Potentially habitable?

The exciting news in astronomy is the discovery of water in the atmosphere of a relatively small planet, known as K2-18b, that happens to lie in what we often to as the habitable zone of its parent star. The result was reported in this Nature Astronomy paper, and also in this arXiv paper that appeared on the same day. I know some of the authors of both papers, have published with some of the authors of the second paper and am currently working with one of the authors of the first.

The reason why this is such a fascinating result is that it’s the detection of water in the atmosphere of an exoplanet, the exoplanet itself is actually quite small, and it lies in a region often referred to as the habitable zone, but that really simply means that conditions could be suitable for the existence of liquid water.

Credit: Rice et al. (2019)

However, some of the coverage has been less than satisfactory. It’s being presented as a planet that could support life. However, this paper reports that it has a mass of M_p = 8.4 \pm 1.4 Earth masses and a radius of R_p = 2.37 \pm 0.22 Earth radii. You can actually put this onto a mass-radius diagram, which I discovered I’d essentially already done. The figure on the right is a mass-radius diagram with all known exoplanets with masses below 20 Earth masses and radii below 2.75 Earth radii. You can clearly see the location of K2-18b, which I’ve also highlighted.

The dashed curves are composition curves. Those shown in the figure include that for a planet that would be 100% iron, one for a planet that would have an Earth-like composition (~25% iron, ~75% silicates), one for a planet that would be entirely silicates, and also includes composition curves for planets in which water would make up a substantial fraction of their mass, and one for a planet with a hydrogen-rich atmosphere that would be about 1% of the planet mass. You can see that K2-18b lies somewhere in the region where it would either have a substantial amount of water (~50%) or it would have a reasonably substantial (~1%, or more) hydrogen-rich atmosphere. In neither case would this be a planet that would typically be regarded as habitable.

Furthermore, the observations are transit observations at different wavelengths. In other words, what we’re observing is the planet as it passes between us and its host star, and comparing the amount blocked at different wavelengths. Given that these differences must be due the atmosphere blocking different amounts of light at different wavelengths, we can use this to say something about the atmospheric composition. However, the signal depends strongly on the scaleheight of the atmosphere, which is larger for a lighter atmosphere (one that contains substantial amounts of hydrogen) than it is for a heavier atmosphere (one that was pre-dominantly water vapour, for example). The paper itself makes clear that a non-negligible fraction of the atmosphere must be hydrogen and helium.

Additionally, a paper published in early 2019 has suggested that the radius of K2-18b is actually R_p = 2.711 \pm 0.065 Earth radii, which would put K2-18b into a region of mass-radius space that suggests it almost certainly retains a substantial hydrogen-rich atmosphere; essentially, it’s a mini-Neptune. Maybe life could exist in the upper regions of such a planet’s atmosphere, but this is almost certainly not what most people would think of when they hear that a planet is potentially habitable.

You might think that this is mostly a problem with the media over-hyping a news story. But, no, it appears to be the narrative presented in the university press release and in the lead author’s The Conversation article. It would be good if the media were to talk to researchers not involved in the actual study, but it’s hard to blame them for presenting a story that is similar to what is being presented by the researchers themselves.

I think that this is all rather unfortunate. This kind of result is very exciting without needing to present a narrative that is probably not true. We also live in an era where it would seem important to not provide more ammunition for those who would like to undermine the public’s trust in experts.

Unfortunately, though, I suspect that these kind of over-hyped stories are likely to continue happening. In the context of habitability on an exoplanet, the public and the media should be dubious of any such claims for at least the next few years, if not longer. These kind of observations are very difficult and although the result presented here is impressive, we can still only do this for planets that are quite a bit larger than the Earth. This doesn’t mean that such planets can’t be habitable, but it’s almost certainly not going to be the kind of life that people think of when they hear such claims. I think it’s important to be clear about this when presenting such results.

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61 Responses to Potentially habitable?

  1. Eabani says:

    What are the colours on the plot, please? Surface gravity?

    Pallab Ghosh for the BBC interviewed several independent scientists:

    A thick water-vapour atmosphere doesn’t sound too habitable to me.

  2. Eabani,
    Thanks, I had seen that article.

    The colours in the plot (i.e., the colour bar on the left hand side) is incident flux relative to that of the Earth. For example, bright yellow would be planets that receive more than 1000 times the flux that the Earth receives (i.e., very hot). Purple, on the other hand, would be planets that receive a similar level of incident flux to that received by the Earth (for example, K2-18b).

  3. Well there is a substantial difference between declaring that a planet could support life and one that is habitable for our kind of living things. This planet appears to exist in the habitable zone and there are suggestions that it might support life of some sort. That’s very interesting, but no one we know is planning a holiday there. We have our hands full trying to keep this planet in a state that can support human life. We can look to the stars with great wonder, but our feet are treading a precious planet with an amazing propensity to bring forth living things. Wondrous things!

  4. Additionally, a paper published in early 2019 has suggested that the radius of K2-18b is actually R_p = 2.711 \pm 0.065 Earth radii, which would put K2-18b into a region of mass-radius space that suggests it almost certainly retains a substantial hydrogen-rich atmosphere; essentially, it’s a mini-Neptune. Maybe life could exist in the upper regions of such a planet’s atmosphere, but this is almost certainly not what most people would think of when they hear that a planet is potentially habitable.

    Why would life only be possible in the upper atmosphere?

    Would not like to land there with a rocket full of oxygen.

  5. Victor,
    The pressure and temperature at the base of the atmosphere will probably be way too high for life.

  6. “The pressure and temperature at the base of the atmosphere will probably be way too high for life.”

    A little too reductionist for my taste. I would change to “life, as we think of it.”

    Here’s an example of things living in spots where temperature and pressures might suggest no life:


    I think precision and careful language are appropriate when we try to describe things to folks who don’t remember too much of the high school science classes. The reality of this planet and the universe is much more interesting than most folks imagine.

  7. smallblue,

    I would change to “life, as we think of it.”

    Except, if this is an atmosphere that is 1%, or more, of the planet’s mass, then that would imply a pressure (and temperature) at the base of the atmosphere that is well outside the range where we would expect life (of any sort) to be possible.

  8. dave s says:

    You mean Hal Clement’s “Mission of Gravity” was just science fiction?
    Ok, suppose it was, but am disappointed.

  9. Dave_Geologist says:

    Interestingly, while we continue to search for Earth-like exoplanets, the definition of “Earth-like” is a moving target. A recent paper in Nature Geoscience finds that Earth’s volatile element composition is much closer to that of carbonaceous chondrites than had been thought. Note that to a geologist, volatile means volatile under magmatic conditions, either in a volcano or during planetary accretion or in magma oceans produced by the Moon-forming collision or the Late Heavy Bombardment. It includes metals like zinc, indium and lead. Oh look, it even has a hockey-stick 🙂 !

    We show that both Earth and carbonaceous chondrites exhibit a unique hockey stick volatile element depletion pattern in which volatile elements with low condensation temperatures (750–500 K) are unfractionated from each other. This abundance plateau accounts for the apparent overabundance of indium in the silicate Earth without the need of exotic building materials or vaporization from precursors or during the Moon-forming impact and suggests the accretion of 10–15 wt% CI-like material before core formation ceased.

    I presume the “too hot” limitation for life on K2-18b means hundreds of °C, where it would be hard to hold organic molecules together (although waxes can be stable to many hundreds of °C, it’s hard to see Nature making a DNA-equivalent out of them). Life is pretty tenacious though. Evidence from sulphur and xenon isotopes indicates that before we had an oxygenated atmosphere and ozone layer, energetic solar UV, sufficient to ionise Xe, penetrated deep into the atmosphere. And yet the dominant life-forms were shallow-water algal (cyanobacterial) stromatolites. There are also several lines of evidence, chemical and biological, that the early oceans were hot (80°C), and had only cooled to about 30-40°C by the time of the Snowball Earth glaciations. That would have made a frozen Earth an even greater extinction threat than it would be today, but life got through it. Primitive, unicellular life, of course.

  10. anoilman says:

    I’m sorry, but with 8X gravity, I’d literally never get out of bed. Indeed Pillow fights would be a lethal experience. Oh gawd what would a rain storm be like?!? RUN!!!

  11. “Except, if this is an atmosphere that is 1%, or more, of the planet’s mass, then that would imply a pressure (and temperature) at the base of the atmosphere that is well outside the range where we would expect life (of any sort) to be possible.”

    I am having trouble accepting/understanding the confidence with which you and others dismiss the possibility of life of any sort. The temps and pressures at the hydrothermal vents are pretty high, yet living things thrive in close proximity.

    I find the following for temps at hydrothermal vents: “400°C Hydrothermal fluid temperatures can reach 400°C (750°F) or more, but they do not boil under the extreme pressure of the deep ocean. As they pour out of a vent, the fluids encounter cold, oxygenated seawater, causing another, more rapid series of chemical reactions to occur.” Isn’t it likely that planets with atmospheres/temps/pressures that would crush or parboil most living things might have niches like the hydrothermal vents where despite our expectations, living things thrive?

    Maybe I am wrong. It happens.



  12. smallblue,
    The point I’m making is that if life does exist on this planet (which does seem implausible to me) then it won’t be anything like what we have on Earth. So – ideally – researchers should be careful when they present these results. This is not an Earth-like planet and if it does host life, it’s not going to be life ss we know it.

  13. Dave_Geologist says:

    mike, the critters don’t live in the 400°C waters. They live in cooler waters, for example where the hydrothermal fluids have mixed with cold seawater.

    Many unique organisms are adapted to life in the harsh environment of an ocean vent. In fact, ocean vents set the current highest temperature possible for life to exist—a fiery 121° Celsius (250° Fahrenheit), found on the Endeavor hydrothermal vents on the Juan de Fuca ridge off the coast of Vancouver, British Columbia, Canada.

    Extending the Upper Temperature Limit for Life

    To learn more about the physiological properties of Fe(III)-reducing microorganisms growing at high temperatures, we attempted to culture microorganisms on Fe(III) from a water sample from an active, “black smoker,” hydrothermal (300°C) vent called Finn, located in the Mothra hydrothermal vent field (at 47°55.46’N and 129°06.51’W) along the Endeavor segment of the Juan de Fuca Ridge, in the Northeast Pacific Ocean. An organism, designated strain 121, was isolated at 100°C in an anaerobic medium … Strain 121 grew at temperatures between 85° and 121°C … Growth at 121°C is remarkable because sterilization at 121°C, typically in pressurized autoclaves to maintain water in a liquid state, is a standard procedure shown to kill all previously described microorganisms and heat-resistant spores.

    Even strain 121 grew an order of magnitude faster at 100-110°C than at 115-120°C, with the maximum growth rate, presumably its preferred living conditions, at about 105°C.

    Interestingly (given my earlier comment about the early Earth’s high temperature oceans and oxygen-free atmosphere), studies have found that “At higher temperatures, and consequent SW:HF mixing ratios <10, anaerobic processes dominate the energy landscape". Catabolic and anabolic energy for chemolithoautotrophs in deep-sea hydrothermal systems hosted in different rock type. The Archaean had life JIm, but not as we know it 😉 . Perhaps one factor in the 1.5 By delay until the Great Oxidation Event was the need for the oceans to cool enough for photosynthesis and aerobic metabolism to compete. It’s also just occurred to me that modelled ocean temperatures from about 2 By to 500 My were in the high thirties (°C), which is where endothermic life sets its thermometer and what ectotherms like to warm up to. Perhaps we’ve inherited a biochemistry that worked at the temperature which just happened to prevail when multicellular organisms evolved. Now that’s what Stephen Jay Gould would call contingency!

  14. I understand that the hydrothermal critters live in the “habitable” zone between the high temperature vents themselves and the surrounding cold dark water. My questions are:
    Isn’t it safe to assume that in this far away planet there will be similar dynamic mixing with a range of microclimate ecosystems that could produce habitable zones where we might not expect them?
    Did we learn anything from the surprising discovery of the life at the depths of the ocean that rely on chemosynthesis rather than photosynthesis as the fundamental driver of life?
    Is there a downside to mainstreaming a limited imagination and reduced expectations about the variability of living things on this planet and through the universe?

    After those questions and reflection, I just think it is smart to think and speak in the nuanced language that ATTP outlined: If life does exist on this planet, it won’t be anything like what we have on Earth. This is not an Earth-like planet and if it does host life, it’s not going to be life ss we know it.

    That is exactly what I think is accurate and should be reported and discussed. It helps open the minds of the readers/thinkers in our species to the actual conditions and possibilities.



  15. thank you, ATTP, your point is exactly the clear, precise formulation that I think it most helpful. I will gently remind you that a few decades ago scientists thought the idea of living things at the bottom of the oceans living in darkness, under great pressure and in close proximity to the hydrothermal vents was very implausible.

    I think we might be in complete agreement with this formulation derived from your post: If life does exist on this planet, it won’t be anything like what we have on Earth. This is not an Earth-like planet and if it does host life, it’s not going to be life as we know it.

    Keeping minds open is hard work and kinda pedantic at times, so I often resist the impulse. I don’t know a non-pedantic way to say, hey, can’t you see what is lurking in your blindspot?



  16. anoilman says:

    Not to mention the concern with Pillows.

  17. When I hear “life”, I am not immediately thinking of cultural achievements like building pyramids, visiting the moon and denying climate change.

    My favourite science channel just released a great video on life under pressure, in the deep oceans. Recommended.

  18. Dave_Geologist says:

    I’m not really disagreeing then mike, in the sense that life may exist in a habitable zone at around 100°C, somewhere in the hot-Neptune atmosphere. I think ATTP’s point was that surface temperatures would be more like Gliese 436 b, 400°C+. With a very thick atmosphere, once you’ve vaporised all the water there would presumably be a huge greenhouse effect. Plus it may, as in that case, be closer to its sun. I suppose you could put it far enough out though…

    If you believe life started with a concentration of organics, liquid water, and some sort of substrate, perhaps clays, whether in Darwin’s warm little pond or Russell’s smokers, it would be hard to get it started high in the atmosphere, even in clouds above the water condensation level. Of course, there’s always panspermia 🙂 . Hoyle may only have been wrong about one of his Big Things.

  19. David B. Benson says:

    Eugene Koonin, in chapter 12 and the appendix of “The Logic of Chance: The Nature and Origin of Biological Evolution”, estimates the extreme improbability of abiogenisis, the origin of life. I do mean extreme.

    For me, having studied that most difficult book — general relativity is much easier — I am quite unimpressed with the pronouncements of the astrobiologists.

  20. Dave_Geologist says:

    David, while the Weak Anthropic Principle (that any planet whose inhabitants can ask about the origins of life is one where life originated, however unlikely that origination may be) means that our own existence is not a useful datum, I think there are some useful pointers from geology. Life appeared a hundred million years or so after the Late Heavy Bombardment sterilised the planet. If you believe that was pre-existing life which had been preserved in material blasted into orbit, that just means that life evolved before the LHB, in the hundred million years or so after the planet cooled enough to have liquid water. Unless there was something chemically or physically special about Earth, which would seem unlikely and be a violation of the Cosmological Principle, you’d expect the origin of life on Earthlike planets to have a PDF spanning billions of years, probably extending beyond the age of the planet. The set of planets satisfying the WAP should be dominated by planets where life started billions of years after they became habitable. We’d be a long shot. Of course that doesn’t prove life is easy to originate, any more than the existence of a lottery winner proves that the lottery is fixed. But we’d be the lottery winner, rather than just a player in a lottery which someone else won, which seems unlikely to me and not a violation of the WAP, which only says that someone won the lottery.

    The get-out clause would be if the PDF was uniform in time. Then 100 my is just as probable as 4 by. I haven’t read Koonin’s book, but most such arguments I have read are that a series of improbable chemical or physical events have to take place in the same primordial soup, and in most primordial soups, that never happens and they remain soup. In that case, the more time you have, the more likely it as that the rare combination will occur. Just as you’re more likely to throw a double six the more often you throw a pair of dice (while the chance of a double six is the same in each throw, the planetary analogy is of a double six occurring in a few throws, vs. a long string of throws). A time-independent PDF would seem most likely if the trigger was some singular event, like the delivery of just the right sort of organics and water by a cometary bombardment. But now we know about migrating giant planets and exoplanets, most planetary systems surely had their own version of the LHB. That means our solar system uniquely had those just-right organics, which would be a violation of the CP. And still leaves open a regression to my original argument if we are not unique among solar systems, just rare. If it’s hard to make life, even if you have the right organics, you’d expect a non-uniform PDF in time-after-LHB, even among the just-right planetary systems (assuming the LHB was the source – the Earth’s volatile inventory indicates a c. 10% contribution from carbonaceous chondrites in the first 100 my, but organics may have been broken down by heat during or after accretion, even without a Moon-forming impact).

  21. David B. Benson says:

    Dave_Geologist, that has nothing to do with Koonin’s argument. Not directly, but I suppose argues even more for the improbability of abiogenisis.

  22. When we talk about about the possibility of estreterrestrial life, we must be very cautious.
    On Earth life emerged very quickly after its formation and it emerged in a very hostile environment, compared to what it is today.
    But in geology it is common to assume the principle of uniformitarianism, according to which the natural processes that have operated in the past are the same that can be observed in the present time and everywhere in the universe.
    According to this principle, not only should life be common in the universe but it should also have generated forms similar to those we know.

  23. Dave_Geologist says:

    From a bit of Googling David, I think I have the gist of Koonin’s argument, and also the distinct impression that he’s regarded as a bit of a maverick on this topic, outside of the consensus. It’s a version of the Drake Equation in which he specifies inorganic synthesis of two RNA molecules with 1000 nucleotides. That leads to an RNA-world, which evolves into a DNA-world. Obviously though it depends on assigning numbers we don’t know to what may or may not be the right process. Yes, RNA is his meat-and-drink so he knows far more about those things than the rest of us, but to a man with a hammer… He seems to favour a multiverse explanation, where we’re in the one universe where that rare chance event did happen. Which is a version of my argument above and has the same flaw. We’re not just living in the lottery-winning universe (fine, the Weak Anthropic Principle requires it), we’re the super-mega-rollover winner because those incredibly rare events not only happened here, they happened in the first 100my of suitable environmental conditions. Not 1by, 5by or 10by later. If we take 10by as Earth’s lifespan, that makes it a hundred times less likely than even Koonin’s calculation.

    So either we and he are missing something, like those before Newton who knew the planets were in more-or-less circular orbits but not what kept them there, or panspermia (but then we’re still a super-mega-rollover winner, because the initial event had to be shortly after the Big Bang to give the spores time to spread at less than light-speed), or God did it. But if God did it, why did He do it by creating primitive single cells on a barren fresh planet at the earliest opportunity, rather than waving His hands in the Garden of Eden? Whichever God did it, She doesn’t sound like the God of the Bible.

  24. Dave_Geologist says:

    Catalysis is the obvious missing ingredient David. I can’t remember if Russell invoked that role in smokers or just used the clays to provide voids the right size for proto-cells. Ah, I see he did envisage some role for catalysts. That takes me back to my undergraduate days, when the asbestos hazard was just being recognised. One of my lecturers had a side business identifying crocidolite, the nastiest, blue form. I remember wondering at the time if the reason it’s so nasty is that one of its unit cell dimensions is almost exactly half the repeat spacing of a DNA helix. Does this remind you of anything, minus the spiral of course?

    The early sea bed, probably very basic like the komatiitic early land magmas, and perhaps with mantle peridotite exposed, would have been replete with long-chain hydrous minerals with dangling hydrogens dying to bond with something, many with unit cells roughly the same size as the repeat length of RNA and DNA molecules. Perhaps that’s no coincidence, and the spacing is inherited from the original catalytic substrates. Other reactions which made different-spacing RNAs or whatever may have been faster or more frequent in the test-tube, but had no suitable catalysts on the early Earth. Dragging myself back on topic, that would mean abiogenesis on an exoplanet not only requires liquid water, it requires liquid water in contact with hydrated minerals on the planet’s rocky crust, so life could not arise spontaneously in clouds high in the atmosphere.

    Drifting off again, one of my other lecturers consulted for a quarry company and they came up with the idea of using crushed anorthosite rather than quartz for sand-blasting (silicosis was also becoming a big deal). I thought that was a really stupid idea. Bad as it is, quartz is about the most benign thing you can use. Chemically unreactive and lacking a cleavage, it grinds down naturally into irregular balls. Feldspar has a strong cleavage and breaks up into sharp-edged fragments with the rough proportions of a house brick. It would shred your lungs. Worse, it’s chemically reactive and would release Ca, Na, and K, disrupting the balance of those biologically active ions.

  25. I haven’t read Koonin’s book, but isn’t the problem with his argument that it’s somewhat similar to being surprised by winning the lottery? It’s virtually impossible to predict who would win the lottery, but if enough people buy tickets, you can be pretty confident that someone will.

  26. David B. Benson says:

    aTTP — Not in the slightest. Obviously abiogenesis occurred here on Terra. It is just that abiogenesis is a highly improbable event.

    For a somewhat related view by two non-biologists, see “Rare Earth: Why Complex Life is Uncommon in the Universe” by Ward & Brownlee, a geologist and a cosmologist, respectively. However, they assume that abiogenesis is easy.

  27. Live on Earth started pretty early. Luck is always possible with only one sample Earth, but this kinda suggests that abiogenesis is not that hard. That we do not detect complex life elsewhere could be explained by assuming other planets also having climate “skeptics”.

  28. David B. Benson says:

    Koonin’s understanding of cosmology appeared deficient. The standard simplified equation for the evolution of the universe, after the first little bit, is FLRW, the initials of the 4 independent discoverers. The solutions are subject to a verifiable parameter describing the density of mass-energy. This is very close to the critical density for “flatness”. Flatness is usually considered to imply infinite. If just close-to-flat then finite and inconceivably large; Koonin’s probability inversed is teeny-tiny in comparison.

    So there is no need to invoke the just-so story of the multiverse. If the universe is infinite then abiogenisis happens infinitely often; if finite then justice moreover timestamp than can be readily counted. In ether case, planets with abiogenisis are rare, much less than one per visible universe sized volume.

  29. Dave_Geologist says:

    A shorter version of my point Victor 🙂 . I do tend to be long-winded 😦 .

    The surprise is not that we live on a planet where life arose, but that we live on a planet where life arose more-or-less at the earliest opportunity. As with the dog-in-a-bar story. The surprise is not that the dog ordered a whisky chaser, but that it could talk in the first place.

    Returning to my catalyst speculation, I see that there are some recent ideas in a similar vein:

    It is somewhat more difficult to understand the “chromosome tangling hypothesis.” We recently found that asbestos fibers including crocidolite are actively taken up by several different kinds of cultured cells. Furthermore, those fibers enter both the cytoplasm and the nucleus. In this situation, asbestos fibers may tangle with chromosomes when cells divide. Whether there is a specificity of tangling for any chromosomal region is the next question to be addressed.

    Finally, as for the “adsorption theory,” it is well known that the surface of asbestos fibers have a high affinity for certain proteins and molecules. This appears to be at least partly associated with positive or negative charges on the asbestos surface.

    They don’t talk about unit cell size, however that obviously bears on the spacing of hydrogen and oxygen atoms in the fibrous crystals making up the macroscopic fibre.

  30. David,

    Obviously abiogenesis occurred here on Terra. It is just that abiogenesis is a highly improbable event.

    Okay, I see what you mean. My own view is that I think life – of some kind – is probably common. I don’t really have any strong evidence for this, other than we know exoplanets are common, we know that water seems to be found everywhere, and we know that there are plenty of organic molecules. Technologically advanced civilisations may be very rare, but some kind of life may well be quite common.

  31. I really appreciate that the “life” around the universe has evolved into this subtle and sophisticated discussion. Big thank you to Dave the G for digging down and sharing his thoughts about this. We have moved away from a species-centric view to the spot that seems intuitively correct to me: that life – of some kind – is probably common. The question of biogenesis is the classic chicken or egg problem: when would we recognize that life has begun as distinctly different from chemical reactions. I am not too interested in spending time on that question. It appears likely to me that we start with chemical reactions and that can lead to consciousness, self-awareness and the ability to gaze up at universe with wonder and develop theories about cosmology that are more than simple origin myths. Truly wonderful.

    Are there bipeds on K2-18B cranking out radio waves and developing sitcoms about life on K2? I think that is not likely. But life is not all about bipeds.

  32. David B. Benson says:

    By analogy to the reasoning of others on this thread, since brooms are common there must be witches.

    I, at least, attempt to stick with the best science. In this case that is the work of Koonin, irrespective of how unpopular his conclusions are.

  33. Dave_Geologist says:

    That’s not my argument David. It’s that if brooms are so hard to make, how come even the most primitive pre-industrial societies had them?

    I tend to stick wieh the consensus on all areas outside my expertise, not just on climate

  34. David B. Benson says:

    Study on consensus:

    But for scientific consensus one has to consider the specialty.
    For abiogenesis just the molecular biologists will do. But the only one to seriously study the matter and write a book is Eugene Koonin.

  35. Dave_Geologist says:

    David, that paper just says lots of people don’t believe the scientific consensus.

    Surprise surprise. Lots of people believe in horoscopes and astrology.

    A molecular biologist who makes the wrong suppositions about the starting environment will be as wrong as a physicist applying Newton’s Laws who assumes that something starts stationary when it is in fact moving. And there seems to be very little molecular biology in his argument. Just an upfront assumption and numerology. Molecular biology would involve saying why that pathway is the only pathway, why the suggested catalysts won’t work (I don’t think anyone serious thinks it can be done without catalysis), and also why no other catalyst we haven’t thought of can work. After asking a geologist what is a reasonable inorganic setting.

    And you don’t need to be a molecular biologist to ask the question: if it’s so hard, why did it happen on Earth at the earliest opportunity, and not billions of years later?

  36. David B. Benson says:

    Dave_Geologist, abiogenesis was a random event.

  37. Dave_Geologist says:

    David, we’re starting to go round in circles but:

    In a universe where abiogenesis is easy, that random event will typically happen early in the planet’s habitable history (about 10 By for an Earth-like planet, while rocks and liquid water are in contact). Most habitable planets will have life, and on most of those planets, life will have originated early. We’d not be unusual.

    In a universe where abiogenesis is hard, that random event will typically happen late in the planet’s habitable history or not at all. Most habitable planets will not have life, and on most of those planets which do, life will have originated late. We’d be unusual, even among the subset of planets where abiogenesis happened.

  38. David B. Benson says:

    We don’t know how long the AI, Abiogenesis Interval, is, viz:

    But irrespective of the length, abiogenesis is so improbable, according to Koonin, that almost all planets with an AI fail to develop life.

    The early vs late argument fails on the above grounds but also some fairly fundamental points of probability; whether or not Terra is typical cannot be determined from a sample size of one!

    I have yet to read a coherent criticism of Koonin’s argument. Life on Terra is unique in the observable universe.

  39. Dave_Geologist says:

    David, neither Koonin nor Gaia touches on the core of my argument: if abiogenesis is hard, and if it did happen on Earth despite being hard, why did it happen in the first of the 100 equiprobable 100My intervals when conditions on Earth were or will be suitable, and not in one of the other 99? Maybe conditions were especially suitable towards the end of the LHB.

    Koonin’s book was in 2011. Has he updated his thoughts in the light of this

    In a lab experiment intended to duplicate the high temperatures and pressures of such an impact, researchers transformed a solution containing a simple pre cursor into adenine, guanine, cytosine, and uracil—the information-bearing nucleobases in RNA, which many believe to have been the first genetic molecule to encode.

    Lots of references, many recent, here: Formation of nucleobases in a Miller–Urey reducing atmosphere.

    Or, if you prefer vents to shock-impacts, Extreme accumulation of nucleotides in simulated hydrothermal pore systems. There’s even a commentary by Koonin: An RNA-making reactor for the origin of life.

    Of course, the results of Baaske et al. (4) by no means put away all of the severe difficulties associated with the origin of life. In particular, at the earliest stages of biogenesis, the formation of mononucleotides, in the first place, remains problematic, and when it comes to the more advanced stages, a ribozyme replicase still is a hypothetical entity (18), and the evolutionary path to the translation systems remains essentially uncharted (19). Nevertheless, the intermediate stage, the transition from a solution of small organic molecules to a population of RNAs, now appears much less mysterious than before. Moreover, the hard combinatorial search for the extremely rare RNA sequences capable of catalyzing complex reactions, such as RNA replication, would be substantially facilitated in this setting through ligation of RNA molecules. Best of all, perhaps, the model of Baaske et al. suggests a straightforward experimental design, and such experiments, if successful, could bring us closer to an actual laboratory reproduction of the origin of life than anything done previously.

    Of course vents and impact shock are not incompatible. The impact simulations make those earliest stages of abiogenesis, the formation of mononucleotides, plausible late in the LHB when it became cool enough for oceans (since there is geochemical evidence for deep mantle compartmentalisation dating back to the first 100my of Earth’s existence, we know the LHB only melted relatively shallow levels, if there even was a magma ocean). Whereas the hard combinatorial search, which seems to be at the heart of his 2011 argument, is made much less improbable by the concentration and compartmentalisation provided in vents. He even seems to be warming to the idea that we could achieve it in the lab! Perhaps that book needs a 2nd edition?

  40. David Benson,
    I bet Dave_Geologist also believes in the theory of abiotic oil, which requires the same recipe — elements such as (C, H, etc), high pressure & temperature, and a long enough time. LOL 😉

  41. Dave_Geologist says:

    LOL. Of course I don’t believe in the theory of abiotic oil Paul. Or is the wink meant for me not David? Gosh, the Interwebz are complicated!

    Anyway that belief would be silly, what with all the biomarkers which can be traced to all the precursor life-forms. For example, terrestrial-derived oils have a high wax content, and land and freshwater plants secrete protective waxes to combat dehydration, osmosis and UV damage. And the process has been reproduced in lab experiments, just like the components of abiogenesis, but starting with kerogen, i.e. partially-broken-down complex molecules, rather than simple models. Indeed I conducted the experiment myself decades ago when I carried a Sourcehound™ into the field and analysed source-rock samples for richness. You have to cook them about 100°C hotter than in Nature. The thermodynamics and kinetics have been well understood for decades and we know it depends on time and temperature. A short time and high temperature achieves the same maturity as a long time at a lower temperature. That’s why we use time-temperature indexes rather than the crude temperature thresholds of the 1960s. Indeed one of the last projects I was involved in before I retired was basin-wide modelling of source rock richness, hydrocarbon generation and expulsion. Oil companies invest billions on the basis of that sort of modelling. Nothing on looking for abiotic oil.

    Back to life on Earth though, now I’m on a roll. There’s been lots of recent progress I was unaware of. Some quite advanced organic molecules could indeed have been delivered by meteorites and comets during the LHB. Apparently you can make complex organic molecules such as nucleosides, nucleic bases, amino acids, sugars, and carboxylic acids from one simple precursor, formamide, simply by exposure to the solar wind. Meteorite-catalyzed synthesis of nucleosides and other prebiotic compounds.

    The formamide-based scenario demonstrates that only one molecule might be the parent for the canonical nucleic bases, sugars, nucleotides, nucleosides, and some amino acids. If we consider life to be a product of chemical evolution with strict rules and determined results, this theory eliminates all of the processes that have the characteristics of a coincidence (complicated pathways that involve a wide range of parent molecules and various conditions). Formamide is abundant in extraterrestrial bodies and it could have been formed as a reaction product of cyanide hydrolysis in the early oceans. …

    The origin of life might simply be a result of a stellar (solar) system evolution processes; that is, the chemical transformation of molecules in the early stellar (solar) system and their subsequent delivery during a heavy bombardment era. Therefore, we are, in a certain sense, aliens.

    Irradiation by high energy protons producing .CN radicals seems to be the key, and that part of the process would be easiest in space rather than on a planet, and given what we know about dust-cloud chemistry, probably happens in many or most solar systems. Meteorite-catalyzed syntheses of nucleosides and of other prebiotic compounds from formamide under proton irradiation.

    Formamide provides a chemically sound starting material for the syntheses of prebiotic compounds; its role in prebiotics is becoming recognized. Kiloparsecs-wide clouds of formamide were observed in the interstellar space. The energy sources for the syntheses explored so far were largely thermal, and the catalysts used were mostly terrestrial. In the presence of meteorites and with high-energy protons, we observe the production of unprecedented panels of nucleobases, sugars, and, most notably, nucleosides. Carboxylic acids and amino acids complete the recipe. These findings extend prebiotic plausible scenarios well beyond our planet.

    Looks like exciting times for abiogenesis, and not just on Earth or in our solar system. Life arising here in the first 100my interval after the LHB rather than in some random 100my interval is beginning to look inevitable rather than coincidental. And spreading out, given technology. Fred Pohl was prescient with his Heechee food factory and CHON food!

  42. David B. Benson says:

    Dave_Geologist, probabilities don’t work like that. If you persist in the belief you have it right, there is a game involving a pair of dice that I would like to play with you for high monetary stakes.

    If you would bother to learn Koonin’s argument you would understand that it already assumes the ready availability of amino acids.

  43. Dave_Geologist says:

    David, OK here’s my bet. I bet I can throw a double six at any point in 100 throws. You bet you can throw a double six on the first throw. We both bet the same amount. I’m up for that game of dice. Are you?

    I can only learn Koonin’s argument by spending a bunch of money on his book, unless someone explains it to me me thoroughly. Given that several years after he wrote the book Koonin appears to have rowed back a long way from the second-hand versions of the argument I’ve read, in his Commentary on the hydrothermal vent paper (CTRL-F combinatorial), I’m not prepared to spend the money on an argument I have a strong hunch Koonin himself no longer adheres to.

    His remaining issue in the Commentary over the manufacture of the required precursors is satisfied by the shock-impact and solar-wind mechanisms. Which are of course not mutually exclusive. Nor the only potential mechanisms. And the “how do you contain them without cell walls” argument was settled by Russell decades ago: vent vesicles.

  44. David B. Benson says:

    Dave_Geologist, yes, if you know the pdf, probability density function. But it is the pdf that is at issue.

    Here is the game: you can see the up faces of the two dice, six on each. You can’t see the other faces. What is the pdf?
    Your argument amounts to optimistically claiming that all the faces are inscribed with a six, so of course two sixes show. The Koonin argument results in the opposite conclusion that only the exposed faces show six.

    As for Koonin’s book, inquire of a local lending library.

    Finally , I see the abstracts of several papers a year regarding abiogenesis or precursors. Yet nothing so far reduces the enormous complexity of assembling a self-replicating biological molecular assembly which incidently also sports a metabolism.

  45. Dave_Geologist says:

    Not at all David. My argument relies on only one face of each dice being a six. I win because I get to throw the dice 100 times and only need to get a double six once, whereas you only get to throw the dice once.

    I’m making the conservative starting assumption (an Objective Bayesian prior, perhaps) that the abiogenesis pdf is flat in time. Of course we know there were times in the past when the delivery of interplanetary organics was much, much more intense, and that they provide potential precursors. In that case the pdf would be front-end-loaded and you could win; but it would be a Pyrrhic victory because you’d only be winning the coin-toss because I’d won the underlying argument.

    I refer you again to Koonin’s PNAS Commentary. I rank that above the book (a) because it’s more recent, and Science Marches On, and (b) because it was written for his peers and published in a learned journal, rather than in a popular book.

  46. According to Markov processes, if the probability of extinction is much greater than the probability of life, then in the steady-state of a “habitable planet”, the uptime of habitability is essentially zero.

    This is apropos to consider in these times of the Extinction Rebellion. That is all.

  47. Dave_Geologist says:

    Short of sterilising the planet, Paul, it seems to be hard to extinguish life once it’s established. It got through several Snowball Earths, and the end-Permian Great Dying.

    Civilisation as we know it – now that’s another matter. And coral reefs. They’ve disappeared several times, for tens of millions of years. It had to wait for another, unrelated (except at the Class level) set of soft-bodied organisms to evolve reef-building.

  48. Just stating the math, not going to get involved in any of the philosophizing.

  49. izen says:

    “if the probability of extinction is much greater than the probability of life, then in the steady-state of a “habitable planet”, the uptime of habitability is essentially zero”

    99.9% of life on Earth IS extinct. That is part of how life is a changing, evolving dynamic process.
    The probability of extinction is not MUCH greater than the probability of life unless conditions become so inimical to life that even bacteria cannot survive. As Dave has pointed out, there is evidence of microbial life just about as soon as liquid water can exist somewhere, and subsequent extreme conditions failed to extinguish it all. It appears that the hard jump is to multi-cellular life, that needs oxygen (or a similar hi-energy chemistry) to sustain it.

    There is also the fact that life acts to perpetuate the conditions that will maintain it as in the Gaia/daisyworld hypothesis.

    The conditions under which unicellular metabolic systems can emerge and survive are less interesting than what conditions can eliminate it entirely.

    There is a reasonable speculation that both Venus and Mars harboured life until ‘recently’ (~1 billion years ago). Venus may still have some form of extremophile bacteria in the upper atmosphere, it would explain some of the changes in albedo and ‘climate’ observed.

  50. David B. Benson says:

    Dave_Geologist, your game is entirely irrelevant as you have assumed a known pdf, the number of spots on the faces of the two dice. So you simply assume optimistic abiogenesis.

    That is not correct. The problem is to estimate the probability of abiogenesis given the evidence. The game I proposed was supposed to be enlightening in that regard, but failed.

    Based on a sample size of one, just Terra, nothing can be said about the probability of abiogenesis other than it is possible. When it occurred on Terra is irrelevant to the question of whether or not it occurs anywhere else in the visible universe.

    As for the commentary in PNAS by Koonin, I will read it if provided with a good reference but preferably a usable link. Whatever you may have tried in an earlier comment failed.

  51. David B. Benson says:

    izen — on the contrary, the difficulty is the creation of something as complex as a bacterium. Or even simpler. For once cynobacteria arise the route to enough oxygen to support multicellular animal life is clear.

    It is establishing a self-replicating biological molecular assembly which incidently also sports a metabolism that is hard.

    For a much simpler, but still difficult task, web search for self-replicating turing machines; no metabolism.

  52. Dave_Geologist says:

    David, here is the Koonin link to the article, in plain text. https://www.pnas.org/content/104/22/9105

    The original was to the .pdf, which I thought was free to all after a certain time-span, but I have a library subscription activated so maybe not.

    On the dice, it’s the pdf over time that matters, not the pdf of the pair of dice.

    Let’s make them billion-sided dice and throw for a double-billion. In 10by I have 100 attempts to throw a double-billion. In the first 100my, you only have one throw. Both very unlikely to hit, but you’re 100 times more unlikely than me. Your chance is the same as mine if I pick another arbitrary 100my interval. But I have the luxury of 100 throws (40 or so to date, the rest before the Sun goes red giant) because none of my 100my intervals is special. Your 100my interval is special. It starts with the delivery from space of a shedload of complex organic molecules.

  53. Dave_Geologist says:

    izen, regarding Venus and Mars, Mars obviously had running water in the past. I’ve seen that claimed for Venus (just the other week in a TV documentary), based on the supposed detection of granite at surface in the highlands. I dug into it a bit and the original reference was not to granite sensu stricto (K-feldspar rich) but to the TTG (tonalite-trondjemite-granodiorite) suite which forms the bulk of the Archaean cratons on Earth.

    It’s not impossible to form true granite in the absence of water – it needs very high temperatures though – but it’s probably difficult. Making TTG doesn’t need water from a geochemical viewpoint, at least no more than the few tenths of a percent which are thought to have been in the primordial mantle. It does need a two-stage partial melting/differentiation process, peridotite to basalt/gabbro, and basalt/gabbro to TTG. Most of the TTGs, Archaean and Proterozoic, seem to have a primordial deep mantle origin, ruling out the third stage of remelting TTG crust, which is the best way to make true granites. A few TTG terrains do show evidence of that, especially the more recent Proterozoic ones, but true granites are very rare anyway in the Archaean. The processes which melt and re-melt are essentially vertical: isothermal decompression melting, and heating by burial. The decompression melting only works in rocks undersaturated with water: wet melting reverses the slope of the liquidus in P/T space. On the modern Earth, apart from in plumes, that vertical movement is ultimately driven by horizontal plate tectonics. But the rocks don’t care about the sideways movement, just the up-and-down.

    There’s a widely held view that plate tectonics requires oceans, although that’s obviously not a sufficient condition because it only got going 3-2.5my ago, a billion plus years after evidence of running water. However there are TTG terrains which predate the generally accepted start of plate tectonics, attributed to convection/advection and density instabilities. Basaltic crust becomes denser than mantle rocks as it cools, especially if it’s thick enough to form eclogite and especially if it’s komatiitic as many early lavas were, and a hot early Earth without plate tectonics would have had more vigorous plumes and convection cells than today’s Earth. Indeed similar processes are invoked for Venus to lose heat today. The geometry of Archaean greenstone belts suggests vertical, not horizontal tectonics. So I would say the jury is out on whether you need oceans, or just vertical motion and bound water in the mantle. The no-water-no-granites paper is 35 years old, and we now know a lot more about the volatile inventory of the early Earth and of the deep mantle today. Both are wetter than we thought in the 1980s.

    Coincidentally, I visited that very subject in my undergraduate geochemistry project. I concluded there had been Archaean plate tectonics, but in retrospect the geochemical signatures I was relying on just reflect the melting and remelting process, and are agnostic regarding the vertical motions which enabled it.

  54. Dave_Geologist says:

    I’m not suggesting early Venus wasn’t wet BTW: just that the Galileo data isn’t the strongest evidence. The recent geochemical evidence that the early Earth was much more volatile-rich than we used to think, dating back to the first 100my of its existence and before the Late Heavy Bombardment delivered more volatiles, would have me saying “so why not Venus too”.

  55. David B. Benson says:

    Dave_Geologist, thank you for the link to

    An RNA-making reactor for the origin of life
    Eugene V. Koonin
    2007 May 29
    PNAS 104, 9105–9106

    Notice that the date is 4 years before the publication of his book. All of these points are encorporated therein.

    For a book review, see
    The Logic of Evolution: Review of The Logic of Chance by Eugene V. Koonin
    Reviewed by Richard D. Emes
    frontiers in Genetics 2012

    The review simply recommends reading the book.

  56. Dave_Geologist says:

    Ah, my mistake on the date David. I thought it was more recent.

    We’ll have to agree to disagree. IMO the fossil evidence trumps ab initio calculations, just as the flight of a bumblebee trumps the engineers who said it was aerodynamically impossible. And I can hardly go with the consensus on climate change, and not go with the consensus on the origin of life, can I?

  57. Dave_Geologist says:

    A late update to this thread: K2-18b got a mention 18 minutes in on the BBC’s The Sky at Night programme at the weekend. They called it a soggy Neptune. They also touched on Jupiter, citing recent evidence that it has a transition from the “core” to the “atmosphere”: Comparing Jupiter interior structure models to Juno gravity measurements and the role of a dilute core. I wonder how robust the conclusion is that Neptune has a distinct rocky core, given that it doesn’t have recent high-precision flybys? Of course Neptune is much colder than K2-8b, so anything affected by factors like the melting point of rock could be different. And the complex organic molecules in the interstellar dust clouds where stars form got a mention. One said they wouldn’t be surprised if we ultimately found amino acids out there 🙂 .

    Going back to the “could K2-18b support life question”: now that I’ve read the arXiv paper, I see their modelled atmosphere reaches 500K between 0.1 and two bars pressure, and rises above the boiling point of water at between 0.1 and 0.5 bars. So if there is a rocky core, it would be in contact with super-hot water vapour, not liquid water, in what both papers say would be a hydrogen-rich atmosphere. So if there is life, it presumably hasn’t developed photosynthesis 😉 (actually, I suppose it could be at a low level, like the early stages of the great Oxidation Event, but with the oxygen being mopped up by hydrogen rather than metabolised). The cloud/rain zone is currently high in the atmosphere, in the 0.05-0.5 bar region. But given that exoplanets seem to have done a lot of wandering, it’s possible that life could have evolved when it was cooler and in a more distant orbit. I would give it a sporting chance of adapting and surviving through slow warming, although nourishment could be a problem without a photosynthesis-equivalent. Thermophiles can be incredibly hardy, surviving cold, X-rays and UV at interplanetary levels: Survival and Adaptation of the Thermophilic Species Geobacillus thermantarcticus in Simulated Spatial Conditions. BTW despite it’s residence it is a thermophile, living in geothermally heated soils on the Antarctic volcano Mount Melbourne, and is not one of the cold-loving critters who live in the ice.

  58. Dave_Geologist says:

    You’ve probably come across this already ATTP, but a recent paper bears on the where-did-Earth’s-water-come-from question: Ruthenium isotope vestige of Earth’s pre-late-veneer mantle preserved in Archaean rocks.

    Geologists had gone cold on the idea that it came from a late veneer delivered by carbonaceous chondrites (along, of course, with organic compounds) because the Ru isotope composition of the mantle is wrong. Ru is highly siderophile and should have been strongly fractionated from a magma ocean into the molten core. Ru in the mantle should be almost entirely from the late veneer, but the modern mantle is well-mixed for Ru and has a different isotopic composition from carbonaceous chondrites. Material balance would argue for a small mass contribution of late veneer, too small to explain all the water. They found evidence from some early, less well-mixed mantle of a Ru isotope composition complementary to carbonaceous chondrites, in that plausible mixing models between the two could give the present-day composition. The clincher would be if we also find patches of ancient mantle with carbonaceous chondrite Ru isotopes.

    The complementary isotope composition matches that expected for the inner Solar System, inboard of Earth. They argue for an early veneer of inner Solar System objects and a late veneer of outer Solar System objects. They don’t go into it but I wonder if that means Earth migrated through the inner Solar System, or just that inner Solar System objects were perturbed by other orbital dynamics and slingshotted around the Sun. It may also bear on the timing of plate tectonics (has to start significantly before 3 b.y., because it happens at tens of cm per year). The mantle seems well mixed after 3 b.y., and plate tectonics is the best way to achieve lateral as well as vertical mixing.

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