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On 300 Million Habitable Zone Planets

We’ve talked about the Drake Equation a good deal over the years, but I may not have mentioned before that when Frank Drake introduced it in 1961, it was for the purpose of stimulating discussion at a meeting at the National Radio Astronomy Observatory in Green Bank, West Virginia that was convening to discuss the nascent field of SETI.

This was in the era of Drake’s Project Ozma and the terms of the SETI debate were hardly codified. Moreover, as Nadia Drake recounts in this absorbing look back at her father’s work in that era, Drake had spent the time immediately before the meeting trying to line up Champagne for UC-Berkeley biochemist Melvin Calvin, who was about to win the Nobel Prize.

So there was a certain ad hoc flavor to the equation, one that Drake assembled more or less on the fly to clarify the factors to be considered in looking for other civilizations. How Drake did all this while trying to locate a sufficient quantity of good Champagne in the rural West Virginia of 1961 is beyond me and adds to his mystique.

Image: Astronomer Frank Drake speaking at Cornell University in Schwartz Auditorium, 19 October 2017. Credit: Wikimedia Commons CC BY-SA 4.0.

Sparkling wine aside, the Drake Equation in various forms has continued to inform discussion. The likelihood of detecting alien civilizations could be approached by multiplying the seven factors Drake came up with, which are shown in the figure below. The number of detectable alien civilizations is N. The Drake Equation obviously relied on guesswork at the time, given that we knew little about the factors involved other than the rate of star formation.

Image: The Drake Equation. Credit: Ming Hsu (UC-Berkeley).

There’s still a lot of play in these numbers, of course, but it’s fascinating to watch the progress of exoplanetary science as we begin to fill in the numbers through actual observation. Notice in particular ne the number of planets, per star, that could support life. This value is what gets play in the recently released (on arXiv) paper from Steve Bryson and a large number of colleagues at the SETI Institute, NASA and a variety of other organizations.

What catches the eye is the figure of 300 million, which is the number the researchers give for potentially habitable planets in the Milky Way. Let’s drill into this a bit: The researchers are computing the occurrence of rocky worlds, defined here as planets within a certain range of radius (0.5 R ≤ r ≤ 1.5 R), orbiting stars with effective temperatures of 4,800-6,300 K. The host stars cover main-sequence dwarf stars from Kepler’s DR25 planet candidate catalog as well as stars in data compiled by the European Space Agency’s Gaia mission. As the authors note: “We base our occurrence rates on differential population models dependent on radius, instellation flux and host star effective temperature.”

This is a change of pace from the norm, so let’s turn to the paper:

Most of the existing literature on habitable zone occurrence rates are in terms of orbital period, where a single period range is adopted to represent the bounds of the habitable zone for the entire stellar population considered. However, no single period range covers the habitable zone for a wide variety of stars…While these period ranges cover much of the habitable zone for G stars, they miss significant portions of the habitable zones of K and F stars, and include regions outside the habitable zone even when restricted to G stars. This will be true for any fixed choice of orbital period range for the range of stellar effective temperatures required for good statistical analysis. Such coverage will not lead to accurate occurrence rates of planets in the habitable zone.

Hence the decision to work with instellation flux, which measures the photon flux on each planet as received from its host star. The authors say that this is the first paper on occurrence rates for habitable zone planets that operates on star-dependent photon output. In terms of effective temperature, G-class stars like the Sun are in the range of 5,200–6,000 K. F-class is 6,000–7,500 K, but as the paper notes, the paucity of F stars in the sampled data leads to the authors setting the temperature limits lower. K-class stars show up at effective temperatures of 3,700–5,200 K. The range used in this paper — 4,800-6,300 K — also excludes M-dwarfs, whose effective temperatures range from 2,400–3,700 K.

Leaving out M-dwarfs could substantially under-count habitable zone worlds, but we also have enough concerns about tidal lock, stellar flare activity and atmospheric loss that we can’t assume M-dwarf planets are habitable. In any case, the authors have other reasons for the decision, including a very practical matter of future observation. After all, an analysis like this may well be useful as we ponder our target lists, and we also have to remember the limits of transit observation Kepler had to deal with:

The reason for limiting to Teff > 4800 K is two fold: (1) The inner working angle (IWA, the smallest angle on the sky at which a direct imaging telescope can reach its designed ratio of planet to star flux) for the LUVOIR coronagraph instrument ECLIPS falls off below 48 milliarc sec at 1 micron (3λ/D) for a planet at 10 pc for Teff ≤ 4800 K, and (2) Planets are likely tidal-locked or synchronously rotating below 4800 K that could potentially alter the inner HZ limit significantly…The upper limit of 6300 K is a result of planets in the HZs having longer orbital periods around early F-stars, where Kepler is not capable of detecting these planets…

So bear this in mind: Excluding what could be vast numbers of habitable planets in M-dwarf orbits, we still wind up with 300 million possibilities in the broad range of K-class through G-class stars. Co-author Jeff Coughlin is director of Kepler’s Science Office:

“This is the first time that all of the pieces have been put together to provide a reliable measurement of the number of potentially habitable planets in the galaxy. This is a key term of the Drake Equation, used to estimate the number of communicable civilizations — we’re one step closer on the long road to finding out if we’re alone in the cosmos.”

Image: This illustration depicts Kepler-186f, the first validated Earth-size planet to orbit a distant star in the habitable zone. Credit: NASA Ames/JPL-Caltech/T. Pyle.

When you go through this paper, bear in mind what Centauri Dreams associate editor Alex Tolley pointed out to me — The Drake ne factor refers to the number of planets per star that can support life. What the Bryson et al. paper takes as its starting point is the number of rocky planets in the habitable zone, and this could mean that the figure of 300 million ‘habitable’ worlds takes in planets that resemble Venus more than Earth. It may also include water worlds, where the likelihood of technological civilization is unknown.

So Drake’s term ne is not the same value as taken up in the new paper. Nonetheless, let’s return to that dazzling figure of 300 million, because when we’re dealing with that many planets of interest, we can afford to lose a number that turn out to be uninhabitable and still consider ourselves overwhelmed with possibilities for life.

Numbers like these have implications for stars relatively near the Sun. The authors look at both the conservative and optimistic habitable zone, with the narrower ‘conservative habitable zone’ bounded by the ‘moist greenhouse’ and ‘maximum greenhouse’ limits, and the wider ‘optimistic habitable zone’ bounded by the ‘current Venus’ and ‘early Mars’ limits. I’m drawing this descrtiption from the Planetary Habitability Laboratory’s summary of work by Ravi kumar Kopparapu and colleagues (citation below).

Image: Habitable Zone of around main sequence FGKM stars. The warm ‘habitable’ zone is divided into a ‘conservative habitable zone’ (light green) and an ‘optimistic habitable zone’ (dark green). Earth is at the inner edge of the ‘conservative habitable zone.’ Credit: PHL.

Filtering their results using calculations for the conservative habitable zone, the authors maintain they can say with 95 percent confidence that the nearest rocky habitable zone planet around either a G- or K-class star is within 6 parsecs (roughly 20 light years). There could be four habitable zone rocky planets around G- and K-dwarfs within 10 parsecs of the Sun.

How to build our small planet catalog to reduce uncertainties in the calculations? The answer is clearly more space-based observations even as new ground-based telescopes come online. Let’s also remember what we lost because of Kepler’s mechanical problems. While we did get a K2 extended mission, the original Kepler extended mission was meant to continue the ‘long stare’ at the original starfield, adding four more years of precision photometric data. The number of small planets in the habitable zone would have been significantly extended.

…by definition, Kepler planet candidates must have at least three observed transits. The longest orbital period with three transits that can be observed in the four years of Kepler data is 710 days (assuming fortuitous timing in when the transits occur). Given that the habitable zone of many F and late G stars require orbital periods longer than 710 days, Kepler is not capable of detecting all habitable-zone planets around these stars.

Given that upcoming missions like PLATO do not include such long stares on a single field of stars (PLATO plans no more than 3 years of continuous observation of a single field), we will need future missions to achieve what the original Kepler extended mission might have done, which would have been a doubling of the DR25 dataset and a large yield of small habitable zone planets.

The paper is Bryson et al., “The Occurrence of Rocky Habitable Zone Planets Around Solar-Like Stars from Kepler Data,” in process at the Astronomical Journal (abstract). The Kopparapu et al. paper is “Habitable Zones Around Main-Sequence Stars: New Estimates,” Astrophysical Journal Vol. 765, No. 2 (26 February 2013). Abstract.

{ 46 comments… add one }
  • Richard November 10, 2020, 9:44

    Thanks as always, Paul. It’s interesting work. Given the immense reaches of time involved in galactic and biological evolution, Drake was perhaps right when he said that the only important term in his equation was L.

    However, I think two other terms may also turn out to be problematic: namely f(l) and f(i). The particular issue here is not (IMO) that we don’t have meaningful data to base our estimates on – it’s in the selection of those two subscripts “l” and “i”. Each of these is effectively an aggregate of many constituent probabilities: for example, one could write f(l) = f (self-reproducing molecules) x f (cellular life) x f (multicellular life) x (survival of extinction events) x … and so on. These may not be the best choices; that doesn’t matter, because my point is simply that f(l) is an *aggregate* figure.

    As such, the accuracy of each individual “f” sub-estimate that contributes to f (l) has a profound impact on the final figure: if there are n items in the aggregate, then each item would need to be accurate to n decimal places (roughly) in order the ensure that the final aggregate f(l) was accurate to just 1 decimal place!

    So, if we think of Drake’s equation in the light of all the possible pinch-points in the human journey so far – from a-biotic reproduction to space and radio technology – and reflect that we would be lucky to estimate the chances of survival at each stage to even 1 decimal place… Then surely we must conclude that f(l) x f(i) is unknowable.

    Admittedly, it is close to unknowable even without making this observation! My point is that it may be more than just ‘unknown at present’, it may be fundamentally unknowable *in principle* – or, at least, it might never be possible to estimate it with sufficient accuracy to provide a meaningful estimate of f(l) x f(i).

    Apply this to full equation, and it seems to me that both L *and* f(l) x f(i) are of immense importance to the final result: both could vary by many orders of magnitude, potentially in opposite directions.

    Research into individual elements of Drake’s equation is still enormously valuable in gauging both how we got to where we are and how other intelligences might evolve and make their mark on the galaxy. But the Drake equation has become an iconic tool – perhaps because it’s easy for lay people like me to grasp – and, like any estimate, It’s only as good as the numbers that are fed into it.

    • Alex Tolley November 10, 2020, 12:10

      Then surely we must conclude that f(l) x f(i) is unknowable.

      f_sub_l may be “unknowable” by theoretical inferences, but it is certainly knowable by direct observation. Whether a small volume of space around Earth, or the whole galaxy, in principle, we can determine life-bearing planets by direct observation, however we define “life”. There may be many variables that are needed to determine whether a specific world is likely to be living, but that is a different issue.

      f_sub_i and L may be unknowable even observational as there may be only a phase when advanced ETI is even observable. It is possible that ETI becomes unobservable at some point in their development. Drake would only have been interested in ETI that was capable of communicating with us, initially broadcasting by radio, but later receiving our radio signals.

      • Richard November 14, 2020, 12:27

        Agreed on all counts, Alex – and your conclusion, as Drake’s, implies that ultimately N=L. My point was only that we have no solid *theoretical* grounds for our estimates of f(l) and f(i). What we might ultimately learn *empirically* is a different issue; but even if we develop the technology to measure these quantities somewhat better, think how long it would take us to acquire a meaningful sample size!

  • Michael Shaver November 10, 2020, 10:03

    Two terms are insignificant fp(fraction of stars with planets) and fi (fraction planets with life albeit microbial). Moons (of Jupiter and Saturn) likely have atleast microbial life.

    Drakes equation is not relevant to SETI:
    Goldilocks highlights significance of stable habitable zones as harbingers of ETI development. Goldilocks equation defined by Earth SOL liquid water radius, by Gas Giant rocky planet habitat radius and by Galaxy SOL evolution radius. Our present SOL system is showing that microbial life has ubiquitous presence thus Drakes (fp, ft) variables have a discard-able value of one. Goldilocks Zone limit L, and enable fc and fi, where as conservation (politics) impacts L as well. Increasingly Drake Equation is sustained by-pernicious paradigm, ‘pernicious’ as in ‘hiding truths’; H. Poincare.

    • Alex Tolley November 10, 2020, 12:21

      Moons (of Jupiter and Saturn) likely have atleast microbial life.


      Our present SOL system is showing that microbial life has ubiquitous presence

      However attractive that idea is, currently there is zero evidence that there is life in the solar system beyond Earth.

      For ETI, life is most likely going to have to be multi-cellular, with evolved brains that can do more than just acts as a signal processing device to connect the environment to the organism’s actions. Therefore if there is a difficulty to achieve multicellularity (which I don’t believe) then the universe could be full of unicellular life, yet devoid of intelligence, bar Earth life.

      Empirical data will eventually be able to determine reality. Until we have it, we are just building edifices on sand.

      • DCM November 11, 2020, 13:14

        Yep. Show me the germs.

      • Joe H. November 11, 2020, 18:54

        “Moons (of Jupiter and Saturn) likely have at least microbial life.” If this is true it will be the biggest discovery in history. However extraordinary claims require extraordinary evidence.

        • Michael Shaver November 12, 2020, 20:34

          Very early Earth had microbial life within 500 million years albeit with water as well, per NOVA ‘Lifes Rocky Start’. One could easily posit that moons of Jupiter and Saturn, having equivalent conditions, could be a habitat for life; And I said ‘likely have microbial life’.

      • NS November 12, 2020, 3:42

        Well, there is something to be said for what could be called informed speculation. It at least gives us an idea of where to start looking. Until the unlikely day that science is given the resources to explore every world for life, deciding which ones to explore (and how) will require a lot of more or less plausible guesswork.

        It certainly seems possible to me that we might discover signs of life in other stellar systems (e.g. atmospheric signatures) before we’re able to explore our own solar system in enough detail to determine whether life exists nearby.

  • Robert Flores November 10, 2020, 10:51

    This study just adds another weight of evidence to a Fermi Paradox solution, in that:
    It affirms the Rare Earth Hypothesis, with the added caveat that we don’t know how likely any specific cultural path leads to a technological civilization.

    I dont think Rare Earth is complete in its critical listings, I can think of
    many additional ones that are becoming apparent now that better data is coming in on exo-planets. Also:
    On earth modern humans have existed for 60K years or so. only for the last 10% of that fraction of time has the march towards technology been decisive, and there have been collapses in spots where the environment played a crucial role, These are random hazards than even we in today world are vulnerable to. how do we factor that into the like propability of ETIs being out there.

    • Ronald November 13, 2020, 8:34

      “It affirms the Rare Earth Hypothesis”
      No it doesn’t, rather the opposite: rocky planets in the HZ around solar type stars are quite common.

  • Henry Cordova November 10, 2020, 11:42

    It took 4 billion years for life to evolve on earth, and another half-billion years for multi-cellular organisms, and eventually intelligence, to appear. Even if we assume this progression may have been much faster on some worlds, it still remains clear that stars formed late in the Galaxy’s history are not very likely to be inhabited. Even systems that are otherwise perfectly suitable are immediately disqualified as potential sites for ETI simply if they are too young.

    Low mass, long-lived, stable main sequence stars appear to be the rule, rather than the exception, and it seems likely star formation rates in the early Galaxy may have been much higher in the past than they are today. No doubt, there are probably plenty of stars and planets suitable for life available. But the fraction of metals in the interstellar medium was generally much lower in the past, (even if we concede local pockets and exceptions due to early supernovae and nucleogenesis) and this must have some effect on the potential for rocky worlds with living things.

    Very old worlds have had plenty of time to evolve life and civilization, but they have also been exposed to the risk of cosmic catastrophe and accident for much longer. And even those who escaped these disasters may have just lost their civilizations through old age. After all, there is no reason to expect even extremely wise and stable civilizations last forever. All complex systems eventually succumb to entropy.

    There are a lot of time-dependent terms that need to be introduced into the Drake Equation (and all its potential successors) before it can be a truly useful tool for modeling the possibilities for life.

    • Michael Fidler November 10, 2020, 22:17

      These early stars especial M Dwarfs would have no or very little radioactive elements in the sense of nuclear destruction. We live on an overly dense planet and most of the early Planets would be made of biologically important organic elements. This would seem to be the best long term environment for life and the majority of these stars are in Halo orbits. This should lead to much calmer and and less flaring in their central stars. I would be inclined to say that first five billion years of our Galaxy would be a much better place for life and civilizations.

      • Henry Cordova November 11, 2020, 9:26

        Disk and halo stars are internally rich in metals due to thermonuclear nucleogenesis during their evolution, but their planets were composed of the relatively metal-deficient material available in the early galaxy at the time those stars formed. They are not likely to be rocky worlds with solid surfaces. It is in the disc population, born in molecular clouds already metal-enriched by previous supernovae and planetary nebulae, where rocky planets form.

        As I have suggested before, there may be exceptions to this rule due to local bursts of star formation and rapidly evolving giant stars in the early galaxy. But in general, it is the Population I stars of the disk which are the most likely to have rocky planets and life. In the Pop II galactic core, halo, and the globular clusters, any planets are likely to be gas giants primarily composed of hydrogen and helium.

      • Michael Fidler November 11, 2020, 10:25

        Here is an opposing opinion that I came across this morning:

        Radioactive elements may be crucial to the habitability of rocky planets.
        Earth-size planets can have varying amounts of radioactive elements, which generate internal heat that drives a planet’s geological activity and magnetism.

        Radiogenic Heating and its Influence on Rocky Planet Dynamos and Habitability.

        This makes sense since we live on the densest planet in the solar system, but again we see it from the earth center point of view.
        We do have a lower density body in this solar system that has many active volcanoes and that is Jupiter’s Io. The early planets around red dwarfs may of had little thorium, and uranium to cause radiogenic heating but because of there close proximity to the central star and other planets they may have been very active. The high magnetic field generated by red dwarfs plus the tidal heat caused by both the star and planets passing near each other would cause heating and internal magnetic fields. Induction heating may keep plate tectonics active on these early planets and kept them habitable.

        Interior Structures and Tidal Heating in the TRAPPIST-1 Planets.
        Solid tidal friction in multi-layer planets: Application to Earth, Venus, a Super Earth and the TRAPPIST-1 planets. Can a multi-layer planet be approximated as a homogeneous planet?
        Magma oceans and enhanced volcanism on
        TRAPPIST-1 planets due to induction

        So again we look at the even early planets as being lifeless and dead but we have our blinders on and can not see natures beauty.

  • wdk November 10, 2020, 12:30

    Perhaps like many on this forum, I think a lot about the Drake equation, but seldom get down in the weeds trying to manipulate or reconstruct it.
    So comments on it are somewhat intuitive. One that arises is response to the figure 300 million habitable planets: Where? Maybe even, when?

    Circa 1960 when Drake and colleagues were originally discussing this issue were they talking about the galaxy or our sphere of detection capability in that era? They probably were not the same historically and might not have ever coincided. After all, there must be gaps in our Milky Way reception and the Magellanic Clouds might be easier than other extremes of this body. Quantifying the number of habitable zones should require a connection to candidate stars, as indicated above by type but what is absent is domain. If we had a Drake equation variant set for the stars within 1000 light year radius of us, the answer might be a little less nebulous in that regard. An estimate of stars would be fixed, even if we have to make decisions about spectral types (color) or age.

    For pragmatic reasons and noted before, in search for habitable zones for planetary stability analyses (e.g. in binary star systems) I have usually used the black body flux relation for most any star: take the surface effective temperature, assume and extend out to a radial effective temperature similar to the sun for 1 AU ( ~ 400 Kelvin) and examine +/- values as well ( 350, 375, 400, 425,..).

    After that, I think a lot of stellar and planetary considerations can be catalogued and examined based on common assumptions or concerns
    ( tidal locking, flares, surviving volatiles, MS life time, gravitation, depth of atmosphere, oceans…).

    It does strike me though that before the Drake equation publication in 1961 there were articles in the newspapers ( when I was in grade school) that would bring such figures up. Perhaps there were some informal sessions on this subject at astronomical conferences? A big difficulty at that point was the absence of exoplanet detections and the “detectability”. But if my memory is not playing tricks on me, I still wonder who it was making claims of millions of habitable planets in the science press back in the 1950s?

    Another historical note: Those that read or remember Heinlein stories, might recall his protagonist in Starship Troopers ruminating on what he would do after he got out of the military service in interstellar war. He was thinking about heading for a habitable planet in orbit around a red dwarf. He was concerned that the UV side of the flux was a little inadequate and might have ill effects on his distant descendants.
    A presumed inability to keep up with the Jones on radiation induced
    mutations. Maybe the local chamber of commerce should have contacted him and explained. No problem at all. Where we live we get our UV and even EUV in big helpings. Not with sun lamps like they do back in Siberia. Please call us back – after the storm.

  • Alex Tolley November 10, 2020, 12:48

    It affirms the Rare Earth Hypothesis

    Not necessarily. The galaxy could be teeming with planets that have well-developed biospheres with millions of species of complex life. Just no technological civilizations.

    Even if complex life does inevitably lead to technological civilization, if the lifetime of that civilization inevitably proves short, then these civilizations may be appearing like cosmic fireflies, but very rarely simultaneously. At any given moment, we may be alone, out of reach by another contemporaneous civilization.

    Contemporary human civilization seems to require economic growth, even if that requires periodic setbacks. At a few percent annual growth (a figure often used to indicate when we could start building interstellar craft) our energy needs would make us a Kardashev II civilization harvesting all the energy from our sun is just 2-3 millennia. The same growth would make us a KIII civilization is just a few millennia more. But we cannot do the KII-KIII transition because the limited of c requires that we cannot reach all the stars in the galaxy in less than 50 millennia, and more likely 500+ millennia. That implies that economic growth requiring energy, population growth, and resource utilization, has to all but stop in a few millennia. There might be a spreading spherical surface of high growth colonies that leave behind stagnating colonies that have reached their maximum energy consumption and GDP.

    If local civilizations can indefinitely cycle through growth and decay, then, depending on that cycle length, the Fermi Paradox may be explained. Growth periods might be very separated in time, or they may occur a few times around any star, then permanently vanish. Either scenario would make the REH difficult to evaluate.

  • wdk November 10, 2020, 18:46

    There are species on the earth that last orders of magnitude longer than our technical civilization. Whether they developed tools is arguable pro and con. For consider how many bird species build nests, or how many mammals dig tunnels or burrows. This activity appears to be integrated into a larger ecology, And considering that we have self awareness, we could presume that collapse or expansion could be modulated at least as well as species that have less. Or else, it might be possible for an exo-planet sentience to do the same. The biggest issues seem to be whether innovations of the last few centuries had to lead to instability elsewhere and everywhere.

    • Alex Tolley November 11, 2020, 12:22

      The paleolithic period lasted over 3 million years. There was tool use, but very little economic advance. Would another 3 million years of the old stone age have resulted in radios? Most of recorded history shows extremely slow economic growth. Life was rather static for generation upon generation. That we live in a period of rapid (but declining?) economic growth is very nice, but it is an anomaly. However, that anomaly has brought us a rapid increase in knowledge and capabilities, and with it a desire by some to settle the cosmos. This was unthinkable a few centuries ago, and even flights to the Moon were considered fantasy even a century ago.

      So yes, there could be preindustrial ETI slogging away in a Malthusian condition, but would they have developed the technology to communicate with us? I don’t believe so, but they could answer the Fermi Question.

      • Kamal Ali November 12, 2020, 4:36

        The more and more I think about Drake, the more it seems we are looking for Radio-telescope Mirrors: civs that have long-lasting radio presence. Failures could include civs stuck perpetually in pre-Renaissance states, planets stuck in uni-cellular stage or planets stuck in low-technology states (they may have some tool-making but never progressed). But my personal favorite explanation for Fermi is that civs encounter not one, but a series of Great Filters: things like supervolcanoes every 1M years, environmental depletion in 1K years, nano-goo attack in 10K years, civilization-ending warfare every 1K years and so on.
        IE: the number n of long-lived civs as a function of lifetime L, hence n(L), is an exponentially decreasing function.

  • torque_xtr November 11, 2020, 4:06

    What strikes me in Drake equation is that it has an implicit assumption that everybody stays on their home planets. While sufficient interstellar colonization means, by definition, that number of colonized worlds is at least equal to the number of homeworlds. But the breakdown begins as early as n(e) – the Earth is thought to be near the inner edge of HZ but still managed to get into snowball state lately. And was heading deep into another glacial if not for humans who have a decent chance in warding glaciation off. There could be equally big fractions of planets which are by all means perfect when seen from afar but in reality don’t have a chance to produce anything beyond bacteria, and worlds which would seem meager to us but evolve complex language and abstract reasoning in a single GYr.

  • James Cambias November 11, 2020, 10:45

    A couple of months ago I did a series of posts on my personal blog about “Great Filters” and the terms of the Drake Equation, addressing some of the issues brought up by other commenters here. Here’s the beginning of the series: https://www.jamescambias.com/blog/2020/08/great-filters-part-1.html

    • Paul Gilster November 11, 2020, 13:46

      Let me add that any of you who aren’t familiar with James’ A Darkling Sea will want to put this on your reading list! What a splendid job. It was great to see you in Huntsville a few years back, when we had the chance to talk about the book at the airport on the way back out.

    • Alex Tolley November 11, 2020, 16:21

      A very enjoyable set of posts on the Great Filter[s] and the Fermi Question. The only thing you leave out because you are uncomfortable with it is that we are alone as an intelligent, civilizational species in the current period (light cone) of the galaxy. But even if that was the case, why no indication of civilizations in other galaxies? Are they all devoid of technological civilizations, even machine ones?

      • James Cambias November 12, 2020, 22:51

        That’s why I wonder if we’re mis-identifying some technological signal as a natural phenomenon. Frankly, it just seems too implausible that there could be whole galaxies devoid of intelligence. As I put it in my blog, 0 is a much more plausible number than 1.

        • Alex Tolley November 13, 2020, 12:24

          0 is a plausible number than 1. However, without 1, there is no way to even ponder the question.

          If the number is >1, then we are also faced with the problem of why no other civilization has emerged that has filled this, or any other of 100 billion plus galaxies with clear signs of astro-engineering. Maybe they have and we cannot recognize it, or these civilizations find some way to transcend and thereby become undetectable other than by their actions (which again, we have to distinguish from naturally occurring events).

          I find it hard to believe that all species or machine ETI suffer the one or more Great Filters ahead of them. As a biologist I don’t want to believe that life is exceedingly rare, although I will obviously have to accept that if that is what the data suggests.
          So I tend to believe the Great Filter is behind us, and that is what has possibly resulted in humans being the first technological intelligence since the birth of the universe, yet the universe is also filled with life, simple and complex. This does seem highly improbable, yet it is also the simplest explanation.

  • ljk November 11, 2020, 12:02

    11 November 2020

    Lauren Fuge

    Mining with microbes in space

    Can bacteria extract useful materials from rocks?

    The first mining experiments in space have revealed that microbes can efficiently extract elements from rocks in zero gravity.

    The tests, performed by astronauts on the International Space Station (ISS), open up possibilities for the human exploration and settlement of the Solar System.

    “On Earth, microorganisms play prominent roles in natural processes such as the weathering of rocks into soils and the cycling of elements in the biosphere,” the researchers explain in a paper in the journal Nature Communications.

    “Microorganisms are also used in diverse industrial and manufacturing processes, for example in the process called biomining.”

    Biomining bacteria can catalyse the extraction of valuable elements like copper and gold from rocks. Here on Earth, they are routinely used to mine rare earth elements (REEs) such as lanthanides, scandium and yttrium.

    The useful physical properties of REEs, like ferromagnetism and luminescence, make them critical components of phones and computer screens, as well as useful in catalysis, metal alloy and magnet production.

    But not only are REEs expensive to mine, they are also rapidly running out. If humans want to explore further into the solar system and build settlements on other moons and planets, we need to figure out a way to mine these elements in situ.

    Full article here:


  • Michael Shaver November 11, 2020, 18:19

    This significance of stable habitable zones as harbingers of ETI development:
    Goldilocks equation defined by Earth SOL liquid water radius, by Jupiter Saturn Resonance clearing-maintaining a rocky planet habitat radius and by SOL-Galaxy evolution radius (Galactic Arm driven ETI evolution). Our present Earth-SOL system (Early Microbial emergence) posits ubiquitous microbial life; Ergo Drakes (fp, ft) variables have a discard-able value of one. Lifetime (L) driven by a conservationist habitat preservation albeit science of Democracy over authoritarianism. Drake’s Equation is marginally relevant, while a Goldilocks Equation is highly relevant.

    • Michael Fidler November 12, 2020, 0:11

      The Goldilocks Equation is very important but one area that has changed and is still changing is the long term effect of large impactors. These impacts are what started plate tectonics along with a close in moon. We see on every terrestrial planets and satellites craters and on the earth we have seen a major impacts in the western part of our world just 12,900 years ago. The geologist look to volcanoes as the active creator of conditions for life but large impacts may of had a much bigger role and also helped in realigning the gene pool for intelligent species. When the Drake equation was created the concept was of a slowly changing stable earth thru out the eons but in the last sixty years cosmic catastrophism has been shown to be the dominating factor.

      A Subterranean Ecosystem in the Chicxulub Crater.

      Satellite images reveal the destructive power of asteroids and meteorites smashing into the Earth’s surface over millions of years in new atlas detailing hundreds of craters carved by extra-terrestrial collisions.

      • Michael Shaver November 12, 2020, 18:35

        Impacters have no periodicity, being random events. With-in the Geologic (Phanerozoic) Time Scale a pattern repeats and is now repeating as well (Time between CB alignment and associated three arms). Also a 419Ma repetition of the Arm Passages indicates a relative SOL-Earth v CB rotation rate. This reveals a evolutionary driver at Galactic our radius is just right in terminating dominant (brute force) species while not exterminating (reseting) life form development. I would posit vulcanism cycling…

        • Michael Fidler November 12, 2020, 23:25

          Periodicity for impactors may be directly to related to passage thru the arm because of a larger number of stellar close encounters. This would disturb the Oort cloud and send in more comets and Jupiter’s effect would create more short period comets which should see a cascade of impacts. The big problem is that 75% of the impacts have been in the oceans and a large percentage have been subducted. The other 25% have been largely destroyed by snowball earth and plate tectonics. When the solar system passes thru the dense cloud nurseries in the arms there may also be higher speed large impacts from interstellar objects and Oort cloud disturbances from forming stellar systems and rouge planets. The very ancient land masses on earth show a vey high number of impacts but 99% of the impacts have already been destroyed. The exploration of the the ancient fossil moon, planets their satellites should give us a much better understanding of the periodicity of impactors.

  • AlexTry November 12, 2020, 4:03

    Drake’s equation was the good argumentation to begin SETI searches, but now, after more that 50 years of fruitless searches, I suppose it is the time to realize that SETI – uses wrong and useless methods to search ETI, and will never bring any scientific results, now it is functioning more like religion movement.
    Drake’s equation is very convenient thing, everyone can play with it’s members to prove every theory. It is exactly like to try to solve System of N equations, when you have only one equation available – i.e. quantity of solutions is unlimited :-)
    Like White noise…

    • Robert Flores November 12, 2020, 10:41

      SETI’s time to shine is starting to run out. It’s relevancy will
      wane within a generation or two

      Once we have the ability to locate Earth Like planets in HZs reliably and at any orbital plane relative to us the game will switch to monitoring exo-biospheres. That will tell us much more clearly how far off or on point the whole SETI effort was/is.

      • Alex Tolley November 12, 2020, 14:30

        That scenario may depend on the findings. Just suppose atmosphere biosignatures indicate N2/O2, and spectrographic analysis of teh surface shows some chlorophyll or some analog. If these are common, this would raise the probability of ETI and SETI search intensity, even if the probability was extremely low. Compared to the cost of space missions and new telescopes, SETI costs are very low and can be piggybacked onto existing astrobiology programs.

        Conversely, if the hunt for biosignatures proves disappointing, then this could end all but the most shoestring SETI program. It could also similarly gut funding for astrobiology programs too, leaving just searches within the solar system, which in turn, could prove disappointing. What happens then?

        I do agree that exoplanet biosignatures are pivotal. If found, that opens up a larger program, including even probe missions of varying cost and complexity. If not found after a survey of thousands of planets, then this could kill astrobiology programs, although not probes (for other reasons). What it might just do is stimulate research on terraforming, perhaps based on volcanic islands that become colonized.

  • Gary Wilson November 12, 2020, 13:25

    Does the Drake equation take into account habitable moons? They could possibly equal or even outnumber the number of possible habitable planets and increase greatly the chance we actually find some form of ETI eventually couldn’t they?

  • AlexTru November 12, 2020, 17:15

    Even if we will detect biosphere on the distant world:
    1. It will be done by astronomical methods, not by radio / EM waves monitoring used in SETI
    2. There is no chance for any communication (using our modern technology), even if you sure there are ETI on distant planet.
    Summary if ETI will be found – it will be found not by SETI as it is today …

  • Spaceman November 12, 2020, 22:58

    Hi Alex,

    Out of this ~300 million planets, I would wonder the following:

    1. What fraction of them have plate tectonics?

    2. What fraction of them have significant quantities of liquid water?

    3. What fraction of them have a moon to help stabilize obliquity?

    4. What fraction of them have experienced abiogenesis AND survived a possible so-called “Gaian bottleneck”?

    5. What fraction of them have a protective magnetic field?

    For all we know, there could be additional considerations besides the ones mentioned above that could spell the difference between a durable biosphere versus a sterile wasteland. Also, when you start factoring in some of these criteria, how many of the ~300 million planets host even microbial life let alone multicellular life let alone intelligent life? Do you think we might be facing a situation where, say, intelligent communicating civilizations exist but in 1 out of every 1000 Milk Way- like galaxies?

    Obviously, this last question is more for kicks and giggles rather than anything else:

    What are Alex Tolley’s preferred values for the various terms in the Drake equation and, if you have the time, what influenced you to choose these values? :-)

    • Ronald November 13, 2020, 9:13

      I would think that 1, 2 and 5 are typical characteristics of terrestrial planets.
      With regard to 3: I have mentioned several times here, with ref. to Barnes and others, that the importance of a large moon for axial tilt (obliquity) stabilization is grossly overrated, and the gyroscopic effect of the planet itself does most of that job.
      4 is still impossible to tell, but I would guess that microbial life is probably quite common, and higher life rare.

    • Alex Tolley November 13, 2020, 12:32

      A few years ago I plugged in some WAGs for f_sub_l, f_sub_i, and L. Mainly because I was more aggressive at reducing probability fo f_sub_i, and keeping L lowish, I ended up with the number of other civilizations in the galaxy as <1.

      The problem of course, is that when this number is multiplied by the number of galaxies in teh observable universe, it would suggest that ETI must exist in at least some of these galaxies. So the Fermi Question is still valid.

      My guess is that we will find prokaryotic life abundant, and complex life abundant, but less so. Intelligence and technological industrial civilization exceedingly rare, if totally absent to the limits of our observational capacity.

      But my guess is probably no better than anyone else's and contains my biases. Data is what we need.

  • Laintal November 13, 2020, 6:03

    Radioactive Elements May Be Crucial To The Habitability Of Rocky Planets

  • Michael Fidler November 13, 2020, 6:58

    All I can say is WOW!

    An approximation to determine the source of the WOW! Signal.
    Alberto Caballero

    In this paper it is analysed which of the thousands of stars in the WOW! Signal region could have the highest chance of being the real source of the signal, providing that it came from a star system similar to ours. A total of 66 G and K-type stars are sampled, but only one of them is identified as a potential Sun-like star considering the available information in the Gaia Archive. This candidate source, which is named 2MASS 19281982-2640123, therefore becomes an ideal target to conduct observations in the search for potentially habitable exoplanets. Another 14 potential Sun-like stars (with estimated temperatures between 5,730 and 5,830 K) are also found in the region, but information about their luminosity and radius is unknown.


  • Ronald November 13, 2020, 9:08

    An extremely interesting paper and, again, a very clear and well-written post.
    A few observations:

    “What the Bryson et al. paper takes as its starting point is the number of rocky planets in the habitable zone, and this could mean that the figure of 300 million ‘habitable’ worlds takes in planets that resemble Venus more than Earth.”
    Well, I don’t think so, if you only consider the conservative HZ, that would leave Venus out.

    The number of 300 million is not in the original paper, where does it come from? Is it in an article related to and/or derived from the paper?
    What the paper concludes is that there are 0.37 – 0.60 rocky planets in the HZ per solar type star. And that the nearest one is (statistically) only about 20 ly away (and 4 within 10 parsec).

    I find the lower bound of 0.5 Re for a rocky planet rather low: with earthlike density this would result in a planet of only 0.125 Me, a bit more than Mars, hardly able to hold on to a decent atmosphere.

    And the authors warn about their own extreme assumption with regard to extrapolation and the power law.

    But this is an extremely relevant study and remarkably roughly along the lines of earlier studies, such as Petigura (2013; Prevalence of Earth-size planets orbiting Sun-like stars, https://arxiv.org/abs/1311.6806) and an early post here on CD: https://www.centauri-dreams.org/2010/03/10/habitable-planets-working-the-odds/comment-page-1/#comments

    Quite encouraging and it makes me look forward to powerfull telescopes and spectrographs, capable to detect and analyze biosignatures.

    • Alex Tolley November 13, 2020, 12:52

      One popular account that references 300 million planets. Others do too, although they may be referencing each other.


      • Paul Gilster November 13, 2020, 14:48

        Actually, the number comes from the SETI Institute, which plugged it into their statement on the work. That’s where I drew it from.

  • Paul Wilson November 21, 2020, 22:27

    300 million habitable zone planets but not a peep from “anybody”.
    What is happening here? As in, something is not right.

  • Chris Lakomy January 15, 2021, 23:00

    The Drake Equation is meant to be a fair calculation of the likelihood of extraterrestrial civilisations in the universe.

    I think that this equation has got to represent the most arrogant base nature of the human condition, similar in kind to the mindset of the eighteenth century Englishmen who assumed that they knew the peoples of the lands that they conquered.

    Similarly, the Drake Equation assumes that any extraterrestrial civilisation must be just like us, living in the habitable zone, communicating with radio waves, even wishing, as we do, to make our presence known.

    Who says so? Does Dr Frank Drake know something that we don’t?

    A colony of worms in my compost bin might think that they’re pretty intelligent, and who is Dr Drake to say that they are not?

    Also, who says life can only develop on planets? And if we can’t detect those “detectable signals”, does that mean they don’t exist? Besides, radio communication has existed here on Earth for less than two hundred years, and already it’s starting to become obsolete.

    Worth thinking about?

    Chris Lakomy

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