliptical galaxies. The galaxies in question are M87 and M32, and the investigation is one that invokes the history of star-forming materials. After all, heavy elements are produced inside stars, so the concentration of metals depends critically on the generations of earlier stars.

From the paper:

First generation stars would have low metallicity, whereas stars that form from material that has been through many generations of previous stars would have a high metallicity. Metallicity is important for planet formation because in the hot protoplanetary disc surrounding a star, the formation of protoplanetary bodies (small planetesimals) depends exclusively on high atomic weight elements since the protoplanetary masses are too small to retain hydrogen or helium. Earth-like planets are composed virtually entirely of compounds that are high in atomic number, Z (silicates) or bound to a high Z atom (H2O). Thus, it is reasonable that the metallicity should correlate with planet formation.

Metallicity can be considered in terms of the iron abundance ratio to hydrogen [Fe/H], which the authors use as their indicator, obtaining this value for a variety of nearby stars including many with known planets. Interestingly enough, the metallicity of exoplanet host stars as shown by [Fe/H] peaks at a value well above that of the Sun, leading the authors to comment “…the Sun may be a typical star, but it is not a typical planet-hosting star… It may be that we are lucky to be here.” The metallicity distribution is then extended to gauge habitability in the two ellipticals.

Why these two galaxies? M87 is an active galaxy (it has an active galactic nucleus, or AGN) along with a supermassive black hole, with the AGN thought to be powered up by the black hole, which is itself marked by two relativistic jets and a wide variety of emissions from the accretion disk. M32 is a compact dwarf elliptical, a low-luminosity satellite of M31, the Andromeda galaxy. Here we have a stellar population that is younger and more metal rich at the core than M87’s. The two are markedly different, giving us the chance to make comparisons between active and non-active galaxies.

Image: A composite of visible (or optical), radio, and X-ray data of the giant elliptical galaxy, M87. M87 lies at a distance of 60 million light years and is the largest galaxy in the Virgo cluster of galaxies. Bright jets moving at close to the speed of light are seen at all wavelengths coming from the massive black hole at the center of the galaxy. It has also been identified with the strong radio source, Virgo A, and is a powerful source of X-rays as it resides near the center of a hot, X-ray emitting cloud that extends over much of the Virgo cluster. The extended radio emission consists of plumes of fast-moving gas from the jets rising into the X-ray emitting cluster medium. Credit: X-ray: NASA/CXC/CfA/W. Forman et al.; Radio: NRAO/AUI/NSF/W. Cotton; Optical: NASA/ESA/Hubble Heritage Team (STScI/AURA), and R. Gendler.

So what do we get in terms of metallicity distribution in these galaxies? The outer layers of M87 from 10 to 15 Kpc from galactic center show metallicity consistent with planet formation. In fact, this region of M87 is shown to be more favorable for planet formation than the distribution of nearby stars. Note that the habitable region here must form well away from the area of the jet, which would have its star-making materials blown outward and would be bathed in radiation.

The younger M32 galaxy has metallicity conditions somewhat less favorable than nearby stars, but still shows a large fraction of stars with Sun-like metallicity values. The distributions the authors are working with are based not on individual stars but on star clusters, with the assumption that the metallicity values are accurate when thus extended. Overall, the older M87 is likely to be rich in planetary systems but both galaxies should support habitable zones:

We have compared the metallicity distribution of nearby stars that have planets to the metallicity distribution of outer layers of elliptical galaxies, M87 and M32. From this comparison, we conclude that the stars in these elliptical galaxies are likely to have planetary systems and could be expected to have the same percentage of Earth-like habitable planets as those in the neighbourhood of the Sun.

The only other paper I know about that studied galactic habitable zones in other galaxies worked specifically on barred galaxies, and to my knowledge this is the first time the concept has been extended to ellipticals. It would be interesting to see the GHZ studied more closely in terms of radiation, because Suthar and McKay are concerned only with planets that would support microbial life, which might not be extinguished by the same radiation that destroyed more complex life-forms. For them, metals leading to planet formation are the story. In fact, note this comment:

Complex life forms are sensitive to ionizing radiation and changes in atmospheric chemistry that might result. However, microbial life forms, e.g. Deinococcus radiodurans, can withstand high doses of radiation and are more flexible in terms of atmospheric composition. Furthermore, microbial life in subsurface environments would be effectively shielded from space radiation. Thus, while a high level of radiation from nearby supernovae may be inimical to complex life, it would not extinguish microbial life.

We’re early in the study of galactic habitable zones, but other factors like nearby supernovae, gamma ray bursts (GRBs), encounters with nearby stars and gravitational perturbations all play into the habitability picture when we start going beyond the microbial stage, studies it will be fascinating to see applied to types of galaxy other than our own.

The paper is Suthar and McKay, “The Galactic Habitable Zone in Elliptical Galaxies,” International Journal of Astrobiology, published online 16 February 2012 (abstract). Thanks to Andrew Tribick for the pointer to this one.


Comments on this entry are closed.

  • henk April 20, 2012, 10:30

    sextrillion habitable worlds in this universe. there are so many of those that we will never see. even if every 10000 galaxie’s have only 1 inteligent life form than still there will be a lot of them.

    We will never meet one of those biengs, maybe it is better that we will never meet them. We will never fight war against them. I think that inteligent life is very rare, but not as rare that we are alone. like what i said above 1 inteligent life form evey 10000 galaxie’s.

    papers like this are very interesting that we can find out that life can exist in most galaxie’s. I like to have more papers about life in other galaxie’s, They are very interesting

    Keep up the good work

  • ljk April 20, 2012, 15:44

    henk said on April 20, 2012 at 10:30:

    “We will never meet one of those biengs, maybe it is better that we will never meet them. We will never fight war against them. I think that inteligent life is very rare, but not as rare that we are alone. like what i said above 1 inteligent life form evey 10000 galaxie’s.”

    It is unfortunate that most often it seems when people think about ETI these days, the first assumptions are about whether they are warlike and conquering or not.

    Before H. G. Wells came along with his War of the Worlds in 1896, most thoughts on alien intelligences was pretty philosophical with little concern as to whether they would come here or not and what their intentions might be.

    Granted, in the old days the view was that anything above the sphere of the Moon was pure and lofty, but even when that ancient view started to go away after the age of the telescope, even the local aliens were busy with their own concerns.

    And as for Messier 87 and its potential habitability, this online paper should be of relevant interest:


    FYI: M87 has about three trillion stars compared to just 400 billion for our Milky Way galaxy. In good optical telescope images of that giant galaxy, you can easily see many globular star clusters hovering around M87 like bees around a hive.

  • NS April 20, 2012, 20:14

    It’s estimated that diseases introduced by visitors from the Old World destroyed 90% or more of the New World’s population. This happened no matter what the intentions of the Old World visitors were. Presumably (peaceful) advanced aliens would be more careful of us, but we can never be sure that something about them or their technology might not be deadly to us, even if it’s unintentional.

  • Michael April 21, 2012, 4:30

    I am on the side that believes intellgent life, capable of technology is very rare, 1 or 2 per Gallaxy, it looks like there are so many conditions required it boggles the mind. I also believe simpler life forms are quite common, there are so many locations they can take root, for example if we took certain bacteria to the oceans of Europa I am sure they would take root there, if they are not already there.

  • andy April 21, 2012, 8:41

    I’ve seen a few suggestions around that the galactic bulge in our own galaxy should be considered as potentially habitable: apparently the supernova rate and the dynamical environment aren’t too hostile. A couple of planets have already been found in the bulge using gravitational microlensing.

    The night sky on a planet in an elliptical galaxy would be quite different from what we see here. A close-up view of the jet in M87 could be quite an impressive sight!

  • A. A. Jackson April 21, 2012, 9:05

    One never knows what to say about the appearance of an idea in the scientific literature and in science fiction.
    I don’t think the idea of habitable zones in galaxies has been extensive in prose SF, but Larry Niven comes to mind and I am sure there are others , and this would have been a long time before Gonzalez, G., Brownlee, D. & Ward, P. (2001).

    They are not hard over on it , but does Chris McKay really subscribe to the Ward, and Brownlee ‘Rare Earth’ thing now? This concept has had a considerable number of criticisms since the appearance of the book , in particular David Darling.

  • ljk April 21, 2012, 10:36

    If you read the paper I linked to above, the author suggests that M87 may be rich in advanced life forms as not only is it a huge and ancient stellar island in its own right, but it is part of the massive Virgo Supercluster of galaxies.


    By comparison, our Local Group (once again astronomers go with the obvious and unimaginative name) of galaxies is at the fringe of the Virgo Supercluster (65 million light years) and consists of only two large barred spirals, us and Andromeda (Messier 31), and one smaller unbarred spiral named Triangulum (Messier 33). The rest are mainly dwarfs and most of them are satellites of the two large spirals.

    So maybe that is another factor in the requirement of life, many large older galaxies in relative proximity to each other.

  • anoooo April 21, 2012, 12:29


    Pretty sure the main problem in galactic bulge is not the supernova, but the radiation from the super-massive black-hole of the galaxy center.

  • Eniac April 21, 2012, 12:39

    There would be very few places in any galaxy where planets could not exist, and even much fewer planets where life could not exist because of interstellar goings-on. Supernovae and gamma-ray bursts do not produce anything not stopped by an Earth-like atmosphere at even a few light years distance. They would have no bearing on microbial or complex life. Perhaps if the stellar density got too large, such as in the accretion disk of the central black hole, or some globular clusters, you could have frequent interactions of stars that strip away planetary systems, but such places would be very special and very limited in extent. The only reasonable condition I have heard of that might define a GHZ is metallicity and its consequence for planet formation. The authors here apparently agree with this, although I would not hesitate to remove the distinction between microbial and complex life in this context.

  • jkittle April 21, 2012, 22:55

    If we were able to observe evidence of life in a nearby galaxy there are two problems. The first is the obvious… it would take a long time to communicate and an even longer time to visit. The second is also worth noting. Anything we observe today is in the deep past! Thus any SETI signal or observation of an inhabited planet was so far in the past that even if we were to INSTANTANEOUS travel there, the civilization would be extinct or perhaps evolved beyond our comprehension. To meet these extra galactic ET’s we would need a time machine!

  • Brasidas April 23, 2012, 0:07

    Could some additional factors make certain zones in galaxies inhospitable, or at least uncomfortable, to life, especailly in the densely populated neighborhoods near the galactic core? For example, could dust clouds from nearby supernovae or other phenomenon impinge on the heliosphere, leaving the planet exposed to radiation from cosmic rays, even if it were in the HZ (I beleive Gary Zank has done some work in this area)? Could debris from other systems cause frequent high-energy impacts, or gravitational forces from more numerous and densely packed nearby bodies lead to unstable orbits or send frequent bombardments of local comets and asteroids? It seems additional factors beyond these could make life in some zones of a galaxy at a minimum less likley.

  • Anthony Mugan April 23, 2012, 5:09

    “maybe it is better that we never meet them”…

    Can I ask your various opinions on the idea presented in very tentative, preliminary and hopefully not too protracted form below…

    This study certainly fits in well with data for our galaxy which suggests that the availability of real estate shouldn’t be a limiting factor. A while ago we had an interesting discussion around the ‘L’ factor in the Drake equation, with most comments being supportive of David Grinspoon’s idea (Lonely Planets, 2003) that if an advanced civilisation reached the ability for interstellar travel the life expectancy of the civilisation would be dramatically increased – perhaps even becoming more of less indestructable. That conversation then ended with the thought that we couldn’t really say anything about the remaining terms in the Drake equation, around the origin of life and the evolution of intelligence / emergence of an advanced civilisation. I’ve been thinking about that and would like to test out some provisional thoughts, as I am sure the commentators on this forum would be both rigourously challenging, spot any major mistakes very quickly but also be balanced in their comments. It’s a three step argument with conclusions at each step which seem interesting…The assumption I am starting with is that DNA based microbial life occurrs on a reasonbably large number (lets say >100) planets elsewhere in the galaxy, I shall leave debate about the origin of life, or its appearance, to another time.

    1) Evolution happens.

    To be a bit more specific:
    a) Evolution always begins relatively small and simple – in fact one would expect it to begin about as small and simple as you can get. At the starting point for a given planet there is zero variance in characteristics (i.e. the first self replicating single celled organism).
    b) Life must be expected to experience natural selection, with chance variations experiencing selection pressure which leads to increased variance in characteristics over time to fill available ecological niches and in reaction to changes in the overall fitness landscape of the environment and ecosystem.
    c) Multicellularity has evolved independently more than a dozen times (e.g. Bonner, J., 1998, Origins of Multicellularity, in Integrative Biology, doi:10.1002/(SICI)1520-6602)
    d) Most terrestrial planets are thought to be significantly older than earth, by on average 1.8 billion years (Linneweaver, 2001)

    Tentative conclusion from step 1 – if life does appear elsewhere we would expect to find mainly complex ecosystems including multi-cellular life rather than simply microbial life alone.

    2. Emergent patterns arise in complex systems

    Any discussion of anything remotely resembling a trend in evolution is dangerous ground, so I’d best just emphasise that I am not talking about any purposeful direction of change, rather I am talking here about the sort of emergent features that arise from the mathmatics of complex systems. As discussed above there is no great surprise that the peak value in more or less any characteristic (genome complexity, encephalisation ratios etc etc) tends to increase over time – its nothing more than would be expected from statistical variance given that evolutions starts small and simple (off memory I think Stephen Jay Gould made that point back in the 1980’s). Such patterns may eventually stop if there are upper limits to a characteristic.
    What may seem a bit more surprising is the regularity that appears in the rate of change in the peak value of many of these characteristics. Genome size has followed a log linear rate of increase in the maximum value present at any one time throughout geological time, whilst encephalisation ratio has followed a log-log linear rate since the Cambrian, for example. (e,g. Russell, D., 1981, Speculations on the Evolution of Intelligence in Multicellular Organisms, in Life In The Universe http://history.nasa.gov/CP-2156/ch4.3.htm#263).

    The consistency of such relationships is not surprising from the perspective of complexity theory. A question I have been thinking about is the extent to which such patterns may be expected to occurr universally. At the qualitative level I would tentatively suggest the answer is we would expect to see a pattern emerging as these are functions of the dynamics of complex systems. At first sight one might not expect to see consistent quantitative values to the slopes but what is most surprising to me (as someone with a background in the earth sciences originally) is the stability of the relationships over time, given the very great changes in the evironment over geological time and in the details of the ecoystems involved over time. I wonder if it might be possible to test the variation in such emergent patterns in evolution using computer modelling. For now I would just raise the possibility that the stability of such unltra-long term patterns may be telling us something fundamental about the dynamics of DNA based ecosystems in terrestrial environments and may therefore be of astrobiological relevance.

    3) Convergent evolution happens
    OK – controversial heading, but if you think complexity theory has a basic point (which I do) then convergent evolution really follows from that.
    If we agree that there are many examples of convergent evolution, including multicellularity, relatively limited numbers of basic body designs, the eye and language etc etc (Conway Morris,. S, 2003, Life’s Solution) this leads me to a rather more radical question….but first a little background to where I’m coming from with this one.

    The trend for encephalisations discussed above is a long term stable emergent feature within evolution – even quite major extinction events seem to have produced quite small blips, followed by rapid recovery to trend (as expected by punctuated equilibrium models). Encephalisation does not equal a species capable of a technological civilisation – that requires high intelligence, certainly and encephalisation may be a requirement for that, but it also at least requires tool making, patterns recognition and language abilites. A question I began to wonder about is. if we expect multicellularity to appear generally (from point 1) and if we would the expect diversification accross available niches, including strategies that involve intelligence, to what extent would the parrallel emergence of a species with all the attributes needed for a technological civilisation just be a matter of time. Professor Conway Morris certainly argues that the evolution of humans was inevitable, based on a very strong interpretation of convergent evolution, but I would argue here for something less definitive, but still very significant if correct.
    There are only so many ways in which a strategy for higher intelligence can provide a selection advantage. As far as i can see these include pattern recognition of sensory inputs for finding things such as food or mates or avoiding predators, better abilities for movement in complex environments, including judging distances etc for jumping or landing (and therefore gripping) or collaboration strategies within the group which involve some means of communication.

    My final point is therefore that the species involved in a strategy of high encephalisation are quite likely to include one or more of the strategies favouring characteristics neccessary for a technical civilisation. If the emergent relationships in evolution rates for the maximum values of particular characteristics have a wider applicability (and they have certainly proved to be stable over hugly long time periods with very variable environmental conditions here on earth) then perhaps the chance combination of characteristics needed is indeed only a matter of time, and perhaps with a much more predicatable time period needed than might have been expected. Again the variance in such trends would need to be modelled by computer simulation to see how differences in simulated evironments, timings and scope of extinction events etc impact, but I would predict a high degree of stability in the relationships, given the actual patterns seen over geological time.

    Even if all the above is viewed as plausible, chance events such as major extinctions may occurr and individual planets may cease to be habitable before things can get to an actual civilisation. For planets in the habitable zone of stable, relatively small (but not too small) stars such as G type stars like the sune, the odds on such a civilisation emerging may actually be surprisingly high. This would also imply that the key variable remaining which may lead to a very low incidence of ETCs would be the origin of life itself, but that’s another discussion..!

    All thought, criticisms and suggestions for further work gratefully recieved, any delay in reply partly reflects work commitments but would probably mean you’ve got me thinking!!

  • Ronald April 23, 2012, 6:45

    I am pleasantly surprised that the giant elliptical galaxy M87 has such good metallicity, even more so than our own MW galaxy. I would have expected that such a galaxy with many old stars would be rather metal-poor, but apparently the situation is this favorable.
    Is this also known for our giant neighbor M31, Andromeda? I would think so. Does anyone know of metallicity data, and preferably even more detailed abundance data for M31?

  • Rob Henry April 23, 2012, 20:04

    Anthony Mugan,

    You are right there was consensus that L tended towards infinity as an ETI started to employ interstellar travel. However the lifetime of a type II civilisation that forgoes such travel is of more importance to those who want to make radio SETI work. I postulated that L for a civilisation that has spread throughout their solar system is at least 100 million years, but other have put it much lower or higher.

    We have previously discussed f(l) much, and all the evidence of our one example (and myself and others would say this is backed by expectations from theoretical considerations), save its early start here, points it it being very rare.

    f(i) is indeed a tough nut to crack.

    I am surprised by your assertion that the average log genome length of the most complex creature on Earth has consistently shown a linear increase with time. As far as I see it, even if there was such a trend, we are only likely to be able to date it from the first appearance of eukaryotes, and I suspect this is once again looking at metazoans over the past 600-700 million years. Anyhow I would love a reference if possible, and you are right to think that this breaks Gould’s model, unless we also add that increases in complexity are often via gene duplication or by the fusion of the genomes of two different creatures. Anyhow if you use Gould’s model you should emphasise how it helps drive complexity upwards by random decreases in genome complexity being limited by the very high minimum complexity required for any freeliving organism.

    Emergent complexity sort of answers every evolutionary mystery, and sort of answers nothing. Anyhow, The Modern Synthesis postulates that natural selection works only in small steps allowed by typical advantageous mutations, so thoughts of anything else are radical.

    The increase of metazoan EQ with time is well know, but what this means for other living worlds is tricker. Note how it only came about after there were high O2 levels, and the increase was rapid thereafter (the same can be said for the rise of metazoans in general). Also look at that weird chart that notes a rise in that EQ on a log-log basis. Many others have noted that also – but look again at the chart and you will notice something incredible. The ordinate is not really log t, it is log(T-t), where T=now. It postulates that some animal alive today should have an infinitely large ration of brain to body! I note that the best fit to actual data puts T at about 10,000-20,000 AD. Also note the Flynn effect and the exploding occurrence of caesareans among the human population. Both these trends are far faster than normal genetic selection can explain. Perhaps something is happening here that we do not understand and their really is something to emergent factors. Then again perhaps not.

  • Rob Henry April 23, 2012, 21:20

    Anthony, I should have also mentioned the following, but labelled it as too radical too take seriously just yet…

    The relationship between brain size and body weight as given in your reference (and as used elsewhere) is the same as if you would get if you postulated that brain size is always as large as it is possible for an animal to carry, because this is the same power relationship as you get between basal metabolic rate and size. What you would then need to add to this is that intelligence per brain mass (as given by the changing typical complexity of its architecture with time in the fossil record) has increased, progressively alleviating its burden with time, and giving natural selection less power to reduce its size.

    In the unlikely case that brain size just increases naturally as a side effect of other processes in metazoa, it would make you question of whether metabolic rate or body size per brain mass is the better fit in a cross phyla comparison. I note that those fire making tool using hobbits ( homo floresiensis) showed that humans with brains the size of chimpanzees are far more intelligent than them. To me that opens the possibility that natural selection allowed human brain size because of its greater sophistication, not the other way around.

  • Anthony Mugan April 24, 2012, 4:21

    @Rob Henry
    Thanks for your thoughts – as always very rigorous.
    F(l) – yes – a tricky one to crack and not one I’m really trying to get into with this discussion. I suspect it is the really key ucertainty in any attempt to assesses the frequency of occurrance of ETCs. If the panspermia model should get to the point where it has to be accepted (which seems to come down now to the long term viability of micro-organisms in deep space environments) then game over, but if abiogenisis is needed on each planet seperately then I would tend to concurr that f(l) would probably be quite low, with the early appearance of life on earth sitting rather awkwardly with that, but quite possible as a statistical fluke.

    I shall come back to you with the reference on the log -log rate of expansion of the genome – the one I have instantly to hand I’d prefer not to use as it is using the data as part of a different argument rather than being the original source as it would add a confusion to the discussion if I used that one…It does include single celled organisms and extends back over 3by though. It’s continuation through the oxygen catastrophe was one of the most striking things in terms of stability over time and accross wide environemental differences, which made me think about a general applicability.

    Interesting point about T on the encephalisation rate chart…One thing I didn’t say much about (it was getting far too long anyway) is that there will often be upper limits to the possible range of variation on a specific characteristic – which may be variable over time and space of course. Humans use a very high proportion of our total energy budget in keeping our brains going (I think off memory something like 25% for a newborn and a bit less for adults). One wonders if we are fairly close to some practical upper limit on this particular characteristic – all trends eventually stop and that one is pretty clearly not going to go on much longer.

    I agree with you about one of the big problems of a complexity theory approach to this is that, beyond relatively simple scenarios the maths becomes horrendous and we can’t adequately quantify the dynamics of the fitness landscapes at the moment, so yes, ‘explains everything but also nothing’ is spot on. My use of it as a conceptual framework for the concept I’m proposing is based on the fit of what the theory would predict qualitatively and what appears to be observed. I intend to try to develop my programming skills, biology knowledge and available computing power to try to begin to explore computer modelling of very simplified simulated environments to try to assess how sensitive the variance on such patterns actually is – suspect that could keep me happily amused for decades though as it would inevitably be very simplistic for the forseeable future.


  • Anthony Mugan April 24, 2012, 4:34

    @ Rob

    Ref for the long term trend in functional genome complexity:

    Sharov, A. A. (2006). Genome increase as a clock for the origin and evolution of life. Biol. Direct, 1, 17. http://www.biomedcentral.com/content/pdf/1745-6150-1-17.pdf

    The online version includes referees’ comments and author’s responses which is quite good. I personally don’t think he’s right to simply extrapolate the trend back as he does despite my general view on this sort of thing, but the data relating to evolution here on earth is interesting…

  • philw1776 April 24, 2012, 16:57

    This paper cited by Nigel Calder claims that nearby Supernovae along with plate tectonics drives the Earth’s climate, sea level and mass extinctions. If so, you want to orbit through galactic zones with moderate but not too frequent supernova flux.


  • Rob Henry April 24, 2012, 22:40

    Anthony, the author of that paper has many misgivings, which he is wise to highlight. I would like to add a little to his extensive list.

    First he rather likes to referring to grades of animals rather than clades, such as fish and worms, rather than gnathostomes and nematodes. This unhelpful trait brings me immediately to an aside. At a the level of phyla, the largest counterexample to the simple clades are primitive idea is “worms”. It now seems that several phyla of worms seem to have originated deep within the morphologically defined phylum of worms that is by far the most complex- the annelids. Here we almost have to make the most primitive forms also the most complex.

    Secondly, while rightfully acknowledging the difficulty of measuring morphological complexity, he then (in understandable desperation), uses the widest definition possible of coding in the genome. This makes me think of Tolkien’s imaginary language of Entish where saying anything at all takes a very log time – ie that sequence may be vital, but the information content of most of it is almost certainly very low. Due to his consistent employing of this measure, he has almost certainly recorded an increase in eukaryote genomic information, and just as certainly has overstated it.

    Thirdly, while again rightfully acknowledging that he should be using only freeliving organisms, he uses parasitic ones to estimate the genomic complexity of bacteria.

    I would love to see the result after an attempt is made to straighten all these problems. Certainly the slope will be much lower, but what interests me is if he still recovers a trend that can go back further than the rise of metazoa? If so, can it be further extended past eukaryotes?

    If the above can be achieved and consistent slope still recovered, then I agree that this has radical consequences for the rise of intelligent life on other planets. Also, it would then be interesting to speculate what anaerobic creatures that are just as complex as our metazoa would look like!

  • Eniac April 24, 2012, 22:41

    … I would tend to concurr that f(l) would probably be quite low, with the early appearance of life on earth sitting rather awkwardly with that, but quite possible as a statistical fluke.

    Fluke is not the right word. Biased sample is a better explanation. We have a sample of one, but it is an extremely biased sample. Even if only one in trillions of worlds were to have life, the one we are sampling has to be one of those few, with 100% certainty.

  • Anthony Mugan April 26, 2012, 7:54

    Rob (and also Eniac)

    Thanks – yes I agree that there are indeed many technicalities around quantifying complexity and identyfying the maximum value present at any one time. I will attempt to do further work on the maths of simulated evolution and watch out for new data on actual patterns…as you say, if the basic proposal holds up to scrutiny then the idea does seem to have radical consequences. It depends on the level of connectivity within the system as to if emergent patterns arise – there seems to be a lot of evidence to support the idea that biological systems are often around the ‘edge of chaos’ where such interesting patterns emerge, although early claims for complexity theory were sometimes overstated by the look of it. It doesn’t actually surprise me too much therefore to find a hint of such emergent patterns on the largest scales, but it really does need more work before the idea would be worth trying out in a paper (hence my floating it here first…!)

    Eniac – yes, biased sample is indeed the problem with f(l) -all we can really say with certainty at the moment is the probability of life appearing isn’t actually zero.

  • Eniac April 26, 2012, 22:23

    Eniac – yes, biased sample is indeed the problem with f(l) -all we can really say with certainty at the moment is the probability of life appearing isn’t actually zero.


    Although, you could argue that we have already sampled the whole galaxy, in a limited way. The fact that our world has not been colonized tells us that, so far, nowhere in the galaxy has a species arisen that successfully went out to colonize the stars the way we are imagining we might in the future. This puts an upper limit on the entire Drake equation, with a factor added for interstellar colonization, but does not really constrain f(l) all that much.

  • Anthony Mugan April 30, 2012, 9:08

    @ Eniac

    no species has arisen that has successfully colonised the stars – absolutely, that is either definately the case (as we are still here) or just conceivably they haven’t got here yet…Not sure i can concurr about the implication for the drake equation. If ETCs exists then each emerging civilisation will eventually have to link into a long established club as a very junior member – the rules of the game will already have been set. Thankfully they either don’t exist or choose to allow emerging civilisations to proceed in the own way until they are ready (I do agree that in terms of the original Drake equation, the second of my options does imply the fraction choosing to communicate with us in any obvious and clear way would be zero)

  • Eniac April 30, 2012, 18:38

    If ETCs exists then each emerging civilisation will eventually have to link into a long established club as a very junior member

    Much more likely: There would be no emerging civilisations because the original ETC have settled all the stars and are not nice enough to eternally quarantine all of the habitable planets. Why would they? Because sometime in the next few billion years or so intelligent life might arise on some of them? Give me a break. Unless, as you point out, two ETCs arise at almost exactly the same time, each before the other can settle their home star. Then, we would have a territorial competition at our hands. Now the galaxy would be filled even faster, and no other “junior civilizations” are likely to follow.

    This latter scenario, however, is unlikely, because if ETCs arise this often, the event would have occurred long ago and we would be former colonists, ourselves.

  • Anthony Mugan May 1, 2012, 14:01

    Very good point about the deep time aspect arguing against the zoo hypothesis…I shall have to have a careful think about that one as it sounds very logical to me.

  • Rob Henry May 3, 2012, 22:45

    Putting aside the Fermi paradox, and assuming that the optimists were right, and that we may expect in the order of a million ETI’s to have arisen by now, given no interference between them, the it is statistically unlikely that the first arising civilisation had sufficient advance of the next dozen to have completed galactic colonisation first. This would provide conflict that must be resolved. The zoo hypothesis should really be seen as a by-product (of the fine print?) of that resolution.