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The Best of All Possible Worlds

I’ve always loved the notion of ‘superhabitability,’ which forces us to ask whether, in our search for planets like the Earth, we may in our anthropocentric way be assuming that our own planet is a kind of ideal. Some scientists have been asking for years whether it is possible that the Earth is not as ‘habitable’ as it might be (see What Makes a Planet ‘Superhabitable’?). The question then becomes: What factors would make a planet a better place for life than our own?

Now Dirk Schulze-Makuch (Washington State University), working with René Heller (Max Planck Institute for Solar System Research, Göttingen) and Edward Guinan (Villanova University) runs through the characteristics of superhabitability, which take in planets that are a bit warmer than ours, a bit larger, and somewhat wetter, not to mention those that circle stars that live longer than our G-class Sun. 24 interesting planets emerge, all more than 100 light years out, but none of those so far identified meet all the specifications for superhabitability enumerated in the paper, nor could they, given the limitations on our current observing methods.

As we dig into this, let’s first remember the significance of target selection as we continue to refine our telescopes. With interesting new resources coming, space observatories like the James Webb Space Telescope and the European Space Agency’s PLATO, we will delve ever more deeply into exoplanet atmospheres.The goal here is to consider the star systems we will want to investigate as we learn how habitability works and how it might be improved upon. Choosing the list carefully is paramount, for time on these observatories will be precious.

We’ve talked before about the virtues of K-class stars, which are less massive and less luminous than the Sun, while also offering a more capacious lifetime, at minimum twice that of a typical G-class star, and in some cases quite a bit longer than that. Given that life on Earth will likely be rendered impossible within a billion years or so due to changes on the Sun, how much more useful to have tens of billions of years longer to allow life to grow and advance. Various other factors likewise come into play, among them a planet’s geothermal heat and magnetic field. The paper runs through the possibilities (and you might also want to have a look at Orange Dwarfs: ‘Goldilocks’ Stars for Life?, which contains several further citations). From the paper:

…studies of solar proxies of our Sun have shown that young dG [dwarfs of spectral class G] stars rotate > 10 times faster than dG stars near the age of our Sun, and have correspondingly high levels of magnetic dynamo-driven activity and very intense coronal X-ray and chromospheric FUV emissions (Guinan et al., 2005), which makes the origin and early evolution of life challenging. Heller and Armstrong (2014) argued that the increased life span of stars with masses lower than one solar mass may allow inhabited planets to build up a higher biodiversity and possibly even a more complex ecosystem…. This argument would lift K- and M-dwarf stars into the realm of superhabitable planet host stars.

But as the authors point out, moving the range of superhabitability into the realm of M-dwarfs is going too far — here we confront problems of tidal locking, flare activity and atmospheric loss due to the proximity of the planet to the star and exposure to a strong stellar wind. Avi Loeb and Manasvi Lingam have also written on the likelihood that K-dwarfs offer the most stable long-lived environment conducive to superhabitability, and it is worth remembering that we have a K-dwarf in the closest stellar system to our own, Centauri B in Alpha Centauri.

What would a superhabitable planet look like? A planet a bit larger than Earth — one with, say, 1.5 times Earth’s mass — would retain internal heating from radioactive decay longer, while the stronger gravity would allow the planet’s atmosphere to be retained for a longer period. A planet between 5 and 8 billion years old seems to be optimum, an age at which geothermal heat and protective magnetic fields persist. Temperature is also an interesting factor:

…higher temperatures than currently existing on Earth seem to be more favorable. The caveat is that the necessary moisture has to be available as well because inland deserts with low biomass and biodiversity are also common on our planet. One example is the early Carboniferous period, which was warmer and wetter (Raymond, 1985; Bardossy, 1994) on our planet than today, with so much biomass produced that we still harvest the organic deposits in the form of coal, oil, and natural gas from it. Thus, a slightly higher temperature, perhaps by 5°C—similar to that of the early Carboniferous time period—would provide more habitable conditions until some optimum is reached. However, this will depend on the biochemistry and physiology of the inhabiting organisms and the amount of water present.

Image: This is Figure 1 from the paper. Caption: Star-planet distances (along the abscissa) and mass of the host star (along the ordinate) of roughly 4500 extrasolar planet and extrasolar planet candidates. The temperatures of the stars are indicated with symbol colors (see color bar). Planetary radii are encoded in the symbol sizes (see size scale at the bottom). The conservative habitable zone, defined by the moist-greenhouse and the maximum greenhouse limits (Kopparapu et al., 2013) is outlined with black solid lines. Stellar luminosities required for the parameterization of these limits were taken from Baraffe et al. (2015) as a function of mass as shown along the ordinate of the diagram. The dashed box refers to the region shown in Fig. 2. Data from exoplanets.org as of May 20, 2019. Color images are available online.

Here is the complete list of factors that lead to superhabitability as listed in the paper. 9 of the 24 planets identified orbit K stars, while 16 of them are between 5 and 8 billion years old. None of these worlds meet all the criteria, in any case. To be fully identified as superhabitable, a planet would meet the following benchmarks:

  • In orbit around a K dwarf star
  • About 5–8 billion years old
  • Up to1.5 more massive than Earth and about 10% larger than Earth
  • Mean surface temperature about 5°C higher than on Earth
  • Moist atmosphere with 25–30% O2 levels, the rest mostly inert gases (e.g., N2)
  • Scattered land/water distributed with lots of shallow water areas and archipelagos
  • Large moon (1–10% of the planetary mass) at moderate distance (10–100 planetary radii)
  • Has plate tectonics or similar geological/geochemical recycling mechanism as well as a strong protective geomagnetic field

As I mentioned, none of the planets of interest identified in this paper is known to have all these characteristics, but we have to keep in mind “the uncertainties in our mostly qualitative model and…the uncertainties in the observed parameters” as we examine the list. Moreover, only two of these — Kepler 1126 b and Kepler-69c — have been validated; the others continue to be planet candidates (Kepler Objects of Interest, or KOIs), and some may turn out to be false positives, while one of the confirmed worlds, Kepler-69c is, at almost 2000 light years, too far out to be a promising candidate for JWST, or even possible successors like LUVOIR.

The near-term question arising from this initial cut at superhabitability is whether we can use these criteria to find worlds closer than 100 light years, and thus accessible for high-quality observations from the TESS mission. The list of criteria presented here should help to establish observing priority for any worlds meeting a large number of these standards that can be identified in this distance range, making them high-value targets for investigating biosignatures.

The paper is Schulze-Makuch, Heller & Guinan, “In Search for a Planet Better than Earth: Top Contenders for a Superhabitable World,” Astrobiology 18 September 2020 (full text). The Lingam & Loeb paper is “Physical Constraints on the likelihood of life on exoplanets,” International Journal of Astrobiology Vol. 17, No. 2 (April, 2018), 116-126 (abstract), but see also the same authors’ later paper “Is life most likely around Sun-like stars?,” Journal of Cosmology and Astroparticle Physics Vol. 2018, May 2018 (abstract). Citations to earlier work from René Heller and colleagues on the question of superhabitability can be found in the Centauri Dreams stories I linked to above.


Comments on this entry are closed.

  • Alex Tolley October 6, 2020, 9:59

    One issue I have with the +5C temperature optimum is that the global temperature on earth has been highly variable and controlled by the the atmospheric gas composition. In turn, this composition, especially CO2 has been controlled by life itself. If life is the controller of atmosphere composition, this is not an independent variable, but a dependent one.

    But let us suppose that the average temperature is dependent on the orbit. Earth is already close to the inner edge of the HZ A higher insolation to push up teh temperature places the planet closer to conditions that could result in a runaway greenhouse effect. IOW, while biomass might increase in a tropical world, the probability of a climate catastrophe increases, and with it extinction events, possibly a permanent one.

    Lastly, while the Carniferous was noted for its lushness, much of this biomass was unable to be reutilized as fungi to break down lignin had not evolved. It was this that helped the formation of the deep coal beds.

    While it is true that warm, wet, tropical forests have the highest biodiversity on Earth today, a planet without other types of biomes would have a reduced opportunity to explore evolutionary space. Without grassland, there would be no large grazing mammalian herbivores. No cold winters would prevent the evolution of hibernation, and animals that migrate over seasons. The Carnoniferous had more surface biomass, but our contemporary world has a greater true diversity of explored evolutionary space.

    • Ronald October 10, 2020, 13:54

      “In turn, this composition, especially CO2 has been controlled by life itself”
      And plate tectonism, in particular mountain building and volcanism. A lot of mountain formation means a lot of erosion resulting in lower CO2. Volcanism adds CO2.

    • xcalibur October 15, 2020, 3:08

      Point taken on variety of biomes. However, climate tends to be self-regulating, so you may be overestimating the risks of catastrophe. Keep in mind, Venus’ runaway greenhouse effect was caused by its lack of a substantial magnetosphere; which led to hydrogen being stripped away, drying out the planet and building up co2, creating the hellscape we know today. Likewise, Mars became a cold desert because it lost its atmosphere to solar wind. It seems that a potent magnetosphere is a key ingredient to habitability.

  • Brett P Bellmore October 6, 2020, 11:49

    “One example is the early Carboniferous period, which was warmer and wetter (Raymond, 1985; Bardossy, 1994) on our planet than today, with so much biomass produced that we still harvest the organic deposits in the form of coal, oil, and natural gas from it.”

    Not just warmer and wetter, but also more CO2. CO2 levels were about 1,500 ppm at the beginning of the Carboniferous period, which terminated as they reached about 350 ppm, modern levels. Then kept dropping…

    Just before the “Anthropocene”, when humans started restoring that carbon to the atmosphere, CO2 levels were actually getting close to the point where photosynthesis stops, as low as 135 ppm. They’re still at starvation levels so far as C-3 plants are concerned.

    We were actually getting close to the end of Earth’s capacity to sustain a complex biosphere, humans came by just in time.

    Now, back during the Carboniferous period, the Sun was a little dimmer, so *maybe* 1,500 ppb would be a little high these days. But I’ve sometimes speculated that, with a little geoengineering, using a wavelength selective sun shield and higher CO2 levels, Earth could sustain a much more vibrant biosphere than today, by concentrating sunlight primarily in the wavelengths efficiently utilized by plants. I suspect we’d get used to the purple sunlight…

    • Alex Tolley October 6, 2020, 19:45

      Just before the “Anthropocene”, when humans started restoring that carbon to the atmosphere, CO2 levels were actually getting close to the point where photosynthesis stops, as low as 135 ppm. They’re still at starvation levels so far as C-3 plants are concerned.

      The lowest CO2 levels were about 175 ppm. C3 plants can photosynthesize at least down to 150ppm. However, this is not an issue. If photosynthesis stops, then CO2 levels rise as respiration becomes dominant. Oceans would start to release their CO2 from their huge reservoirs, and volcanism would add to the CO2 levels.
      We know that all the major biomes existed 10 millennia ago.

      IOW, the idea that CO2 levels were at a perilously low level is not true.

      • Brett Bellmore October 7, 2020, 12:49

        “The lowest CO2 levels were about 175 ppm. C3 plants can photosynthesize at least down to 150ppm.”

        Yes, they can just barely survive at such low levels. The distinction between “not dead yet” and “healthy”.

        • Alex Tolley October 7, 2020, 13:46

          And yet there were extensive tropical and boreal forests. How would this be possible if what you are claiming is true? If C3 plants were in dire straits, the world would have been covered in grassland. Yet this was not the case.

          Humans were numerically a tiny group with very limited impact on teh bisophere for many thousand of years. The levels of CO2 did not start increasing due to human use of fossil fuels until the industrial revolution barely 3 centuries ago. Yet the CO2 levels were more than adequate to support the C3 plants prior to this time.

          Suggesting thatanthropogenic CO2 emissions is useful is just disinformation in the service of the fossil fuel industry. The results of global heating will more than offset any benefit of CO2 on plant growth that you can get in a controlled greenhouse experiment.

    • H. Floyd October 7, 2020, 1:08

      “They’re still at starvation levels so far as C-3 plants are concerned. We were actually getting close to the end of Earth’s capacity to sustain a complex biosphere, humans came by just in time.”

      The idea that carbon dearth poses a greater problem than carbon-induced climate change is timely and reassuring — conspicuously so — but not without a lot of informed critics. If there’s majority support for that argument among earth, plant and climate scientists, I’d be genuinely interested in seeing it borne out. Cf., https://tinyurl.com/yxj8j8lm

      • Alex Tolley October 7, 2020, 16:13

        I had forgotten that the the US administration had put a climate heating denier on a climate committee. With so many pro-industry lobbyists running various agencies it becomes harder to keep them all in memory, especially as the dismantling of regulations and institutional expertise makes news.

        The media is really responsible for ensuring that “truth sandwiches” are published to counter the nonsense propaganda these people push out.

    • Ronald October 10, 2020, 14:03

      “CO2 levels were about 1,500 ppm at the beginning of the Carboniferous period, which terminated as they reached about 350 ppm, modern levels. Then kept dropping”.
      Well, that is too simplistic: in fact the CO2 level has fluctuated a lot since then. It was also much higher (about from 1000 – 1800 ppm) during much of the Cretaceous.
      Atm. CO2 level largely depends on the balance between mountain formation, which removes CO2 through erosion and mineralization, and volcanism, which adds CO2.
      The recent intense ice ages were probably largely a result of the formation of the Himalayas which meant enormous erosion and CO2 binding.

      • Gary Wilson October 16, 2020, 14:14

        And for the industrial age atmospheric CO2 is increasing due to the human contribution currently amounting to 2 ppm to 3ppm a year and rising. The human contribution is huge and rising and it is causing a huge negative change in global climate. This is undeniable to anyone who will read the literature and is not a fossil fuel apologist (or possibly working for or receiving money from fossil fuel companies). Denialism is a powerful and dangerous problem in our human society. It was in effect for decades during the denial of the connection between cigarette smoking and lung cancer, just to give one other example. I watched someone I loved die of lung cancer and I don’t want to see my own race die due to its addiction to fossil fuels.

  • DCM October 6, 2020, 12:45

    Another effort at generalization without information. Till we examine worlds with life up close we can’t say there are any more habitable than Earth. We do know Earth is inhabited and has been for a very long time.
    Speculation is good, but it’s likely taken for fact. Consider the reports of UFOs that have generated bizarre histories in which aliens are responsible for all human advances not made by European whites.

    • safely anonymous October 6, 2020, 21:35

      We need more exoplanet missions

  • James Essig October 6, 2020, 12:48

    Awesome! Prospects for future settling of super-habitable planets around orange colored Suns is whimsical.

    Since there have been proposed exotic stars that burn on other processes aside from main-sequence stellar fusion processes, these might work as well.

    Also, I just thought to ask if anyone knows of credible literature on habitability around red dwarfs. I have read the usual stuff about how stellar flares can be a biosphere evolution ending mechanism for red dwarf systems. Perhaps stars that lie along the border between red dwarfs and K-class stars can work.

    • Brad in Dallas October 6, 2020, 14:51

      If we can shake off the mental habit of thinking we can only ever live on planetary surfaces, orbital Island 3-style colonies can be built in orbit around any source of radiant energy. Having asteroidal raw materials with the necessary elements would be almost the only other criterion.

      • James Essig October 6, 2020, 19:45

        That is a good point. Since K stars can live a lot longer than G class stars, they are a great place to set up orbiting space-habs around.

        There is something romantic about the idea of living in a space colony with an orange sun.

  • Michael Spencer October 6, 2020, 13:03

    The notion of a ‘better’ earth is so intriguing. A couple of questions:

    1. Wouldn’t the high oxygen level make the planet prone to fire?

    2. Could such a planet be imagined to retain other desirable features without the gravity penalty? Getting out of our gravity well has been a very difficult issue for us as it is. And of course inhabitants would have compensatory features.

    • Robert October 6, 2020, 16:33

      A planet 10% larger but 1.33 times as massive as Earth would have the same surface gravity as we have.

      • Thomas R Mazanec October 6, 2020, 18:22

        But it would have a higher escape velocity, making it harder to get into space.

      • Brett Bellmore October 6, 2020, 18:47

        And would be basically impossible to leave without nuclear rocketry. If Earth were even a little more massive, chemical rocketry wouldn’t be capable of getting you into space.

        • Michael October 10, 2020, 3:42

          Not strickly true, multi staging, but it’s a lot harder.

    • Robert October 6, 2020, 18:21

      I just realized that even though surface gravity is the same in the case above, other things such as orbital velocity are not. So, there would be a penalty in this case as you say.

    • Alex Tolley October 6, 2020, 19:17

      Wouldn’t the high oxygen level make the planet prone to fire?

      Yes it would, although the dampness will mitigate this. As we are seeing with the western US fires, as the forests fry out, the number and size of the wildfires increases. In Brazil, the Amazon rainforest has been burned by natives for small scale farming. Now cattle ranching and soy farming is resulting in large areas being burned, which changes the local climate to drier conditions allowing for more frequent natural fires in the future.

      But note that the fires return the CO2 and O2 back towards less flammable conditions, as well as reducing temperatures by adding smoke to the skies which reduces the insolation at the surface. This is a self-regulating situation.

  • Michael October 6, 2020, 15:13

    They seem to have missed off a massive atmosphere, a thick dense atmosphere retains heat better and allows for larger creatures to get about easily.

  • Christian G October 6, 2020, 16:25

    Definitely interesting but there’s something on that list you didn’t mention – are we still going with the idea that a large moon is indispensable? I seem to remember one or more papers arguing that the greater precession wouldn’t have enough of an effect on climate to eliminate the possibility of life coming about.

    • Alex Tolley October 6, 2020, 19:49

      Interestingly the paper notes that the Earth’s obliquity might be stable without a moon, and also that instability is not necessarily a problem for life. Why they stuck with the need for a moon is not clear to me.

    • Ronald October 11, 2020, 7:46

      Yes, I think Barnes et al.
      A large moon is grossly overrated as a stabilizer of Earth’obliquity and hence climate. Without our moon the Earth climate would probably only vary significantly in the course of something like 50 – 100 my, which isn’t faster (in fact slower) than many present major climate changes (even without humans I mean).
      The gyroscopic effect of the Earth itself may actually have a greater stabilizing impact.

  • Jer October 6, 2020, 19:20

    Of course, we may be engaging in a bit of hyper-sentimentality, since it is very likely that humanity itself will start to engineer better organisms, better animals, richer and denser ecosystems, types of ecology of such a varied nature that any world could support them, and ultimately the means to create a level of complexity (the ultimate test and judgment of biodiversity) way beyond what simple ‘Nature’ herself could ever hope to accomplish. Further, it is entirely likely that older and more sophisticated exo-cultures have done just that with their ‘home world’. Perhaps, even before they became a space-faring entity. A world that is most likely to achieve the base conditions for ‘intelligent life escape velocity’ might be all that’s required to spawn a ‘complexity cascade’ – whether such a base condition could achieve its intelligent inhabitants a billion years sooner could be of interest and may be part of the Fermi.

    • Robin Datta October 6, 2020, 22:30

      “create a level of complexity (the ultimate test and judgment of biodiversity) way beyond what simple ‘Nature’ herself could ever hope to accomplish”

      Indeed so, except that biodiversity may then be passé: IBM’s Deep Blue was designed to learn and self-program: by playing against itself for a few hours, it attained skills beyond the world’s top chess players. Likewise Alpha Go trained against itself and then made moves beyond the conceptualization of the world’s top players. Admittedly these are one-trick dogs, but that does not preclude integrating more dogs with more tricks into a greater whole.

      And just as common practical concepts in one specialty seem novel in another when transferred by a physician switching specialties, so too may unimagined cross-fertilization occur unawares when not just dogs but whole kennels are integrated. One scenario may be that humans do not even realize that they are being controlled by their creations.

    • Michael Spencer October 7, 2020, 8:47

      “it is very likely that humanity itself will start to engineer better organisms, better animals, richer and denser ecosystems”

      Indeed. The Nobel in Chemistry went properly to the discoverers of CRISPR. I remain surprised the we’ve not seen the misuse of this tool, yet; but it is coming.

      • Alex Tolley October 7, 2020, 11:30

        I remain surprised the we’ve not seen the misuse of this tool, yet; but it is coming.

        We have had restriction enzymes to cut DNA for half a century. They are just not as precise as CRISPR but are widely taught the in biology courses. In that time there has been reasonably good control of bioengineering as far as I am aware. Maybe we were lucky.

        But bear in mind it is DNA sequencing and CRISPR that are the tools that were crucial in delivering a Covid-19 vaccine, as well as tracing virus mutations both to track the animal source and to trace the pandemic. Like most technologies, CRISPR is a two-edged sword, but it is up to our better angels to manage the risks.

      • charlie October 7, 2020, 17:55


        On hearing that they had been awarded a Nobel Chemistry Prize for their groundbreaking work on gene-editing Jennifer Doudna and Emmanuelle Charpentier said they hoped it would inspire a new generation of women in science.

        Charpentier and Doudna are the first all-woman team to receive a Nobel science prize and become the sixth and seventh women to be honoured for their research in chemistry since the first awards in 1901.

        Pernilla Wittung Stafsheden of the Royal Swedish Academy of Sciences, which is responsible for selecting the Nobel laureates in chemistry, said the prize to two female laureates was “a historic moment”.

        The Nobel is for the pair’s development of CRISPR-Cas9, a tool that allows scientists to snip DNA and edit the genetic code of animals, plants and microorganisms.

        congratulations to both; well deserved prize !!

      • tesh October 8, 2020, 13:40

        I still think that the use of CRISPR and other cell biology tools to (even more) nefarious ends is in its infancy. We are probably using it to generate better killing agents (based on viral and/or bacterial systems) but are not at the stage where manipulating the genome is an advantage. I wonder whether there will ever be a situation where someone (some entity) will feel the need to go down this path?

        A couple of scenarios – both of which require routine use of far more sophisticated and intricate bio-technology than we have now:
        1. a country lacking resources start to experiment to enable its people to withstand a specific challenge(s)
        2. a country lacking resources creates something that will disrupt the rest of the world to “even” things up
        3. Plastic particulates become so ubiquitous that they pervade everything at toxic levels and eventually require humans to have the ability to digest plastic – I used plastic but it might as be some thing different.
        4. Due to lack of resource we engineer the ability to (also) photosynthesis

        These are all of course quite outlandish and require highly specific pressures and circumstances but maybe this is evolution in an “evolved” form. It will no longer be based on local geography, bio-pressures and take 10s of thousands of year to come to fruition. Maybe going forward it will be faster and based on global/non-(classically)gia pressures that are created by our need to consume.

        • Alex Tolley October 8, 2020, 15:53

          …but are not at the stage where manipulating the genome is an advantage.

          I suspect that you may not be conversant with the use of CRISPR. 2 examples:

          In medicine it has already been shown to successfully treat genetic diseases in animals (e.g. dogs) and will in due course be allowed in the clinic.[1]
          CRISPR has been used in several ways to reduce the spread of mosquito borne diseases like malaria. Whether to cause a self-induced population crash or the inability of the plasmodium parasite to live in the females using a “gene drive” [2]

          1. genetic disease treatment with CRISPR
          2. gene drive

          • tesh October 9, 2020, 8:43

            The problem here may be my poor attempt to write down what I meant clearly – or – its interpretation.

            Taking the whole sentence you highlight:

            “We are probably using it to (*) generate better killing agents (based on viral and/or bacterial systems) but are not at the stage where manipulating the genome is an advantage. ”

            I concede we are currently endeavoring to make use of the tech for positive reasons with a (long term) view of treating humans with rare disease and to that end maybe i could have added “try and treat certain rare diseases and…” at the indicated asterisk (*). Yes, it can be a useful tool in treating a rare disease or two but the technology is currently too clumsy to correct these diseases with surety and at the level of safety required.

            I will however stick with the bit you highlighted. It is more an inference to changing a healthy (human) genome to gain a perceived advantage – which is where the rest of my post meanders towards.

            p.s. “Conversant” is a broad term I guess, but I have heard of CRISPR, I know of it’s uses and have indeed made use of it! Will I be using it tomorrow on my children to “cure” the unlikeliness of them growing to 180 cm, their inability to see in the infrared, etc, – nooooo.

            • Alex Tolley October 9, 2020, 12:48

              I think I see what you are getting at. Biotechnology is still in its infancy and has hardly started to remake the world. Because life is so much less controllable than inanimate mechanisms, we have to proceed carefully. Any technique that is used in the clinic requires nearly 2 decades of testing first. I think in practice if we want to destroy nanoplastic particles, even in our own bodies, that we will engineer bacteria to do that, not our own genomes. Similarly, I do not see any value in being able to photosynthesize CO2 and water in our our skins, rather than externally, as the surface area has to be very large, comparable to a small field to support our respiration.

              Development in organisms is more like baking, it is easy to adjust basic ingredients to make either a bread or a cake, but you cannot convert a bread to a cake after the bake. In that sense, gene editing of the human genome will not be used to cure diseases directly. You can extend telomeres to extend cell replication in adults. But most uses of DNA editing will be used to modify the development of short lifespan cells, like white blood cells, or to change the genomes of symbiotic organelles like mitochondria, or the microbiome. But I think the major impact of CRISPR will be to modify single cells like E.coli and yeasts to insert code to create specific products, whether complex biological molecules or small compounds. Much of this will be invisible, just as human insulin manufactured by gene engineered bacteria is not visible to diabetics. CRISPR is far more precise than the library of restriction enzymes in terms of insertion or deletion of DNA sequences, which will make designer organisms far more like engineering than stochastic tinkering.

              There are dangers to be sure, although because the control of bioweapons is so poor I doubt we will see their use against populations. I suspect that it will be the unforeseen consequences of some engineered organisms that will prove problematic. The science fiction episode Beachhead from the old BBC “Out of the Unknown” series is an example.

              • tesh October 9, 2020, 18:51

                I believe that we will eventually pry open pandora’s box – we will get to a stage where we start to interfere/manipulate the genome of healthy humans. The plastic and photosynthesis examples were (mostly) facetious but similar efforts will take place – though thankfully/hopefully in a future distant from now. Maybe increased/decreased pigmentation in the skin, regrowth of adult teeth, greater muscle growth etc. are slightly more plausible in the near term. Indeed, what’s to stop a similar procedure to introducing healthy genes into diseased cells in vivo not being applied to augmenting a specific (even a vain) phenotypes such as hair/eye/skin colour, weight and height?

                I agree, the first and major uses of sophisticated bio-technologies will be on bacteria, crops and simple organisms (which in fact is broadly practised already) but eventually it will progress to a point where we will no longer be able to resist using it on humans.

                I somehow find it inspiring, sad and unsurprising. Inspiring because i believe we will master the tech give enough time and sad because we will misuse it – unsurprisingly.

  • John walker October 6, 2020, 20:54

    The merits of superhabitability as a concept and of any associated desirable planetary or stellar characteristics is a solid topic for discussion. But, I do wonder why the authors included the above list of “candidate” planets. They have little to offer to the conversation. As Paul noted only 2 are confirmed. And it’s superfluous to add that none of the candidates fulfill all the benchmark parameters since 5 of the 8 parameters are currently impossible to measure.

  • Robin Datta October 6, 2020, 21:54

    Superhabitability may be viewed in a variety of ways. If diversity is in the running, a contender may be the mouths of snakes and other reptiles that swallow prey without attempting to kill it: the prey defecate in their mouths. Compost heaps/piles/bins could be another consideration.

    Feces being 50%+ microbial forms, the terminal part of the gut of most metazoa would be a consideration for superhabitabilty if sheer abundance of organisms were a consideration.

    Still unresolved is whether a superhabitable world would indeed become inhabited through abiogenesis or panspermia (which pushes abiogenesis to some remove) or whether that world might remain sterile – till we showed up!

    And if materials akin to lignin and later microbes akin to lignin-digesting fungi were in play, deposits of energy-dense coal-like material deposits would be a one-off event, putting a crimp in the possibility of there arising of a new civilization after a preceding one has consumed the coal-like deposits.

  • Michael Fidler October 6, 2020, 23:46

    Refining the transit timing and photometric analysis of TRAPPIST-1: Masses, radii, densities, dynamics, and ephemerides.

    “We have collected transit times for the TRAPPIST-1 system with the Spitzer Space Telescope over four years. We add to these ground-based, HST and K2 transit time measurements, and revisit an N-body dynamical analysis of the seven-planet system using our complete set of times from which we refine the mass ratios of the planets to the star. We next carry out a photodynamical analysis of the Spitzer light curves to derive the density of the host star and the planet densities. We find that all seven planets’ densities may be described with a single rocky mass-radius relation which is depleted in iron relative to Earth, with Fe 21 wt% versus 32 wt% for Earth, and otherwise Earth-like in composition. Alternatively, the planets may have an Earth-like composition, but enhanced in light elements, such as a surface water layer or a core-free structure with oxidized iron in the mantle. We measure planet masses to a precision of 3-5%, equivalent to a radial-velocity (RV) precision of 2.5 cm/sec, or two orders of magnitude more precise than current RV capabilities. We find the eccentricities of the planets are very small; the orbits are extremely coplanar; and the system is stable on 10 Myr timescales. We find evidence of infrequent timing outliers which we cannot explain with an eighth planet; we instead account for the outliers using a robust likelihood function. We forecast JWST timing observations, and speculate on possible implications of the planet densities for the formation, migration and evolution of the planet system.”


    Stellar Flares versus Luminosity: XUV-induced Atmospheric Escape
    and Planetary Habitability.

    Space weather plays an important role in the evolution of planetary atmospheres. Observations have shown that stellar flares emit energy in a wide energy range (1030-1038 ergs), a fraction of which lies in X-rays and extreme ultraviolet (XUV). These flares heat the upper atmosphere of a planet, leading to increased escape rates, and can result in atmospheric erosion over a period of time. Observations also suggest that primordial terrestrial planets can accrete voluminous
    H/He envelopes. Stellar radiation can erode these protoatmospheres over time, and the extent of this erosion has implications for the planet’s habitability. We use the energy-limited equation
    to calculate hydrodynamic escape rates from these protoatmospheres irradiated by XUV stellar flares and luminosity. We use the Flare-Frequency Distribution of 492 FGKM stars observed
    with TESS to estimate atmospheric loss in Habitable Zone planets. We find that for most stars, luminosity-induced escape is the main loss mechanism, with a minor contribution from flares. However, flares dominate the loss mechanism of ∼20% M4-M10 stars. M0-M4 stars are most likely to completely erode both their proto- and secondary atmospheres, and M4-M10 are least likely to erode secondary atmospheres. We discuss the implications of these results on
    planetary habitability.


    The large majority of stars are M4-10 Dwarfs and are very likely to host life. TRAPPIST-1 planets spectrums will give us how likely the M4-M10 planetary systems are “The Best of All Possible Worlds”.

    • torque_xtr October 9, 2020, 8:06

      https://arxiv.org/pdf/2010.01074.pdf – wonderful results if correct. It illustrates how carefully should initial TTV results be considered and how important are precise prolonged observations and refined calculations. Totally inconclusive before but now fits beautifully into a single picture – all seven worlds formed from material dried out by initial high-luminosity phase. So what we see is not full analog of Galilean moons system – all seven worlds are analogs of Io and maybe Europa, probably with additional ice worlds and maybe even neptunes way farther out, beyond initial snow line – now as cold as Pluto. Should be able to see them with more TTV observations and astrometry! (there also should be a cold debris disk, although I haven’t heard about it yet)

      PS it also means worsened prospects for habitability – even if there was considerable delivery of volatiles from outgassing and outer system, it was likely gradual, with all volatiles freezing on the nightsides because of initially poor hear redistribution. Unless build-up was enough for glacial flow onto daysides and runaway melting due to improved heat transfer.

      PPS of course there could be surprizes and coincidences. Total mass is very close now to universal satellite/primary mass ratio of 0.02% for giant planets, and it should be much lower if these planets formed only from rocky part of circumstellar material.

  • Geoffrey Hillend October 7, 2020, 1:29

    Another problem is that a K dwarf star might be tidally locked or it might be difficult for the exoplanet to have a Moon. I am assuming that the K dwarf star was picked because gives the exoplanet a longer life in the life belt, than a G class star. I agree with everything on the Superhabitable contingency list accept for the K star.

    • Ronald October 11, 2020, 7:58

      Well, maybe it should have been better specified (as I will put in my longer comment) that the authors chose early K stars, plus (late) G stars.
      As for the underlying principles, which come from Cuntz & Guinan in the referred ‘Goldilocks stars’ post, and from the earlier Heller et al. paper:
      the optimal stars are considered by the researchers to be late G and early K, about from G8 – K2 or so. In the mentioned paper by Cuntz & Guinan, they clearly state that at or beyond K3 tidal locking becomes a problem.

  • david lewis October 7, 2020, 11:04

    A larger planet could very well mean it ends up being a water world. On earth, despite being 70 percent of the planet’s surface area, oceans only account for 1-2 percent of our biomass, and that biomass tends to be near the continents.

    On earth, if there was double the CO2 in the atmosphere, the oceans would tend to be higher, but we would also have a whole other continent for life to flourish on. Higher temperatures could also mean more nitrogen-fixing bacteria to fertilize the soil, possibly meaning higher plant growth. And winters would be less severe, again potentially meaning more biomass.

    Of course, that would require CO2 levels to be stable at those levels, and for life to be adapted to those levels and that temperature – not for something to be whiplashing the CO2 levels around. Nor would it necessarily be more ideal for intelligence life. Better conditions for life also means better conditions for disease and parasites.

  • charlie October 7, 2020, 14:27

    One thing that continually nags at me is the issue of gravitation. We have an entire Earth’s worth of organisms that have adapted to not only a certain radiation intensity, magnetic fields intensity, etc. etc. etc. but more importantly have been exposed for billions of years to a relatively stable gravitational field.

    Now I’m all for the idea that we can go and settle into other planetary systems provided a lot of our needs are met but we are not capable of (as far as we know) adjusting and manipulating gravitational fields willy-nilly. A few years ago I came across an article in Discover magazine (by the way, a far now superior magazine to scientific American). Any who, the article got into the issue of adjusting orbits using gravity assists through the use of asteroids. I won’t get into the details here. But it would seem to behoove us to be able in foreign stellar systems, provided the conditions are met to use asteroids to take underweight (but potentially habitable) bodies and bulk them up (if you will) such that their mass will be added to by asteroids to bring the primary body up to the mass of our current earth. That way the new body gains a significant fraction of mass and maybe the asteroids could also be used to do orbital adjustments such that we have a near as close to earth new home as we do here. Just a thought.

  • Geoffrey Hillend October 7, 2020, 16:54

    Another problem as Alex Tolley has mentioned is the 5 degrees C or centigrade warmer everywhere because that would be ten degrees F warmer at the equator and that is a little too high some species. It is possible that the exoplanet could be in a warmer period like a single supercontinent phase and still have ET life especially with the K dwarf star model which has a longer lifetime for the life belt in at the same distance from the star. If it is that warm with separate continents with the same distance apart as on the Earth, then I would think something has to be wrong there because that is not be normal. Either the K star is moving off the life belt and getting brighter or some kind of ET made greenhouse gases have changed the climate there. Look for the spectra of CO carbon monoxide in the atmosphere. I.e., the land positions of the continents have to be considered because the long term climate is based on them and does not stay the same over 500 million years. If the exoplanet is 5 to 8 billion years old, it should not be 5 C warmer than ours especially if we include high oxygen levels of 25 to 30 percent. A high oxygen level implies a low CO2 level and photosynthesis had a long time to increase oxygen levels with a lot of plants, forests and phytoplankton.

    One thing is for certain, the life in the carboniferous period did remove a lot of carbon dioxide, so there is no way the carbon dioxide levels will ever go back to a time when life began or before it such as 7000 ppm of CO2 even if we consider the possibility of a longer lifetime in the life belt around a K dwarf star because photosynthesis or life and the carbon cycle removed the carbon dioxide. The carbon cycle still includes high periods of volcanic activity.

    • Ronald October 10, 2020, 13:48

      “A high oxygen level implies a low CO2 level”.
      No, not correct: the CO2 level is and has always been, since higher life emerged, very low compared to the O2 level.
      Even if CO2 level is 2000 ppm, this is only 0.2%.
      During large parts of the Carboniferous and the Cretaceous both CO2 and O2 levels were higher than today. These can go together, especially because CO2 is added by volcanism and converted to O2 by plants.

    • Ronald October 11, 2020, 8:04

      “5 degrees C warmer (…) I would think something has to be wrong there because that is not be normal. Either the K star is moving off the life belt and getting brighter”.
      No, not necessarily: as I pointed out above, Earth has been much warmer AND lush during much of its history, e.g. the Cretaceous. This has nothing to do, obviously, with moving off the HZ on the hot side, but with atmospheric conditions, in particulat CO2 levels (which in turn depend on volcanism, etc.).

  • Graham Knott October 8, 2020, 17:14

    “Earth is already close to the inner edge of the HZ”

    Just how close to it is Earth at the present time? One hears that bio-sustainability levels (as they exist today) will remain largely unchanged for “at least” another 500,000 years. Is this likely, or will conditions for complex life on Earth become intolerable long before that? Just a thought.

    • Ron S. October 8, 2020, 19:49

      There is an alternative form of the rule: the edge of the HZ is close to Earth. That is, it’s not so much that Earth got lucky than that the HZ is inadequately defined. At least the HZ is defined to include Earth since it would otherwise look sillier. We don’t really know as yet what an HZ should be since we have, so far, just one data point.

    • Ronald October 11, 2020, 8:21

      Very good question.
      Answer: according to most recent research, in particular Kopparapu et al., the inner edge of our (conservative) HZ is estimated at about 0.97 – 0.99 AU.
      In other words: scarily close for long term evolution of higher life.
      According to the referred Cuntz & Guinan paper, Earth will get too hot for higher life in about 300 my.

  • AlexTru October 9, 2020, 3:38

    We know almost nothing about planets of our own Solar System, we know nothing about distant planets, so I can accept discussed article as Sci-Fi speculations (or common narrative for the new sci-fi story), there is nothing related to real facts or science.

  • Michael Fidler October 9, 2020, 6:33


    Here’s a Clever Idea, Looking for the Shadows of Trees On Exoplanets to Detect Multicellular Life.


  • torque_xtr October 9, 2020, 8:27

    There should be a distinction between worlds habitable for us humans, and worlds which favor biodiversity, evolution, etc. For the first, one greatly underestimated issue is the biochemistry compatibility problem. The best worlds for humans are sterile planets, young earths (maybe even with abiotic oxygen atmospheres), which could be seeded with Earth’s lifeforms. Not older than 1 Gyr. Even Mars or the Moon could turn out easier for colonization than Pandora.

    For biodiversity, it’s another thing, a delicate balance of stability and evolutionary driving factors. I guess that as far as the atmosphere is not prone to escape, the correct evolution-driving factors and the abscence of “brakes” (such as great reductive potential of Archaean oceans or diminishing of atmospheric CO2 by waning volcanism and continuing sequestering) are of more importance than the exact amount of volatiles, insolation, planetary mass, etc. The Earth, with it’s failed start at multicellularity at the beginning of Great Oxygenation, and the Boring Billion which ensued, is definitely not at the top :-).

  • Geoffrey Hillend October 9, 2020, 14:22

    We do know what a habitable zone should be: The life belt is the zone around a star where water can remain liquid without being frozen or evaporated into a gas. If the planet is too far away, water remains frozen if it is too close water remains a vapor which could result in a runaway greenhouse effect which is why we want a planet in the life belt. This is based on the temperature and brightness of the star. A K star is not as bright as a G class star so the habitable zone is closer to the star which could result in some tidal locking. Venus is tidally locked. A Earth sized exoplanet in the life belt around an M dwarf star is certainly tidally locked which makes it impossible for it to have a Moon like our Moon. The class of star matters and there is physics which supports the life belt idea which is infallible which allows scientists to make a prediction that will turn out to be true or match observations.

    The Earth will not move out of the life belt until after one hundred million years because the Sun increases its brightness approximately 7 percent even billion years. The Sun’s brightness will increase less than one percent in 100 million years. A smaller star will burn it’s hydrogen fuel more slowly, so it’s brightness increases at a slower rate, and an exoplanet will be in the life belt longer in a smaller star. The smaller the star, the closer the life belt so tidal locking becomes more of a problem. Since there is more mass and gravity in larger stars, the gravitational compression in the core is higher so larger stars burn their fuel more quickly and have shorter lives.

  • Gary Wilson October 9, 2020, 16:32

    I love this concept of super-habitability but I’m going to say the obvious. Now we need hard data to back the idea up. We need data on real exoplanets that support the idea. And of course reality will be much more complex than any model can be. So it will depend on when we can get some atmosphere measurements from candidate planets among other types of data.

  • charlie October 9, 2020, 16:52

    A planet being in the habitable zone is not the only thing required for life.
    Life also needs Phosphorus and Phosphorus is relatively rare in the universe, the rarest of the six elements required for life as we know it.
    They are assuming Earth-like life. But rationally, it seems to me there is nothing more Earth-like than Earth.
    If you are going to say life in general in the Universe, we simply have no way to say what is possible, common or favored.

    If we had solid information on the atmospheric compositions, the land and seas, temperature range and stability, distributions of elements (especially those needed and those that are toxic), planetary rotation, effectiveness of ozone layer/magnetosphere, we could say if Earth life could survive there without advanced technology. But we could not say much more than that.

    On a 25,000 to 100,000 year window of ‘interest’, one could, without resorting to near-science-fiction near-lightspeed transport, one could traverse the stars in the local 100 LY radius sphere. ⁴⁄₃πR³ … = 4.2 million cubic light years of space. Turns out there are know to be at least 1,800 F, G and K stars within that volume. More likely at the K and M (especially) side, but they are usually also encumbered by heavy x-ray flare activity. Best is between F (somewhat hotter-and-yellower) and K (somewhat more orange and cooler) than our G star.

    1800 candidates to go visit. Maybe all of them can be well imaged in the next 100 years by fabulous space-based kilometer-scale interferometer objective ‘telescopes’ (bad word, but they clearly are kind-of-optical instruments). We determine there are 5% having ‘interesting planets’ in well regarded orbits.

    90 stars to VISIT. Up to 10,000 LY to ‘get there’; maybe speed-of-light high-spectral purity laser back-communication to relay what’s found. Images. Spectrography. Hêll, who knows, even atmospheric sampling!

    Within that 25,000 years, we learn of a few percent of those, maybe 10 in whole, which actually have abundant conditions conducive to supporting life. And that DO.

  • Geoffrey Hillend October 10, 2020, 15:54

    The idea of having a model is to make some predictions and extrapolations when we don’t yet have the data. None of the exoplanets we have discovered is an Earth twin the the life belt around a G class star. We do know that such an exoplanet must have an orbital period of one year based on real physics like general relativity. Due to the mass of a G class star, the exoplanet can’t have an orbital period of two years because it would be where Mars is and out of the life belt and and it can’t have an orbital period of too much less than a year like Venus with only 225 days because that would be a tidally locked planet with a runaway greenhouse effect and also too close to the star and not inside the life belt. I argue that in order for the superhabitable idea to be an effective predictor of observations, it really has to have all of the contingencies of our own Earth Moon system and G class star.

    We know that none of the exoplanets that we have discovered so far meets this criteria. Consequently, I predict that none of these will have all of the biosignature gases and therefore will be ruled out. It is good to know this because if that is what happens after we get all the data, we won’t get discouraged and will have and refine the search. We might have to look at hundreds of thousands of exoplanets or even one million before we find a superhabitable exoplanet with this model which is why it is not popular. It is always good to be optimistic and I am, but I will be surprised if we find all the biosignature gases in the spectra of an Earth twin around an M dwarf. The age of the star is also important because without a magnetic field, the long term chances of the survival of life are diminished. The K dwarf may be tidally locked because due to the K dwarf star’s smaller mass than our Sun, so the life belt and orbit of the exoplanet must be closer to the K dwarf.

    • Ronald October 11, 2020, 8:16

      Also with regard to youre previous comment (9 Oct.) and tidal locking:
      If you read the referred post about Goldilocks stars and its underlying paper by Cuntz & Guinan, you will find that the researchers have therefore been looking for the sweet spot (or rather sweet range), where stellar evolution is slow enough AND tidal locking not yet a problem: late G tot early K stars.
      So, the researchers never included all K stars, but only the early ones, about from G8 – K2, defined as the optimum.
      The paper says it all, truly very interesting.

  • wdk October 11, 2020, 12:59

    It was Robert Pirsig in “Zen and the Art of Motorcycle Maintenance” who came up with the notion of carving up a motorcycle with “an analytical knife”. Similarly, in the absence of a second habitable planet (possessing species), we are in search of means to carve up a planet in habitability’s name. A simple example is that you or I live in a “house”, but maintain a “garden”, we find ourselves more comfortable in the house than in the garden on account of the diversity of life encountered in the garden beside that which we wish to harvest. If we lived in the garden instead of the house, our temporal mileage as a species would “vary” from the one with which we became accustomed. So that’s at least an index to consider with regard to super-habitability. It could be so superhabitable locally that no one can get anything done constructive: lots of lettuce and grubs – miles high, but no one working on SETI communications.

    Other considerations: Considering that planets are spheres, radial, latitudinal and longitudinal considerations should be taken into account at some point. For the Earth we have a troposphere, surface and oceans that seem to bound much of our HZ. That’s granted. Another planet might have a different depth in that regard just as atmospheres have different opacity, temperature and pressure gradients.

    There are rain forests and deserts on Earth and their qualities vary with latitude. The Olympic Peninsula and the Amazon for example offer different opportunities for life. Each have tree growth, varying numbers of canopies… So that suggests that HZ regions on a planet could be double-decker, triple,… It depends on how high a sentient creature could roam, plunge in oceans or glide in an atmosphere.

    Tree houses are fascinating, and I would suppose a number of us have
    tried our hand at that sort of thing. But here on Earth we as humans have not really proliferated very much that way. With a greater amount of vegetation, that might be another way in which a biosphere might
    be more habitable than the one we subsist in – if a sentient life form
    were to proliferate amid the growth.

    These are not basic chemistry considerations. Maybe they are more akin to science fiction, space travel expectations. But our recent tendency with habitability, taking in anything cellular, prokaryotic or eukaryotic – this wide usage might be a little unnerving if standing at a star gate threshold.

    Planets detected via transit in combination with doppler give us a running start with mass and diameter to determine atmopsheric consequences with an orbital radius, eccentricity… plus stellar luminosity and spectral features. Determining planetary atmospheres will be difficult, but it is becoming more doable every year. Consequently, the spherical envelopes, the latitude and longitudinal
    considerations will come more into play too.

  • Geoffrey Hillend October 11, 2020, 16:34

    I stand corrected on the idea that the large K stars are tidally locked. They are not tidally locked. There is still the size of the star to be considered. There is less gravity and a possible a smaller protoplanetary disk and smaller amount of gas and dust which plays a role in the planetary early and late bombardment periods because there will be less gas and dust to make planets like Gas giants and small rocky planets and planetesimals . Consequently, there will be less collisions and less possibility or chance of a large collision like Theia with Earth to make a Moon. With a lower probability of making of Moon and without a Moon higher forms of life like animals will never get started due to the loss of atmosphere, water vapor, etc., from solar wind stripping and photodissociation without a magnetic field.

    Also gas giants are needed to deflect asteroids and a K dwarf will have to have a smaller solar system with less total of planetary mass than our system which might make it less superhabitable. All the factors must be included, and it is surprising what contingencies for superhabitable model can be found just by considering the different mass in different classes of stars.

  • Ronald October 12, 2020, 11:23

    “I’ve always loved the notion of ‘super-habitability’ ”
    So have I! In fact it is my favorite astronomical topic (‘Goldilocks stars’), besides of course the search for Earth analogs around solar type stars and solar twins.
    As usual, very clearly written summary of matters.

    A few remarks:
    The first bullet: In orbit around a K dwarf star.
    I think that should be ”around a late G or early K star”, with reference to the Cuntz & Guinan paper, in which the basis for the selection is described.

    As Alex Tolley and others have also pointed out, 2 of the 8 bullets (temperature and O2 level) depend at least partly on life itself. And these two plus another bullet (land/water distribution) will vary through geological time.

    The requirement of a large moon will greatly reduce the occurrence rate of super-habitability, but this requirement is probably overrated and outdated: see among others Lissauer et al.: Obliquity variations of a moonless Earth.

    What I miss in this otherwise great concept of super-habitability, I mean besides what causes it, is what *defines* it. In other words, clear criteria for what super-habitability is, how it is measured.
    Like an index, consisting of a number of measurable factors.
    (Ok, there is some mentioning of biodiversity and biomass).

    I can think of a few factors myself:
    – Lifespan of the star’s CHZ (determined by stellar characteristics, such as stellar mass, temp., luminosity, spectral type).
    – Geological lifespan of the plant (determined by planetary mass, radioactive elements, etc.).
    – Total biodiversity (e.g. expressed as total number of species, and others).
    – Total biomass.
    – Total primary productivity.

    The first two will together constitute another one: potential lifespan of the planetary biosphere (for higher life).

    As mentioned, the other three are highly time- and life-dependent.

  • Geoffrey Hillend October 12, 2020, 16:02

    An exoplanet cannot have a magnetic field without a Moon. It is the angular momentum or fast rotation of the planet which causes the charged particles or the liquid iron to move in circles which creates a magnetic field which is how Earth’s magnetic field works.

    • Ronald October 13, 2020, 6:53

      I think that statement is blatantly wrong:
      The Earth’s magnetic field is generated by electric currents due to convection currents of molten iron (and nickel) in the (outer) core, due to heat escaping from the core; also known as geodynamo.

      Besides, even without a magnetic field, life might not be as impossible as some would think: although the magnetic field protects us from a lot of charged particles and in particular hard UV from the Sun, a sufficiently dense atmosphere would also do most of that job.
      Plus: a late G/early K star would probably emit less hard UV.

      But all that is beside the main point here, which is: the moon has little or nothing to do with that.

  • Geoffrey Hillend October 12, 2020, 16:32

    In order for an Earth sized planet to have a Moon it has to have a collision with a large Mars sized body called Theia as in the giant impact hypothesis. Without such a collision, our Earth would not have a fast rotation and a magnetic field to protect our atmosphere from solar wind stripping. Consequently, without a Moon, there is a lot more atmospheric mass loss due to solar wind stripping, but I will admit I am using a different super habitable model which requires a Moon for there to be higher forms of life. I also will admit that my super habitable model is different, the a priori idea or intuitive hunch that all exoplanetary systems must be exactly like ours, with a G class star and an Earth Moon system in the life belt.

  • Geoffrey Hillend October 13, 2020, 22:20

    Ronald, Iron does not make a strong magnetic field unless magnetized. The idea of convection currents in Earth’s magnetic dynamo always must include the idea of charged particles moving in circles. It is the fast rotation of Earth’s magnetic field which causes twin rotating cylinders of liquid iron in the core which forces the charged particles to move in circles which causes the magnetic field. The same effect is created by wrapping a wire many times in a circle around the entire length of a nail and attaching both ends of the wire to battery which makes an electromagnetic. The electrons in the circling in the wire are like the electrons circling in the iron core which produce the magnetic field.

    The UV rays of the Sun are photons from the electromagnetic spectrum. The entire electromagnetic spectrum is transmitted by the photon. The UV radiation, x rays and gamma rays are also photons which go right through the Earth’s magnetic field. The reason is photons have no charge and also are bosons which are not subject to the Pauli exclusion principle. The ozone and nitrogen in the atmosphere stops the UV and shorter wavelengths, the x rays and gamma rays.

    An exoplanet without a Moon also does not have a stable inclination of axis like Mars, so it experiences a wide change in it’s angle over time which will result in a rapid change of climate every fifty thousand years. I first saw this on youtube under the title what would happen if there was no Moon. I don’t argue that life could not evolve on an exoplanet without a Moon, but not the higher forms of life like animus and humanoid, intelligent life like us. This is just a theory, but as I have already pointed out, if it is correct, then it might take us a long time to find a planet with all the biosignature gases because intelligent life would be rarer in our galaxy. Life is superhabitable once the right, perfect conditions are met like here on Earth. Afterwards, it can move and adapt to a more hostile environment like the thermophiles in boiling hot seawater. If the environment is hostile of less adaptable from the start, then life might never move beyond primitive single celled form or not get started at all. Ultra violet does penetrate sea water by a certain amount of feet, but has life stared in our deep oceans or shallow pools which makes a big different considering the harmful UVC.

    • Ronald October 14, 2020, 7:22

      Hi Geoffrey, no offense intended, but, with regard to;

      Your first 2 paragraphs: all fine and OK, but where is the moon in this whole magnetic field story?

      Your 3rd paragraph: as I also mentioned above, the alleged role of the moon in stabilization of the Earth axis (tilt) and hence climate, is probably grossly overrated and overstated.
      See for instance: Lissauer et al.: Obliquity variations of a moonless Earth.
      Barnes is also one of the authors, a well-known ex-moons researcher.
      The gyroscopic stabilization of the Earth itself is already enormous. Significant climate variations without our moon would rather be in time-frames of 50 – 100 my and within the amplitude of Earth’s known climate changes.

  • Michael Fidler October 13, 2020, 23:46

    [Submitted on 13 Oct 2020]
    Detection of simplest amino acid glycine in the atmosphere of the Venus.

    “Amino acids are considered to be prime ingredients in chemistry, leading to life. Glycine is the simplest amino acid and most commonly found in animal proteins. It is a glucogenic and non-essential amino acid that is produced naturally by the living body and plays a key role in the creation of several other important bio-compounds and proteins. We report the spectroscopic detection of the presence of the simplest amino acid glycine (NH2CH2COOH) with transition J=13(13,1)–12(12,0) at ν=261.87 GHz (16.7σ statistical significance) with column density N(glycine)=7.8×1012 cm−2, in the atmosphere of the solar planet Venus using the Atacama Large Millimeter/submillimeter Array (ALMA). Its detection in the atmosphere of Venus might be one of the keys to understand the formation mechanisms of prebiotic molecules in the atmosphere of Venus. The upper atmosphere of Venus may be going through nearly the same biological method as Earth billions of years ago.”


  • Geoffrey Hillend October 14, 2020, 15:43

    I am using the general principle about all twin Earth Moon systems have to form the same way, the giant impact hypothesis which is my intuitive hunch. With this idea, the Moon was not captured, but formed from the ring orbiting around the Earth from the break up of Theia from it’s collision with Earth at a grazing angle. The fragments reformed into our Moon. The collision caused the Earth to spin rapidly, the angular momentum for the spinning of our Earth was gained by the collision of Theia. Also Theia’s iron core went into our Earth giving Earth a larger iron core.

    Furthermore, without a Moon, there can be no fast rotation of an Earth sized exoplanet, and therefore no magnetic field. Without a magnetic field, there is more solar wind stripping of the atmosphere so an exoplanet with this problem looses much more atmosphere than a exoplanet with a magnetic field.

    As far as the over rating of climate change caused by a lack of Moon, I agree with you only if we don’t have to worry about migration and animal life which I don’t think life will ever get started on exoplanet without a Moon. Also there is a lot of atmosphere lost over a deep time due to solar wind stripping, but I will admit I don’t know how it will affect the start of early life and micro organisms. It also depends on the chemistry of the atmosphere or how much Nitrogen the atmosphere has because that is also a gas which blocks ultra violet radiation and how much Nitrogen so it depends on how much Nitrogen is in an atmosphere over time.

    • Alex Tolley October 15, 2020, 4:41

      If, a big if, a 2nd abiogensis is found on Mars, then this would invalidate the need for an Earth-Moon analog for abiogenesis. Similarly, if we detect unambiguous biosignatures on exoplanets without large moons, this would similarly falsify this hypothesis.

    • Ronald October 15, 2020, 11:38

      “Furthermore, without a Moon, there can be no fast rotation of an Earth sized exoplanet, and therefore no magnetic field.”

      Again, I disagree, and I would even say: on the contrary, the Earth is slowing down (among others) because of momentum transfer to the moon.

  • Geoffrey Hillend October 16, 2020, 1:14

    Its fine to disagree, but take in mind your disagreement is besides the point, since every planet with a Moon and a magnetic field always slows down over deep time due to momentum transfer caused by both gravity of the Moon and the oceans, the older books say its only the oceans. The Earth and Moon will eventually become tidally locked in the distant future as the result of momentum transfer, and the Earth will not have any magnetic field or rotation at all which is the future of all planets with Moons, but that future is distant or long after the Earth gets too hot and has a run away greenhouse effect like Venus and even looses all it’s atmosphere due to the brightness and heat from of our Sun in the red giant phase. A planet can’t have a magnetic field without a fast rotation because there will be no charged particles or electrons moving in circles. All planets get their rotations from collisions and I don’t see any other way for them to gain angular momentum. I also don’t expect you to agree with me Robert since new ideas are often found by one’s freedom to disagree, so I welcome them.

  • Geoffrey Hillend October 16, 2020, 1:16

    Pardon, me I meant Ronald.

  • ljk October 20, 2020, 12:48