≡ Menu

Are Planets with Continuous Surface Habitability Rare?

Put a rocky, Earth-sized planet in the habitable zone of a Sun-like star, and good things should happen. At least, that seems to be the consensus, and since there are evidently billions of such planets in the galaxy, the chances for complex life seem overwhelmingly favorable. But in today’s essay, Centauri Dreams associate editor Alex Tolley looks at a new paper that questions the notion, examining the numerous issues that can affect planetary outcomes. Just how long does a planetary surface remain habitable? Alex not only weighs the paper’s arguments but runs the code that author Toby Tyrrell used as he examined temperature feedbacks in his work. Read on for what may be a gut-check for astrobiological optimists.

by Alex Tolley

The usual course of the discussion about planet habitability assumes that the planet is in the habitable zone (HZ), probably in the continuously habitable zone (CHZ). The determination if the planet is inhabitable concerns the necessary composition and pressure of the atmosphere to maintain a surface temperature able to support liquid water. As stars usually increase their luminosity over time, we see charts like the one below for Earth showing the calculated range of average surface temperature. Atmospheric pressure and composition can be modified to determine the inner and outer edges of the HZ or CHZ.

Image: Life on an exoplanet in a globular cluster. Credit: David Hardy (Astroart.org).

However, these charts say nothing about the various factors that may upset the stability of the climate, especially the geological carbon cycle, where volcanic outgassing of carbon dioxide (CO2) is approximately balanced by weathering of rocks to ultimately sequester carbon as carbonate rocks such as limestone. Imbalances can create significant changes in greenhouse gas (GHG) composition of the atmosphere with temperature impacts [4].

Figure 1. Possible bounds of Earth surface temperature over 4.5 By of changing solar output and the impact of atmosphere. The early Earth would have required a different energy trapping atmosphere to maintain an inhabitable temperature during its early history to prevent freezing. Continued increase in solar luminosity will render the surface too hot for life even with no atmospheric trapping of the sun’s heat. Source: Kasting 1988 [3]

We have much evidence of widely fluctuating average surface temperatures for the Earth, from a possibly hot Archaean eon, several local temperature maximums including the end of the “Snowball Earth”, the Permian/Triassic extinction, and the Eocene thermal maximum. Earth has also cooled, most notably during the “Snowball Earth” period that lasted millions of years and several extreme glaciations over its history.

Before even considering the many other possible factors that may preclude an inhabitable planet, there is a question of just how stable are planetary surface temperatures, especially when subjected to shocks due to excess CO2 emissions from very active volcanism, or conversely from excess weathering depleting the atmospheric CO2 pressure.

The Journal Paper

A new paper [1] by Prof. Toby Tyrrell looks at this question in a very different way. He posits that for the many planetary types and conditions in the galaxy, we should assume a wide variety of possible temperature feedbacks and simulate the average surface temperature of a planet over a long period of geologic time (3.0 By) to simulate how frequently planets could continuously maintain an inhabitable surface temperature throughout this period.

Tyrrell on randomly configured feedbacks:

“It is assumed that there is no inherent bias in the climate systems of planets as a whole towards either negative (stabilising) or positive (destabilising) feedbacks. In other words, it is assumed here that the feedback systems of planets are the end result of a set of processes which do not in aggregate contain any overall inherent predisposition either towards or against habitability.”

His model is very simple. He assumes a randomly chosen number of feedback values (change in temperature over time for a specified temperature) within an inhabitable temperature range. Figure 2 below shows one example of the model showing random feedbacks, calculated temperature attractors, runaway temperature zones, and a time course of temperature impacted by random temperature perturbations. The values for temperatures between each feedback node are interpolated from the 2 surrounding nodes. If the feedback slope is negative and if the 2 points straddle a 0 feedback, the model calculates a temperature attractor at that point, so that temperatures between the 2 feedback nodes tend to stabilize the temperature at the attractor position.

If, however, the slope is positive, the temperature will be destabilized and driven towards the upper node if above the 0 feedback level, and conversely to the lower node if below the 0 level. He sets a minimum (-10C) and maximum(60C) surface temperature range that if the calculated temperature extends beyond those boundary temperatures or is in feedback that will lead to runaway feedback to either a very low or very high temperature, the model assumes the planet is no longer habitable on the surface. The model adds 2 other important elements. Firstly there is a long-term forcing (e.g. increased solar output), which for the Earth is a positive one as the sun continues to increase its output over time. The second is to introduce small, medium, and large temperature perturbations (i.e. shocks) that introduce noise into the model and can flip the climate between attractor temperatures and also into runaway temperature conditions where the feedbacks positively reinforce the temperature change. Figure 2 below is extracted from the paper to indicate an example. [Annotations added for clarity.]

Figure 2. Extracted from the Tyrrell paper and annotated. The left chart shows 9 randomly created feedbacks. Where 2 adjacent feedbacks are connected by a negative slope and cross the zero feedback line, a temperature attractor is created, in this example there are 2 attractors. At either end are zones where the temperature would cause a runaway increase or decrease in temperature and these are indicated by grayed areas. The chart also indicates that over the long term, there is a negative forcing that reduces the average temperature over time. The right-hand chart shows the temperature over time. The 2 attractors are shown, as is the starting temperature [blue square]. The various perturbations are indicated both in time and size by the red triangles below the chart axis. The gray bands show the runaway temperature conditions and the black bands the start of uninhabitable conditions. The 500 My display shows the temperature flipping between the 2 attractors, with each flip due to larger temperature perturbations. A few temperature perturbations approach but do not cross into the runaway temperature zones.

With the parameters he uses, the model demonstrates that with repeated runs, only a few percent of planet runs enjoy a 3 billion year period where surface temperatures stay within the inhabitable temperature range. Once the range is exited, surface life ends and the planet becomes lifeless on the surface.

Figure 3 shows how rarely planets can maintain inhabitable conditions over the entire 3 billion year time period.

Figure 3. The probability of a planet always surviving as inhabitable over 3 billion years over several runs with the same feedback conditions but with no temperature perturbations and with random temperature perturbations. The gray (H1 – chance alone) hypothesis is pure random perturbations withwout feedbacks and the red (H2 – mechanism alone) – feedbacks but without large perturbations – are compared with the simulation results. The most important result is that a planet that can maintain surface inhabitable conditions is quite rare.


“The initial prospects for Earth staying habitable could have been poor. If so, this suggests that elsewhere in the Universe there are Earth-like planets which had similar initial prospects but which, due to chance events, at one point became too hot or too cold and consequently lost the life upon them. As techniques to investigate exoplanets improve and what seem at first to be ‘twin Earths’ are discovered and analysed, it seems likely that most will be found to be uninhabitable.”

His conclusion is ominous for astrobiologists. Even if we discover many planets that are in the HZ, and confirm that their atmospheres could support an inhabitable surface, those planets are either frozen or too hot to allow life to exist on the surface. The vast majority of apparently suitable worlds will prove lifeless and appear as if abiogenesis (or even panspermia) has failed to ignite an evolutionary progression to complex life and even possibly technological civilization.

This conclusion is enough to dampen any astrobiologist’s day and suggests that the search for biosignatures may be as disappointing as the results of SETI.

While the paper shows the results using the values of the published model and code, the supplementary information includes a considerable analysis of the model, for example, extending the inhabitable range, and several other parameters. However, the broad conclusion remains robust. Maintaining an inhabitable temperature over 3 billion years is unlikely.

Tyrrell acknowledges the simplicity of his approach and suggested in a recent SETI Institute webinar that he hopes to apply his approach with a more sophisticated planetary climate model to determine if his findings hold up.

Given the proxy indications of Earth’s paleotemperatures (see figure 4 below) showing wide ranges and some close misses to survival shown by the mass extinctions, why did Earth life survive? Tyrrell argues that the anthropic principle has to be invoked. Just as the universe we live in needs the exact constants for life and we couldn’t be in any universe without those conditions, so we technological humans cannot investigate unless the Earth had maintained a continuous inhabitable surface temperature.

A Critique

Figure 4 below shows the estimated temperature fluctuations in the paleotemperature proxy data. Have we just been lucky that there do not appear to be any clear multiple attractor temperatures?

Figure 4. A chart of paleotemperature of Earth. For 3 billion years Earth’s average surface temperature has fluctuated in a range of less than 30C.

An obvious question is whether his model reflects reality. The random nature of the feedbacks coupled with the temperature perturbations might lead to many situations where even small temperature perturbations will tip the surface temperature beyond the acceptable range. As we can see from Figure 2, the upper-temperature attractor is within 10C of a runaway temperature increase, making habitability susceptible to even relatively small temperature perturbations.

Fortunately, the model code has been placed online and the source code available to experiment with.

Observing several runs it became clear that the model would quickly fail if the current temperature at an attractor temperature was near the boundary range so that even a modest temperature perturbation could push the temperature outside the range. How serious was this effect?

Figure 5. The most benign model. The 2 feedbacks are at the temperature range extremes and result in an attractor at 25C that is maintained across the temperature range. The long-term temperature forcing is set to 0. Only the infrequent large temperature perturbations, average size 32C are operative.

I created an experiment (also suggested at bottom of page 4 of the paper) where the planet would always have the most favorable conditions for a stable surface temperature. Just 2 feedbacks were created at each end of the range, with the calculated attractor in the middle of the range at 25 C, so that any perturbation would have to exceed 35C in either direction to exit the inhabitable conditions. I removed the long-term feedback too. I also removed both the small and medium-sized perturbations, leaving just the rare, large perturbations. The probability of timing and size of the perturbations was left as per the model. By starting the planet’s temperature at the attractor, the inhabitable conditions would be maintained at the attractor temperature unless a random large perturbation exceeded the 35C size.

The results for different perturbation probabilities are shown in figure 6. The average survival time of planetary runs and the %age survival plotted against perturbation probability demonstrate what might be intuitively guessed. Tyrrell’s purely mechanistic run (H2) with optimal feedback and no perturbations had all planet runs complete the 3 By survival. This is consistent with figure 6 where expected large perturbations = 0.

Figure 6. Survival times and %age survival of planets without an attractor and with a single attractor at 25C. With an attractor, survival times are greatly enhanced, especially as the expected number of perturbations increases.

Inspection of the model’s large temperature perturbation distribution indicated that the average size was 32C with a standard deviation of 16C. For the stable model planet I was testing, the temperature would be perturbed beyond either range boundary by a value of just 0.25 standard deviations, i.e. that about 40% of all randomly selected sample perturbations would trigger a surface temperature outside the inhabitable range. When that happened depended on the random timing and would dictate the survival time of inhabitable temperatures. As a control to determine the frequency of perturbations, a model world was created with no attractor temperature so that it sat on a temperature knife edge. Any perturbation would cause runaway heating or cooling. The impact of the lack of the stabilizing temperature attractor on survival time and average % of planets surviving for 3 By is evident.

Given the importance of the large perturbations, just how reasonable are the size of the perturbations and the maximum inhabitable temperature range.?

It is hypothesized that the “Snowball Earth” temperature ranged from deepest glaciation to a temperature maximum could have been as high as 100C (-50C to 50C). The chart of paleotemperature suggests for over the last few billion years an average surface temperature range of 26C (-10C to16C). That the Cryogenian glaciation period encompassing the “Snowball Earth” could have had a surface temperature of -50C, yet life quickly reemerged more vigorous than ever (the Cambrian “explosion”) after the glaciers melted, suggests that the lower temperature bound of -10C may be too conservative. As for the upper bound, it has been suggested that the Archaean eon may have had surface water temperatures of 70-80C. While most complex life has an effective upper limit of 60C, extremophiles have been found at 122C. For complex life, while the resilient tardigrades can withstand extreme temperatures for short periods, the inhabitable surface range is reasonable for complex life. However, we should bear in mind that ecological refugia can provide safety for complex life, for example around undersea vents to resist freezing, and migration to the poles to escape the equatorial temperatures and hence live in regimes that remain below the average surface temperature.

These points were acknowledged in the Tyrrell paper that discussed the limitations and caveats to the model.


Geographical variability implies that more extreme average global surface temperatures might be required to force extinction everywhere. Microbial life can potentially survive periods of inhospitable surface conditions within refuges, such as in subsurface rocks or deep in an ice-covered ocean at hydrothermal vents, emerging later to recolonise the surface; evidence from Neoproterozoic Snowball Earth events suggests however that eukaryotic photosynthetic algae persisted through the events and therefore that surface habitability was maintained at some locations. Other environmental conditions can affect habitability, but only temperature (and therefore water availability) are considered here.

Dynamic models are often unstable without tuning. The simplest example is Wolfram’s linear cellular automata with 3 cell states determining the next cell state. With just 8 possible rules for the 3 state combinations, there are 256 combinations of rules, yet just 6 (2.3%) do not converge on static states. The random feedback combinations may reflect a similar outcome, but where the majority of conditions will easily slip out of the inhabitable temperature range, rather than the benign experimental planet conditions I tested.


Dr. Malcolm (Jurassic Park):

“Life Finds a Way”

Given the results from my experiment with the optimal feedbacks for a stable climate, if feedbacks are more stabilizing on average than the hypothesized randomly assigned feedbacks, planets with inhabited surfaces possibly may not be quite as rare as the author’s model indicates. Tyrrell notes that average surface temperatures hide the variability of temperatures and exclude possible refugia, such as undersea hot vents, and lithosphere life warmed by the planet’s core. If we can accept that the Earth was populated by unicellular bacteria and eukaryotes for most of its history, and that the Earth’s complex biota may have even taken a major loss during the Cryogenian period, it seems likely that inhabitable worlds will have some sort of life assuming abiogenesis is easily achievable. While our climate history may be a lucky chance, history does not seem to indicate some attractor temperatures, but rather a single attractor that is subject to GHG source and sink imbalances that last for some time. The hypothesized extreme volcanism that ended the Permian resulted in the greatest extinction event in the fossil record and lasted for 2 million years. Our current fossil fuel burning that is increasing the atmospheric CO2 levels while very much like a temperature shock is not believed to be able to cause a runaway heating as happened on Venus. However, it is suggested that sometime in the next billion years, the Earth’s atmosphere will need to have no CO2 to stay habitable. Well before then, autotrophs will not be able to fix carbon and the complex life biosphere will collapse.

Once life starts, it is tenacious. A reset back to extremophiles may well be recoverable given time allowing new complex life forms to emerge under the right conditions and genetic “accidents”. However, there may be many more possible wrinkles to the sustainability of habitability, and eventually, surface life may be unable to survive. For subsurface life, the story may be very different. The lithospheric life might survive all other life until our sun destroys Earth billions of years in the future.

It would be interesting to modify the model so that rather than stopping when the temperature is outside the range, that the instances of these periods are recorded as possible reset conditions for refuge (e.g. lithosphillic) life to restart the evolutionary process rather than assuming the planet is “sterile”.

Time will tell when astrophysicists have cataloged and characterized a statistically useful sample of Earth-like worlds in the HZ that can test the model hypothesis of rare survival of surface inhabitablity over billions of years.


1. Tyrrell, Toby. “Chance Played a Role in Determining Whether Earth Stayed Habitable.” Communications Earth & Environment, vol. 1, no. 1, (2020), doi:10.1038/s43247-020-00057-8.

2. Ibid Supplementary Information

3. Kasting, James, et al “How Climate Evolved on the Terrestrial Planets”, Scientific American, (1988)

4. Berner, R. A. & Caldeira, K. “The need for mass balance and feedback in the geochemical carbon cycle”. Geology 25, 955–956 (1997).


{ 81 comments… add one }
  • Brett May 28, 2021, 15:31

    Well before then, autotrophs will not be able to fix carbon and the complex life biosphere will collapse.

    I’m not so sure on that. Maybe they’ll get much more efficient at fixing carbon, or maybe they’ll figure out a different way to absorb it from stored carbon in the soil or decaying organic material on the ground instead of the ambient air.

    • Alex Tolley May 28, 2021, 21:15

      Autotrophs fix inorganic carbon. Fungi and animals feed on dead and live organic matter. The fungi already occupy the niche you suggest, but this does not ensure new carbon is fixed. Perhaps they can use carbonate rocks as their carbon source directly, without atmospheric CO2. The CO2 levels falling to keep the Earth cool is a Gaia hypothesis. If the geologic carbon cycling is dominant, the Earth will just heat up.

  • Mike Serfas May 28, 2021, 16:01

    This idea confuses me a bit. I’m not very clear on the feedback mechanisms in relation to the basic physics. The equilibrium temperature of Earth is around 260 K – see https://www.astro.princeton.edu/~strauss/FRS113/writeup3/ . The rest is “greenhouse effect”. I don’t especially like the presentation, but the following explains that 60% of that greenhouse warming is water vapor – and the amount of water vapor in the air depends on the temperature (vapor pressure)! https://www.acs.org/content/acs/en/climatescience/climatesciencenarratives/its-water-vapor-not-the-co2.html Taking that into account, Earth (300 K) couldn’t really be cooler than 284 K or so without some kind of reflective covering like SO2 clouds, and it has been nearly that cold recently. Making it warmer can be done with any of the greenhouse gasses we know and loathe, but what are the odds that will happen without organized industries on the job? Is there a feedback model within a vaguely habitable temperature range for massive CO2-emitting volcanic eruptions?

    When the Sun was young it was dimmer, but if you approximate that Earth radiates as a black body, then the temperature scales as the incident light to the 1/4 power. (See https://en.wikipedia.org/wiki/Planetary_equilibrium_temperature) As I understand it, the Sun was only 70% as bright back then ( https://en.wikipedia.org/wiki/Faint_young_Sun_paradox ) but that means the Earth was 91.5% of its current Kelvin equilibrium temperature = 238 K, before any other atmospheric issues are considered. I’m five degrees over the graph with that, but some sites say the present equilibrium temperature is 255 K.

    In any case, an 8.5% or 22 K difference doesn’t seem like an existential crisis for life as a whole – it’s less than the difference between Arctic and Sahara. Life made it through the “Snowball Earth” phase and came back stronger than ever. We’ll see if we can cap our own greenhouse influence before that much of a change is made, or find out if life can do that again.

    • Alex Tolley May 28, 2021, 21:49

      I’m not a planetary climatologist, but let me try to answer some of your questions and comments.

      1. Looking at figure 1, you can see Kasting’s model results for average surface temperature over time with our current atmosphere. Obviously, the Earth needed a lot more GHG heat-trapping with the early Earth. However, the usual GHGs CO2, CH4, H2O are insufficient to raise the Earth’s surface temperature above 0C. This is called faint young sun paradox. The proxies indicate the Earth was warm, but the calculations suggest it couldn’t be. The various solutions include adding other unlikely gases, like H2.

      2. The black-body calculations assume a certain albedo. A world before the continents formed would be covered in water and the albedo would be low. Conversely, a frozen world would have a high albedo. Both of these cases would have positive, rather than negative feedback (I think).

      3. Tyrrell’s model is not just for the Earth, but for any number of different worlds with unknown climate feedbacks. As I showed with my model experiment with strong negative feedbacks to the mid-range temperature that the survival probability was much higher.

      4. You can see in figure 4 that the labeled first cooling (The Huronian) wasn’t as deeply cold as teh later Cryogenian (includes Snowball Earth). This despite the cooler sun but likely far higher GHG mixing ratios in teh atmosphere. Consider what might have happened if photosynthesis evolved earlier, or that there was a decline in volcanic emissions. It is possible that the Earth could have cooled even more and entered a much longer frozen state with no liquid water even in the deep ocean. And complex life would likely have perished, requiring a “reset” to evolve it again. Other planets with differing conditions could have been in a more precarious state than our Earth and erased all complex life permanently.

  • David Slaton May 28, 2021, 16:27

    “A reset back to extremophiles may well be recoverable given time allowing new complex life forms to emerge under the right conditions and genetic “accidents”.”

    If resets are very common and Earth got lucky not in the sense of maintaining life for over 3 billion years, but in the sense of no total wipeout of complex life occurring, then this could still be significant in another way, as it provides an explanation to the Fermi Paradox.

    Life finds a way, but if most of the time it is extremophiles carrying the banner, adaptions would not be able to build on previous adaptions so successfully as they did on Earth. We might be not so lucky as to have life but so lucky as to have a limit to how severe the interruptions were, so complex life could increase its complexity further until you get lineages that have large brains, language, and precision grip manipulators.

    Throw in the gamma ray burst galactic phase transition theory and you have extra reasons still why no one has beaten us to the stars (at least in this galaxy).

    • Alex Tolley May 28, 2021, 21:54

      What you are describing is Robin Hanson’s Great Filter to explain the Fermi Paradox. In your scenario, the filter is behind us, rather than in front of us.

      Starward, ho!

      • Robert May 29, 2021, 10:45

        Since you brought up the subject of Robin Hansen, has CD discussed his new ‘Grabby Aliens’ model yet?

        • Alex Tolley May 29, 2021, 18:58

          Not directly, although there has been discussion about alien expansion. I would also suggest that any ubiquity of von Neumann replicators is a similar argument.

          This seems to be a good video of Hanson’s Loud/grabby model with a Q&A afterward.


        • Robin Datta May 31, 2021, 23:01

          Greg Laughlin has mentioned it:
          Grab all they can.

          • Robert June 1, 2021, 13:16

            I have to admit I don’t really understand the Grabby Alien model or why Hansen thinks it’s so important. Seems like a lot of big conclusions based on negative evidence.

  • Andrew Palfreyman May 28, 2021, 17:56

    The role of deep ocean throughout the 3 By period seems to be all but discounted.

    • Alex Tolley May 28, 2021, 22:02

      If you mean that the deep ocean stays liquid and complex life can continue to huddle around hot vents, then yes and no. Worlds with less water, fewer continents, and shallow seas may not have deep oceans. However, both Tyrrell and I do state that ecosystem refuges may ensure that life continues. I maintain that life in the lithosphere cannot be sterilized under all but the most extreme disruptions (e.g. a collision like that with the hypothesized Theia that formed our Moon). Complex life is more fragile and exposed.

      Arthur C Clarke described hot vent life (and civilizations) at the bottom of Europa’s oceans (2010: Odyssey 2 ?). Each civilization was trapped around its vent which would eventually cool. He describes many civilizations rising and falling to the tempo of the vents.

  • Brad in Dallas May 28, 2021, 18:02

    What role might our plate tectonics play? And our seemingly unusually large satellite?

    • Alex Tolley May 28, 2021, 22:04

      Tyrrell’s model does not cover any of this explicitly but rather implies that different planets may/will have different feedbacks. Some have only positive feedbacks or poor starting conditions and quickly become lifeless.

    • Andrei May 31, 2021, 5:50

      Plate tectonics is of utmost importance to make a planet inhabitable for any longer time span.
      Even if it’s not conclusively proven, there’s several reasons to think that a large moon is important both for the appearance of life:
      But also for keeping Earth from flipping the rotation axis, and perhaps also maintaining the magnetic field. I am personally with the camp that a large moon is beneficial for life, though I might be biased as I regularly see the effect the Moon have on life on Earth.
      The same effects could of course be found on a satellite orbiting a larger planet, so I find the idea of life on such moons plausible.

  • Geoffrey Hillend May 28, 2021, 18:46

    If we start out with an unscientific premise like temperature is random or chance, then it is going to be hard to make accurate predictions. Nothing is mentioned here in Prof. Toby Tyrrell idea about what causes the temperature to change. If we don’t include the cause, then naturally, it will appear to be random. I am not adverse to looking at only temperature as the only variable, but the chart showing the paleotemperature of Earth in this paper does not seem to reflect what is generally accepted the history of climate or the temperature changes in deep time. At 250 million years the temperature should be high and earlier it should be going down as more carbon dioxide is removed from the atmosphere by photosynthesis, carbon cycle and continental drift. Also the temperature as written in this paper is very high after the first snow ball Earth period which should be higher than time after the snowball period, but that is not reflected in the paleotemperature chart shown here in this paper. Compare to the Geologic temperature record chart Wikipedia: https://en.wikipedia.org/wiki/Geologic_temperature_record

    It’s o.k. to keep things simple, but we have to explain why temperature changes to make accurate predictions. We have already limited the planet size to near Earth, so Jean escape still applies, but also Timing does matter because we have to consider what kind of star the planet is around since small stars have life belts that stay in one place for a long time like M dwarf stars. K and G class stars will have Earth sized planets which will move out of the life belt faster than M.

    I agree with the author’s conclusion but for the reverse premise, or the Gaia hypothesis that our planet is finely tuned for advance forms of life like plants, animals and humanoids. I don’t think that life especially intelligent life happened by chance and we don’t even need a teleological or religious view to see that since it is supported by physics that our planet is fine tuned for life.

    I assume that the higher forms of life animals won’t evolve on an Earthlike planet unless is has a moon roughly the size of our Moon for the following reasons. Without a Moon, there most likely won’t be a magnetic field since the Earth needed a giant impact to give it the fast rotation and angular momentum. A planetary magnetic field won’t work unless charged particles in a liquid, iron core move in circles. Also our Moon gives us the right gravitational tidal forces to make plate tectonics possible. A Moonless Earth might not have plate tectonics or at least efficient, continental drift and subduction which is crucial to having a carbon cycle and without that the planet’s atmosphere would be depleted by carbon dioxide by small micro organism, cyanobacteria, algae, and plants. Without plants there will be no photosynthesis or oxygen. Without oxygen there can be no animal life. Also a magnetic field is needed to deflect the solar wind which otherwise would strip a away a lot a planets atmosphere so there would be a loss of oxygen.

    Earthlike planets without a Moon still might hold onto microbial life for a while. A lack of oxygen spectra might not rule them out, but it would make it hard to prove that life exists there. Hopefully if it does there will be oxygen in the spectra.

    An important point is that we don’t need to have a runaway greenhouse to cause mass extinctions which have happened several times in the past due to the loss of carbon dioxide from less volcanism and the photosynthesis of plants, continental drift breaking up the last supercontinent with more coastline and mountains and exposed basalt to remove the CO2 than before, so the carbon cycle which has become well balanced in the past three million years to keep our temperatures cool. Consequently our man made climate change is an existential threat since our biosphere has adapted to the cooler temperatures.

    Also our Earth will look like Venus only 250 million years from now due the increased brightness of our Sun which changes brightness by ten percent every billion years. Venus was in the life belt four billion years ago and it has moved out of the life belt due the main sequence burning or conversion of hydrogen into helium which causes the increased brightness. The Sun fuses 600 million tons of hydrogen into 596 tons of helium every second. The lost four million tons is turned into energy or light due to E equals MC2. The Sun has less hydrogen in this fusion process, so it has slowly to burn hotter and brighter in order to balance the gravitational contraction. The speed of a star’s main sequence hydrogen burning is based on the mass of the star so the larger the star, the shorter the life because larger more massive stars will burn their hydrogen fuel faster since there is a stronger gravitational contraction in the core, so a planet in the life belt of a larger star will move out of the life belt faster which is why we have to include the star size in the cause of the temperature changes in deep time.

    • Alex Tolley May 28, 2021, 22:30

      Nothing is mentioned here in Prof. Toby Tyrrell idea about what causes the temperature to change.

      The temperature is controlled purely by its initial (random) state and the various feedbacks, both positive and negative. This is the essence of his simplified, toy model.

      …the chart showing the paleotemperature of Earth in this paper does not seem to reflect what has generally accepted the history of climate or the temperature changes in deep time. At 250 million years the temperature should be high and earlier it should be going down as more carbon dioxide is removed from the atmosphere by photosynthesis, carbon cycle and continental drift. Also the temperature as written in this paper is very high after the first snow ball Earth period which should be higher than time after the snowball period, but that is not reflected in the paleotemperature chart shown here in this paper. Compare to the Geologic temperature record chart Wikipedia:

      The included paleotemperature chart (figure 34) is not Tyrrell’s. I selected it from a paper by Douglass and Vacco (no reference provided). It was one of the very few that extended back further than the Pre-Cambrian, which is as far back as the Wikipedia entry chart goes.

      I don’t think that life especially intelligent life happened by chance and we don’t even need a teleological or religious view to see that since it is supported by physics that our planet is fine tuned for life.


      That is not a widely shared opinion. Vertebrates appeared in the Cambrian as one of the major phyla. However, it was not especially common according to the fossil record. Stephen Jay Gould has argued that if we rewound the evolutionary clock, the vertebrate phyla might not even have appeared or survived. That does not preclude another phylum evolving intelligence, but none have evolved our level of intelligence and certainly not technology – however adorable living with an octopus is.

      I assume that the higher forms of life animals won’t evolve on an Earthlike planet unless is has a moon roughly the size of our Moon

      If that hypothesis is true, then complex life is going to be extremely rare in the universe. However, Tyrrell’s paper only models temperatures, not the myriad other factors that might be important for complex life to emerge. This is Ward’s “Rare Earth” hypothesis and is a discussion best left to another post.

      The potential of a planet that is edged into a hothouse condition is covered in Tyrrell’s model with long-term forcing. His long-term forcings are random in size and direction, but any planet with positive forcing and positive feedback that pushes temperature towards the upper end of the habitable temperature range will rapidly tip into that runaway feedback (see example planet in figure 2 where the temperature range is grayed).

  • Robert Flores May 28, 2021, 21:33

    The more we look at the path the Solar System / Earth, physics/biochemistry took, the more I see a very complex set of parameters that were necessary to arrive at to our present state. It’s not divine, but extremely unlikely.
    Just think at how crucial ATP is, think how limiting it would be for cells to work with 1/13 the amount of energy available on earth. Nothing very complex can arise, Or if maybe an animal arose it would be so sluggish It will most certainly not have time to evolve to develop technology before a major cataclysm destroys its biosphere.

    • Alex Tolley May 29, 2021, 8:19

      I remain strongly in Dawkins’ “Blind Watchmaker” and “Climbing Mount Improbable” camp rather than Behe’s “Irreducible Complexity” camp.

      It may have been serendipitous that photosynthesis evolved, but I don’t doubt evolution’s capability of finding solutions to exploit the free O2 with aerobic metabolism. Rather than looking at the intricacy, consider the components of photosynthesis, how they may have evolved from simpler molecules and arrangements for energy transfer.

      Biologists may be able to replicate how life evolved from organic molecules. In the distant future, they may even discover worlds in various stages of evolving life to study these steps that led to our own abiogenesis and last universal common ancestor (LUCA). We may discover alternatives to our biology, both very similar and very different, or we may find that life has remarkably similar biology wherever we look.

      • Robert Flores May 30, 2021, 20:49

        I look forward to the day when Quantum Computers can
        simulate organic chemistry in full accuracy.
        We maybe able to find how likely certain chemical pathways
        are statistics wise. And Maybe they can find another pathway that
        is close to the efficiency of ATP for energy transfer. Curiously there
        does not seem to be a Non-ATP molecule in living oxydizing cells today that is say 1/2 as efficient as ATP. ATP is an order of magnitude (plus More) improvement of the prior biology. if its
        rarity is one per million habitable worlds, it would explain much.

        • Alex Tolley May 31, 2021, 11:54

          Rather than computational simulation, why not run experiments? Cronin’s group is experimenting by mixing many possible precursor compounds together and seeing what comes out. If Stuart Kauffman is correct, any large enough randomly selected compounds will create simple metabolic systems. This is “getting order for free”.

          Phosphorus will be present as phosphate from weathered rocks and therefore may well be part of the metabolic set early on.

          There is a reason that one would not expect to find inefficient non-ATP energy transferring molecules in life. Evolution will have selected for the organisms with the more efficient ATP and the organisms will have gone extinct.

          You may recall the flurry of excitement in 2010 when Wolfe-Simon announced that her team had found bacteria in Mono Lake that substituted arsenic for phosphorus. This was part of the “shadow biosphere” search. It turned out the experiment was flawed and the results widely debunked.

          So points for thinking about alternatives to phosphorus but the reality is that there excitement that NASA created collapsed as the findings were released too early for the usual scientific review and attempts at replication.

        • Robert June 1, 2021, 13:24

          I’m still a bit skeptical of quantum computers and wonder of they are not just a more materially sophisticated form of analog computation we already have. I understand some people claim it’s all been proven and maybe it has but I still wonder…..

          • Alex Tolley June 1, 2021, 14:52

            I don’t think one can be skeptical of the concept. However, I think you can be skeptical of the performance and usefulness. There still seems to be some doubt over when/if “quantum supremacy” will appear, with the number of qubits increasing to meet this goal. As to whether this type of computer will be useful is another matter. Entanglement is fragile, and at this point, I don’t see it being available as a compressor chip anytime soon. Quantum computing is suited for certain types of computation and not for most computational needs, and that for these special tasks the technology will mature and be available, perhaps even to the population as a web available service.

  • AlexTru May 29, 2021, 2:26

    I am very pessimistic when we are talking about SETI chances to detect any sign of life (or intelligent life), but on same time when I read this article I had a feeling that author have serious problem with elementary school mathematic.
    The problem that author and critics try to find solution for single (the Earth) equation that has huge (unlimited) number of variables.
    In this case the numbers of solutions for single equation is unlimited… So analyzing of every possible solution – it is area of Sci-Fi…
    To be closer to real solution of “life” equation we need more habitable planets examples, i.e. to add more equations to our equation system – but meanwhile science do not have any data about habitable planets – only Earth…
    So using proposed by authors approach, we can get any result we want, using same approach it is easily to prove that life is rare as well as life is widespread in our Universe.
    Summary discussed article is nonsense, additional speculation of Socratis paradox: “I know that I know nothing”

    • Alex Tolley May 29, 2021, 7:59

      To be closer to real solution of “life” equation we need more habitable planets examples…

      I agree. I would say that Tyrrell’s paper could be considered advice not to be too discouraged if most worlds that look like Earth analogs prove disappointing with regards to biosignature searches. Pessimistic but open to surprises if biosignatures prove common.

      My take is that it reinforces the likelihood of worlds being living, but confined to organisms of the archaea/bacteria/unicellular types. This might make biosignature searches harder especially if photosynthesis has not evolved and life remains anerobic.

  • torque_xtr May 29, 2021, 5:42

    3 GYr stability on few per cent of earthlike worlds still does not present a bottleneck. It means the nearest continuously habitable world could be 40 parsecs away instead of 15, but over the Galaxy, this still gives impressive numbers.

    And the Earth is probably not the best example of stability. We actually have at least one destabilizing feedback that warmer worlds don’t have: ice-albedo. Without surface ice, Milankovitch cycles mean nothing. An ice-free world could allow for considerable eccentricity and axial tilt variations without dramatic fluctuations of climate. The mass of Earth is quite at the lower end of habitable range in the chassical HZ, and the Sun is a bit too short-lived and too rich in ionizing radiation.

    In my imagination, the Safe Haven World is a 1.5-2.5 Me super-earth around an early-to-middle orange dwarf, with 0.6-0.8x Earth insolation, several times more nitrogen in the atmosphere, and axial tilt of 35-45 degrees.
    This includes protection:
    -From Snowball events and impact winters – by redder spectrum of it’s star (water ice and aerosols reflect less in the infrared)
    -From all kinds of abrupt glaciations also – by heat capacity of it’s oceans, kept at 20oC throughout it’s volume by inert gas greenhouse (the similar effect is theorized to allow liquid water on rogue planets with 1 kbar H2 atmosphere and geothermal heating only)
    -From atmospheric and water loss – just by higher escape velocity and vanishing XUV irradiation (no magnetic field needed),
    -From HZ drift – by longer lifespan of it’s star ,
    -From runaway water greenhouse – by lower insolation and cloud-albedo feedback,
    -From climate variations in general – by thick atmosphere. If most of greenhouse is provided by major inert gasses, CO2 and water variations mean less than on Earth (of course it depends on major and inert gas absorption actually covering at least some CO2/H2O bands). No large moon is needed to stabilize axial tilt – denser atmosphere and shorter seasons damp variations while some day/night and seasonal differences are still allowed.

    CO2 cycle is trickier, but strong volcanic activity always means surface renewal, and this always means some burial of sediments and carbonate sequestration – not necessarily by plate tectonics. More, in the presence of volcanoes and weathering, but in the absense of plate tectonics, buried carbonates are less likely to reach mantle and decompose to CO2 again, so if there is strong volcanism, there will be some CO2 equilibrium anyway.

    In short, heavier planets with thicker atmospheres seem more stable against most factors except stellar lifespan, but it is difficult to see what could counter-act all of this.

    On the other hand, right kind of climate variations could stimulate evolution; indeed, humans as biological species appeared in one of the coldest and least stable periods of the whole Phanerozoic. These fast and pronounced glacial cycles are dire compared to lush and balmy Mesozoic and Eocene. Global temperatures at glacial maximums second only to the Cryogenian, and even at interglacials actually still shivering – yet we evolved intelligence just in this period!

    • Alex Tolley May 29, 2021, 7:50

      GYr stability on few per cent of earthlike worlds still does not present a bottleneck. It means the nearest continuously habitable world could be 40 parsecs away instead of 15, but over the Galaxy, this still gives impressive numbers.

      My BoE calculation gives a similar result – about 1 star (F,G,K) with a 3Gy surviving Earth-like world (in HZ) gives 1 world within 100 ly.

      Your safe-Haven world being heavier might mean that it is hard (but not impossible) for a civilization that emerges to become space-faring. It might be that such a civilization is trapped in its homeworld, unable to physically explore its system, let alone manage interstellar flight.

      As you point out, each environmental stressor has resulted in evolutionary “pushes”. The Cambrian explosion after the Cryogenian glaciation. The rise of Dinosauria after the great Permian extinction. The rise of mammals after the Cretaceous extinction. The emergence of humans from the forests, to Savanna, and radiation to Asia during various dry and cold periods. The eventual dominance of H. sapiens during the last ice age.

      Human-created stressors – technological jumps also happen as a result of wars, most notably the wars since the industrial and scientific revolution.

      All these stressors increase competition, resulting in the emergence of new dominant life forms, and with humanity, new social systems, and technology. We see a similar spurt in innovation with increased competition within the corporate world (e.g. Schumpeter’s the “gale of creative destruction”, and Christensen’s “Innovator’s Dilemma”).

      A safe, stable, world might have a very slow pace of evolutionary change, possibly remaining in the unicellular state given how late complex life emerged on Earth.

      • torque_xtr May 30, 2021, 5:58

        Agreed, it seems that in order to go all the way from prokaryotes to a civilization, a delicate succession of instabilities within some robust margins is needed. But we’re not optimal at this, either. Earth had it’s Boring Billion when evolution was much slower than after and before it. And there was that suspected fail-start of multicellularity around Great Oxygenation Event, some 2.1 GYr ago. If not for faint young Sun and deep oxygen sinks, the descendants of Francenvillian biota could have been the first Moon walkers, more than billion years before humans have done the same. Instead, we’ve got Huronian Snowball and the Boring Billion after it. I guess the Galactic record from life inception to tool using is below a single billion of years.

        I think that higher escape velocity is a mixed blessing rather than impassable barrier. It means the inhabitants will only have one way to go – nuclear thermal propulsion. NERVA had Isp of around 800 and thrust-to-weight much more than 1 – the same design could achieve orbit even from a massive super-earth where escape velocity is surely beyond any chemical fuel. There were visions of lifting helium-3 from ice giants which are harder than almost all super-earths. Likewise, later they won’t spend their thinking on space elevator, knowing that no material would withstand the tensile stress and concentrating on Lofstrom loops instead.

        And once they are in space, they’ll have much less objection to Daedalus or Orion-type propulsion. Even detonating bombs in a km-wide superconducting-loop magnetic nozzle is just a quantitative step for those who made their first orbit by nuclear rocket. There was a calculation that up to hundred thousand tons of payload could be launched to LEO by optimal-Isp nuclear-salt rockets before the average radioactivity of the whole atmosphere rises by 10 per cents (assuming all spent fuel stays in the air).

        The number that sounds very encouraging to deep well dwellers!

        This all sounds like I’m just arguing against Rare Earth, but I think it’s more complicated. Conditions that allow for civilization development may be rare, one in a many thousands-to-million of earth-mass planets in the HZ, and the step from low orbit to mature interstellar colonization might be as hard as going from oceans to land. Multiple Small Filters combined can amount to one mature spacefaring civilization per billion worlds that had a chance to evolve life. But regarding the Paradox, it’s the outliers that matter – the ones who evolved early, remained expansive, and spread out quickly.

        • Mike Serfas May 30, 2021, 14:54

          The “Boring Billion” might be a bit dull for geologists, but I doubt we could have done without it in terms of the origin of complex life. Multicellular organisms are vastly more complex than a colony or biofilm of bacteria. Our nerves, our muscles, or the pollen tube cell that grows up a strand of corn silk… these cells are well over a decimeter long! Bacterial and archaeal cells aren’t equipped to transport proteins over that sort of distance. Humans and paramecia share a method of meiosis for sexual reproduction, which required chromatin to have evolved to package the DNA and microtubules to move it to the poles of the cell. We have a wide variety of organelles far more complex than the “intracytoplasmic membranes” of prokaryotes. We build a nuclear membrane and do a complicated splicing process on RNA before it is permitted to pass out. The organization into exons allows us to do some amount of meaningful evolution despite absurdly long generation times – though even so we are in no position to pioneer basic biochemistry and find ourselves losing basic necessities like vitamin C and essential amino acid biosynthesis.

          As these examples illustrate, eukaryotic cells had to become tremendously advanced before they could make a go of evolution as complex, long-lived multicellular individuals. Something like the lens and retina-like functions of dinoflagellates (see http://www.sci-news.com/biology/science-warnowiid-dinoflagellate-plankton-eyes-02973.html for a cute illustration) should give us an idea of how close single cells during the “Boring Billion” were able to come to exhibiting advanced behavior on their own. The fossil record for single cells is still scanty, but I think we’ll find the time was put to good use.

  • Michael Fidler May 29, 2021, 6:17

    Wow is me, I see we are collapsing back into the earth centered universe!
    All life on earth was probable extinguished several times and renewed by panspermia. The idea of the earth being a stewing pot for 4 billion years with out any outside influence is ludicrous. Catastrophism is the new religion, the idea of a slowly evolving earth died not to long ago. Our oceans crust is all less then 250 million years and the little bit of ancient earth still around is what’s left of the craters rims of super impactors. If we had found earth and had never seen a planet like this one, that would be the most likely conclusion. Every time we swing thru the embryonic gas clouds of the galaxy arms we are clobbered by giant super impactors and at the same time inundated with panspermia. These clouds are factories creating suns and planets, and the life that may have been blown out from the massive dying stars systems that formed them.

  • Alex Tolley May 29, 2021, 10:34

    All life on earth was probable extinguished several times and renewed by panspermia.

    Is there any evidence whatsoever for this contention?

    I would argue the evidence is that this has not happened, at least since the Hadean.

    1. Once life penetrates the crust it becomes extremely hard to eradicate it, even with impactors. The crustal environment is much more stable than the surface and not subjected to climate change.

    2. From 1, if panspermia was common, we would expect to see evidence for this with organisms with different biologies appearing on Earth and surviving alongside the earlier forms. We see no evidence for this despite looking for this and Paul Davies’ suggestion of a “shadow biosphere”. In particular, the last 2 revolutions of the galaxy have not in any way shown disruptions in the fossil record that would need to be explained by panspermia, whether by independent organisms like bacteria or parasites like viruses (c.f. Hoyle & Wickramasinghe) of which the latter would need the same DNA/RNA and genetic code as terrestrial organisms to successfully replicate.

    The most interesting part of the suggestion that the galaxy is replete with clouds of bacteria or similar organisms of sufficient density to reliably enable panspermia is that possibly all galactic life is based on the same biology, due to the success of some early abiogenesis resulting in the spread in that organism around the galaxy. We might just be able to test this idea by analysis of incoming interstellar dust and comets, looking for evidence of life. Another comet similar to 2i/Borisov could be investigated with a probe outfitted with life-detection instruments possibly including a sample return to look for life. The further out such interceptions are made, the lower the likelihood of contamination by any outgoing terrestrial life.

    • Michael Fidler May 30, 2021, 10:25

      I would say there is plenty of evidence, three quarters (3/4) of the earth is less then six percent (6%) of the age of earth. The earth surface was stripped clean 635 million years ago, that is only fourteen percent the age of earth. Anything beyond that time period (635 Million) is pure conjecture, that would not stand up in a court of law, and has very little evidence.
      But let me be clearer, 3/4s of the earth was destroyed some time ago to just magma oceans from planet size impactors, the original crust after the impact was all in one location. Later impactors changed the ancient ocean convection pattern of its thin lithosphere set up by the original impactor. That is when tectonics started to tear apart the single large continental land mass. All most all of this has been destroyed by the recycling of the oceans crust. What is left of the ancient core continental crust has been changed by metamorphism from heat and other impactors. What we see today is mostly volcanic chains eruptions in the oceans that have merged with the ancient cores.
      Any sign of these impacts would of quickly been recycled in the conveyor belt of oceanic crustal spreading. What impacts on land where mostly destroyed by snowball earth.

      • Michael Fidler June 1, 2021, 22:17

        Interesting report on the Hiawatha crater in northwestern Greenland.

        JUNE 1, 2021
        Research sheds light on origins, age of massive impact crater.

        The impact of the asteroid thought to have created the Hiawatha crater would have produced so much heat that the ice sheet would have released a massive volume of meltwater “in pretty much an instant,”
        “Because it is very well preserved, it points to a possibly very young age, as young as the onset of the Younger Dryas period (between 11,500 and 14,500 years ago).”


        Effect of ice sheet thickness on formation of the Hiawatha impact crater.

        “The discovery of a large putative impact crater buried beneath Hiawatha Glacier along the margin of the northwestern Greenland Ice Sheet has reinvigorated interest into the nature of large impacts into thick ice masses. This circular structure is relatively shallow and exhibits a small central uplift, whereas a peak-ring morphology is expected. This discrepancy may be due to long-term and ongoing subglacial erosion but may also be explained by a relatively recent impact through the Greenland Ice Sheet, which is expected to alter the final crater morphology. Here we model crater formation using hydrocode simulations, varying pre-impact ice thickness and impactor composition over crystalline target rock. We find that an ice-sheet thickness of 1.5 or 2 km results in a crater morphology that is consistent with the present morphology of this structure. Further, an ice sheet that thick substantially inhibits ejection of rocky material, which might explain the absence of rocky ejecta in most existing Greenland deep ice cores if the impact occurred during the late Pleistocene. From the present morphology of the putative Hiawatha impact crater alone, we cannot distinguish between an older crater formed by a pre-Pleistocene impact into ice-free bedrock or a younger, Pleistocene impact into locally thick ice, but based on our modeling we conclude that latter scenario is possible.”


        I would think that this would also be true for 3/4’s of the impacts on the earth’s oceans, possibly not leaving any crater signature in the deep ocean abyss. Just to make clear that the deep ocean abyss is also less then about 180 million years old. That means I need to correct that 70 percent of the earths crust is only less then FOUR percent (4%) of the earths age and most of it a lot younger. So we have a very limited image of the number of large impactors craters, most of which have been destroyed on the continental land mass by snowball earth and even the last ice age 11,500 years ago. If we include Greenland and Antarctica ice sheets that is 75 percent of the world that has no or limited ability to record large impactors. So when you see the super bright flash… )

    • Michael Fidler May 30, 2021, 11:03

      If DNA is the universal language for life and RNA for viruses how could we tell, since we have none from the ancient fossils. Genetics is still learning and we have no idea what would be the result from infinite panspermia. Large stars that create the clouds to form suns like ours and smaller stars may also have a large number of planets. These stars in the red giant phase expel their gases before exploding and that may slowly strip those planet’s atmosphere. Life may also be stripped with this and be carried into the large molecular clouds, Wolf-Rayet stars are an example of what may take place.
      Just as on earth the interstellar medium has deserts and oceans of material. It is the ocean of the thick clouds, like the Orion Nebula that we will find life and impactors that can both create and destroy life on earth. Every time we pass thru these arms of our galaxy we will not always encounter the heavenly clouds, but they may be the only area where panspermia can survive and also where impactors are at their highest density.
      I think that the moon and Mars may give us more clues and possibly fossil records of life via panspermia and giant impactors…

      • Alex Tolley May 30, 2021, 17:26

        If DNA is the universal language for life …how could we tell, since we have none from the ancient fossils.

        As Dawkins once remarked, we could determine the evolution of life on Earth even if we had no fossils, as we can sequence the DNA of existing life forms.

        For quite some time biologists have been building phylogenetic trees by comparing DNA sequences. The correspondence between these DNA sequence trees and the fossil record we have is quite remarkable.

        Your argument is that because we do not have fossils much older than the Cambrian, and that we have no DNA sequences of that age or older, how can we be sure that DNA, or at least the genetic code, has not changed in 3.5+ billion years. (I hope I have paraphrased your argument correctly.)

        There are several lines of evidence showing that there has been no total removal of ancient life forms entirely and that panspermia has replaced the tree of life.

        1. All living multicellular organisms can be assigned to the phyla (basic body plans) that appeared in the Cambrian. The lineages for each modern representative species can be traced back to their ancestors in the Cambrian using both physical fossils and DNA sequencing. Therefore, we can be sure that multicellular organisms must be using the same DNA structure, bases, and code throughout that 550 million years.

        2. DNA sequences of the most fundamental biology, such as ribosomal subunits, can show a connection to bacteria and archaea, demonstrating that multicellular organisms are connected to even the earliest types of life.

        3. Viruses require the host organism to provide the replication and translation machinery to create new viral DNA/RNA and proteins. This means that the viral information storage and instruction set must be the same as that of the host. This further connects viruses to bacteria and multicellular life.

        4. We can transplant genes from multicellular organisms, e.g. to make insulin, into bacterial plasmids. If the bacteria used a different DNA->RNA->protein instruction set, this would be impossible.

        So all the evidence points to the common lineage of all life on Earth. Furthermore, if there is a shadow biosphere of different biology on Earth, we have so far failed to find any evidence of it. It is not impossible it exists in some refuge, such as the crustal biosphere, but so far none of the samples have discovered anything that would suggest this. Any such discovery would be guaranteed to make the front cover of Nature or Science.

        Is it possible that early bacterial life forms were replaced with a new version that became our ancestor? It is possible, but this must have happened before we find the earliest fossil life and the connection between those forms and all subsequent life. IOW, during the Hadean late bombardment, it is possible there were several serial abiogenesis events, each succeeding one replacing the earlier one entirely. We have no way to knowing that. What we can be sure of is that since about 3.5+ bn years ago, there has been no replacement.

        Is it possible that viruses from space continue to enter our biosphere and impact extant life, for example, retroviruses that insert their genes into animals and plants, altering them? If they do, their DNA and genetic code must match the terrestrial version common to all life. If this was the case, it would imply that at least one other world has exactly the same genetic makeup as we do. That would tell us some important things. If we were to retrieve samples of life from interstellar material that we could be sure was uncontaminated, then analysis of those organisms would tell us if there are one or more genetic instruction sets. This sort of approach is a direct extension of taking aquatic samples and sequencing all the shed DNA to detect the organisms they came from. Such a test was done in Loch Ness and turned up no unexpected, e.g. plesiosaur, DNA, just the organisms known to live in and around the loch.

        In summary, once the late heavy bombardment was over, the life that appeared in the earliest fossils and whose age can be estimated by the DNA clock, there is no room for any catastrophism to indicate a replacement of the terrestrial lineages. Terrestrial life has been continuous despite the various extinction events that have made their mark in the fossil record.

        • Michael Fidler May 30, 2021, 23:49

          Ok, I understand your point, but as a simple example lets say the first civilizations created panspermia and sent it thru out our galaxy. Could we tell the difference if it was the same as our DNA on earth? Would it diverge or stay the same? Lets say they sent it out in the tough and hearty Tardigrades for safe keeping?

          • Alex Tolley May 31, 2021, 12:21

            Assume that the civilization was directing panspermia in a way that perfectly preserved the DNA of the organisms. Or using the Thomas Gold hypothesis of alien visitors to Earth shedding their DNA in some way.

            1. When would it have arrived on Earth?
            2. Could we detect that DNA.

            As all Earth life uses the same DNA, any panspermia must have arrived to have become the ancestor of all life. This means that a complex organism like a tardigrade wouldn’t work as it is too late. It would also need an ecosystem to thrive in – plants to feed on etc.
            So any panspermia or accidental shedding of bacteria must have happened at the very dawn of life on Earth so that the successful bacteria/archaea were at the base of our tree of life.

            What about determining if DNA in interstellar space is the same or different than our DNA?
            We have structure (how the bases are connected with a deoxyribose sugar), the bases (A,C,G,T) used, the chirality of the helix, the genetic code (does translating it to a protein work with our code). the isotopic ratio of the elements (e.g. C12/C13 ratio). All these tests would indicate similarities or differences. One issue is that Earth might well be shedding DNA in bacteria out into space. (We had a post by Zubrin suggesting that our bugs have already contaminated Mars). An exciting finding would be if the tests clearly showed the DNA was different to terrestrial DNA in some way.

            It is the same problem if we find life on Mars. If the bacteria have the same DNA as Earth, does it indicate shared abiogenesis or 2 geneses that happened to get the same result due to some attractor state? [I would go for the former as the most parsimonious explanation]. What we want is a different biological base – live organisms that at least use a different genetic code, or amino acids, or some feature that clearly distinguished them from terrestrial organisms. Bacterial spores in a comet or asteroid that can be cultured and experimented on.

            A controversial finding would be detecting DNA from samples from space near another star with the same features as terrestrial DNA but with a very different C12/C13 ratio, indicating a different origin. Another abiogenesis that resulted in the same DNA features as Earth’s, or indications of a common origin (c.f. Mars), perhaps from a 3rd origin.

            • Michael Fidler May 31, 2021, 22:00

              Let me get this strait, if we are the only life in the galaxy and in the future we send out our DNA thru panspermia, would we be able to tell if it was ours when we reach the other side of the galaxy?
              This brings up a interesting thought, if we find life on Mars how can we tell if it was expelled from earth during a solar storm and reached Mars? Could we even tell if it was from space craft contamination or naturally created on Mars? The main point is if panspermia is common and the DNA is like ours could we even tell where the life on Mars came from??? Would not the C12/C13 ratio depend on the local environments? We may be seeing more of this problem if we do find life on Mars, but fossils will make it much more controversial.
              Saw some interesting images of thick water ice in protected glaciers on Mars so maybe DNA could be found.

              Exposed subsurface ice sheets in the Martian mid-latitudes.


              Newly discovered glaciers on Mars may help humans settle on the Red Planet one day.


              So what if we found a variety of types in the Martian ice? I’m sure Elon Musk will want to know what is in the water they drink from Mars glaciers. You have read about the ancient life melting out of the Siberian permafrost? )

              • Alex Tolley June 1, 2021, 10:13

                Let me get this strait, if we are the only life in the galaxy and in the future we send out our DNA thru panspermia, would we be able to tell if it was ours when we reach the other side of the galaxy?

                If we do the analyses suggested, then we could say that it is the same as terrestrial DNA. If our stellar probes had not turned up other life or that life all had different DNA in some way, then likely a Bayesian inference would suggest it is ours. It would be much more uncertain if our interstellar probes found that any life found used exactly the same DNA. Probes will eventually inform us about life in the galaxy, how different vs similar its biology is to terrestrial biology. If it proved very similar, then sequence analysis might even tell us if the lineage is connected to terrestrial life and when they shared common ancestors. But all this is far in the future.

                This brings up a interesting thought, if we find life on Mars how can we tell if it was expelled from earth during a solar storm and reached Mars? Could we even tell if it was from spacecraft contamination or naturally created on Mars? The main point is if panspermia is common and the DNA is like ours could we even tell where the life on Mars came from?

                If we found living organisms on Mars, then DNA sequencing and experiments would tell us if life on Mars was of recent contamination or ancient. If the life found had DNA sequences that were very close matches to known Earth organisms, then most likely it was contamination. Sequence analysis would likely tell us whether the organisms were connected to Earth’s tree of life and when the divergence occurred. Infecting them with a terrestrial virus would show whether the genetic code is the same as Earth’s or different – different indicates likely separate abiogenesis.

                So what if we found a variety of types in the Martian ice?

                That seems unlikely unless each type flourished in some separate period of Mars’ habitability and then was entirely wiped out and replaced by the next organism that drifted by when Mars was habitable again. However, I think that the latter idea is unlikely as bacterial spores in deep subsurface glaciers would be protected from damage at least for many millions of years and spores would therefore be the most likely to be dominant once conditions were clement again, resulting in their presence alone in the most recent glaciers. But I don’t want to be dogmatic about this.

                It would certainly be prudent to treat and test all water on Mars before drinking it. Distillation would be the best method to ensure the water was entirely safe, if rather tasteless. The colonists would probably already have drunk water recycled urine, humidity from sweat, and bathing water on the voyage over.

                If any organisms were discovered, it would be important to know how similar or different they are to determine whether they might be potential pathogens or not. Bacteria using different bases would definitely not. Most bacteria on Earth are not pathogenic either, so I would expect most Martian bacteria to not be pathogenic either. But…

            • torque_xtr June 1, 2021, 10:21

              Isotope ratios will tell little, living organisms are insensitive to all of them except D/H. Any living matter will quickly assume local values, to the precision of living/external ratios, including skeletons after a single generation.

              The task of figuring out the origin of hypothetic martian DNA-based organisms is quite straightforward after we get a detailed look at their genomes. All it takes is to measure informational difference between martian and terran life. If the difference is much greater than the natural spread in Earth’s biosphere, then the origin is likely separate. If not, then molecular genetics will tell the time of divergence, like animals from plants 1.6 GYa. But I think if martian life has separate origin, then there will be unmistakable differences in biochemistry. Like, this enzyme could not evolve from terran organisms, and these signal molecules are unlike anything we see on Earth. “This, combined with other biomolecular evidence, supports the separate origin to the confidence level of at least 19 sigma” (Journal of Martian biology, 2045)

              The delicate thing is, due to lithopanspermy and relative isolation of Martian habitats from each other, we absolutely can not be sure that the first encountered samples of martian life will represent the whole planet, be they our relatives or not… On Earth, we search for shadow biosphere; on Mars, it could be difficult to tell which one is dominant.

              • Alex Tolley June 1, 2021, 14:58

                Isotope ratios will tell little, living organisms are insensitive to all of them except D/H. Any living matter will quickly assume local values, to the precision of living/external ratios, including skeletons after a single generation.

                I think you are saying that any spores we find will have the isotopic ratios of the comet/moon/planet and therefore hide the origin whether alien or terrestrial. Fair point.

                If so we will have to rely on the other methods of analysis which should be definitive.

            • Alex Tolley June 2, 2021, 11:44

              By a coincidence, listening to the audiobook of KSR’s “Ultima”, the question of Martian biota’s linkage to Earth is mentioned, as well as interstellar panspermia. Almost everything we have mentioned is included in an exposition by an AI in the book. KSR had clearly researched the literature about this.

      • Michael Fidler June 1, 2021, 23:02

        Evidence Suggesting that ‘Oumuamua is the ~30 Myr-old product of a Molecular Cloud.

        The appearance of interstellar objects (ISOs) in the Solar System — and specifically the arrival of 1I/’Oumuamua — points to a significant number density of free-floating bodies in the solar neighborhood. We review the details of ‘Oumuamua’s pre-encounter galactic orbit, which intersected the Solar System at very nearly its maximum vertical and radial excursion relative to the galactic plane. These kinematic features are strongly emblematic of nearby young stellar associations. We obtain an a-priori order-of-magnitude age estimate for ‘Oumuamua by comparing its orbit to the orbits of 50,899 F-type stars drawn from Gaia DR2; a diffusion model then suggests a ∼ 35 Myr dynamical age. We compare ‘Oumuamua’s orbit with the trajectories of individual nearby moving groups, confirming that its motion is fully consistent with membership in the Carina (CAR) moving group with an age around 30 Myr.
        We conduct Monte Carlo simulations that trace the orbits of test particles ejected from the stars in the Carina association. The simulations indicate that in order to uniformly populate the ∼106 pc3 volume occupied by CAR members with the inferred number density, n=0.2AU−3, of ISOs implied by Pan-STARRS’ detection of ‘Oumuamua, the required ejection mass is M∼500 MJup per known star within the CAR association. This suggests that the Pan-STARRS observation is in significant tension with scenarios that posit ‘Oumuamua’s formation and ejection from a protostellar disk.”


        Could ‘Oumuamua be both a destroyer and a creator from the nearby Carina—Vela moving group?
        “The closest members of the Carina—Vela moving group are just 30 pc from the Sun. The young open cluster IC 2391 is a part of the moving group.”


        I found this on the Interstellar Research Group page “Interstellar Updates” which keeps a current updates on papers. “These are items of interest to the interstellar exploration community that we’ve found in our quest for information that will help us advance toward our goals.”


        Paul Gilster our mentor, is on the Board of Directors of Interstellar Research Group.


  • wdk May 29, 2021, 17:07

    About a decade back, I would sometimes encounter articles about the Edicaran period and some remarkable fossil finds in British Columbia, which led me to think that the Edicaran Hills were on NA rather than Australia. But there was also suggestion that the Edicaran had so little connection with the Cambrian that followed, that perhaps the Edicaran was a cul de sac from which the Cambrian did not emerge.

    Why mention it at all? Perhaps it is an era in which there is some evidence for how resilient life is.on Earth – and could be on another
    world experiencing period disruptions. But of late, I have seen little follow up on that one-time interpretation.

    • Alex Tolley May 29, 2021, 20:26

      There is little fossil material other than mud impressions concerning the Ediacaran fauna. My book on phyla doesn’t even mention them. There is also an even earlier, but controversial complex biota. My 2 cents is that the Ediacaran biota are contiguous to the Cambrian biota, although the connection is not yet known (and may never be).


      • wdk May 30, 2021, 16:12

        Sorry about my misspelling, if anyone else was looking into this.

        My introduction to the subject was in passing in the pages of Science,
        journal of the AAAS, But some other summaries are on line.



        The Ediacaran Period (635-542 million years) represents a turning point in the history of life with the advent of the first large and complex multicellular soft-bodied organisms. These include sponges and cnidarians, as well as a number of problematic groups represented by both macrofossils and microfossils. Some of these fossils have traditionally been regarded as the remains of forerunners to Cambrian (and modern) animals, while others have been seen as a completely extinct kingdom.

        The exact affinity of these organisms is still debated, but many researchers agree that they display a wide range of morphologies, suggesting they might belong to different groups at the base of the animal tree of life.
        Graphic showing origins of different Ediacarans

        Possible positions of various types of Ediacarans at the base of the animal tree of life. Dotted lines represent the probable range of particular groups of animals. Solid lines represent fossil evidence. Extinct groups (taxa) are represented with a circled cross. (modified after Xiao and Laflamme, Peterson et al and Dunn et al.).

        Perhaps the most iconic of these enigmatic fossils belong to a group known as the rangeomorphs, found in late Ediacaran (575-542 million years ago) rocks. These are feather- or bush-shaped and show self-repeating (fractal) growth patterns that resemble the outlines of some modern fern fronds. They are not, however, related to plants – rangeomorphs lived deep in the sea, far below the depth where light could penetrate to allow photosynthesis. The rangeomorphs lack any evidence of a mouth or gut – or indeed any other complex internal organs typical of most animals. Their affinity remains ambiguous. Some researchers have suggested rangeomorphs could represent a grade of organization that evolved before the sponges, making them very primitive metazoans.

        Another illustrative report from AAAS publication Science.
        Modular Construction of Early Ediacaran Complex Life Forms

        Guy M. Narbonne
        Science 20 August 2004, pp 1141-144
        Newly discovered, exceptionally preserved, soft-bodied fossils near Spaniard’s Bay in eastern Newfoundland exhibit features not previously described from Ediacaran (terminal Neoproterozoic) fossils. All of the Spaniard’s Bay taxa were composed of similar architectural elements—centimeter-scale frondlets exhibiting three orders of fracticality in branching. Frondlets were combined as modules atop semi-rigid organic skeletons to form a wide array of larger constructions, including frondose and plumose structures. This architecture and construction define the “rangeomorphs,” a biological clade that dominated the Mistaken Point assemblage (575 to 560 million years ago) but does not appear to be ancestral to any Phanerozoic or modern organisms.

        • Alex Tolley May 31, 2021, 13:07

          Thankyou for the link. I was mistaken about the mention of Ediacaran in the textbook: “On the Origin of Phyla” (James Valentine, 2004). There is a very mention concerning whether any of this fauna may be related to the Cnidaria (jellyfish), while the link you cited seems to suggest at least one lineage closer to the sponges. [phylogenetic trees may place either phylum as the more ancestral]. By 2004, the Ediacaran fauna was still rather mysterious, based on impressions left in sediment, and uncertain whether they were separate lineages, even an animal kingdom, or whether any were ancestral to a Cambrian phylum.

          Then there is the even more intriguing Francevillian biota that may have existed 2 billion ya before going extinct. Or they may not be biota at all (c.f. the controversy over the Martian meteorite fossil life ALH84001).

  • Gary Wilson May 29, 2021, 17:35

    The time frame considered (3 By) is just to staggeringly long to me. We don’t know enough about refugia, or exactly how the survivors repopulated the surface of the earth or what other factors other than temperature are important. We don’t know whether an Earth-like moon is required (some of us are just hypothesizing it is existentially important). We don’t know the range of worlds complex life can arise on. We think complex life leading to humans arose here about 600 My ago. So evolution fairly rapidly generated an extremely large range of complex organisms fairly quickly. If refugia exist just about everywhere that abiogenesis occurred (and we don’t know anything about how often abiogenesis occurs) then possibly 600 My is sufficient for some form of intelligent life to occur, not 3 By. It’s just hand waving until we have significant data from worlds in other habitable zones and that won’t be for quite a while. Speculation is fine, models are fine, but you can’t go from that point to beyond hypothesis and to what appears to be in general true until you have large amounts of the critical data points.

    • Alex Tolley May 29, 2021, 20:20

      There is definitely a lot of hand-waving involved. A lot of bias due to beliefs affects what one’s thoughts on the matter are. Abiogenesis could be easy based on the almost immediate presence of life as soon as the surface was habitable, or it may be hard so that life must be seeded from elsewhere. We just do not know. While I would argue that lithospheric life is going to ensure continuity almost regardless of surface factors, whether this ever allows more complex life to evolve after a surface sterilization is anyone’s guess – a billion years? More?

      As ever, we need data. Getting that data and interpreting it is the hard part. Recall we are not even going to look at icy moons as there are no current means to even observe biosignatures for such worlds. What if those worlds proved by inspection to be the most common living worlds? Telescope time is precious and so the searches will most likely focus on worlds that are closest to Earth if only to get the jump on the fame for the first discovery of a living world. Ultimately we are going to want to get statistical data from surveys like Keppler and Gaia.

  • Geoffrey Hillend May 29, 2021, 17:50

    Quote by Alex Tolley: “The temperature is controlled purely by its initial (random) state and the various feedbacks, both positive and negative. This is the essence of his simplified, toy model.” I can’t see anywhere in meteorology or even in physics this statement applies. I am not saying it is not true, but words like feedback process, perturbation, etc. have no meaning if they are abstracted from the concrete, physical process they describe or predict, i.e. they cannot function independently from physical processes and take a life of their own without a physical cause and it is very hard for me to not draw that conclusion from looking at this paper. The feedbacks in our past and present climate are not random and must have a cause which is not random and makes them predicable. The cause is not shown here..

    Temperature is controlled by the vibration of atoms and molecules which is the energy we add to them due to collisions with each other or with high energy EMR, the molecules vibrate and rotate faster. If we define the specifics of the system, and this paper has and Earth sized planet in the life belt around at star. This initial state is not random. It might be in a widely spaced, interstellar gas cloud with different densities before part of it began to collapse into an a proto star.

    The temperature of a fixed system like our Earth which has a Moon give it a long term climate which only an Earth Moon system can have. For example. If the Earth did not have a Moon it would have a large difference in axial tilt over time than Earth so every fifty thousand years would change as much as 25 degrees instead of only 2 degrees. We can use Mars as an example because it does not have a large Moon to balance it’s axial tilt or obliquity with only a small variation of only two degrees. Consequently the precession of the Earth is roughly 25,800 years because it has a Moon to balance it, but Mars precession is 175,000 years due to the larger wobble of its axes over time since it does not have a Moon to balance it.

    The Milankovitch cycles are the result of the Earth having a Moon to give it a small obliquity change over time. Also The Milankovitch cycles did not affect our climate more than fifty million years ago because the carbon dioxide levels were much higher than today. The polar ice caps appeared about 20 million years ago and it is the carbon cycle, continental drift and the photosynthesis the helped remove the carbon dioxide. About three million years ago we stared to have ice ages due to the Milankovitch cycles which has balanced the temperature so we always have ice ages about every fifty thousand years. Climate change has changed our long term climate, for according to NASA we will miss the next ice age or skip one of them so the next one will be over one hundred thousand years in the future.https://www.climate.gov/news-features/understanding-climate/climate-change-atmospheric-carbon-dioxide

    The general accepted view by climatologists and scientists with a knowledge of geology, and the all of the physical which allows us to understand our past climate think the idea that the Milankovitch cycles contribute to our ice age is absolute fact which is not debatable. They will of course be happy to debate it. The reason being is the science which supports it is infallible and there is a lot of concrete, physical evidence. The ice core samples give us not only the Co2 levels from bubbles of atmosphere trapped inside the ice a million years ago, but we also get the oxygen isotopes from that time as well and the support the carbon dioxide levels which rise when the temperature goes up and decrease when the temperature goes down. The oxygen isotopes give us the temperature, the precipitation and ice covering of a period. http://www.ces.fau.edu/nasa/module-3/how-is-temperature-measured/isotopes.php

  • Geoffrey Hillend May 29, 2021, 18:37
  • Alex Tolley May 29, 2021, 20:08

    I think you may be getting too much into the weeds here.

    The model is for all earth-like planets. Milankovitch cycles apply to Earth and not necessarily to other planets. The feedbacks are set for various temperatures. Yes, they are random in direction and number, but that is to reflect our incomplete knowledge of climate and the likely difference on exoplanets. For example, Dr. Ramirez’s (?) post on CO2 and temperature shows that M_dwarfs do not behave as one would expect from extrapolating from Earth due to the spectrum of light at the top of an exoplanet’s atmosphere. A planet with shallow seas may fall into a runaway cooling more easily than Earth. A planet with more active volcanism conversely may fall into a runaway greenhouse. As planets will vary in their place in the HZ, any runaway temperature change may start quite close to the stable planetary temperature. Some planets will have a strong Gaia effect, others a weak one, possibly non-existent.

    The reality might be the reverse of this. My experiment was to assume a single attractor where the feedbacks at the extreme temperature range were always negative. This resulted in a much higher survival rate as would be expected.
    I commented on the size of the perturbations and explained how the large temperature perturbations distribution was responsible for the failure to survive even in the most stable of climate feedbacks. Reducing their size and distribution, as well as frequency, increases survival rate. I would say that @AlexTru’s comment upthread is most pithy as regards the author’s model.

    The model is out there to play with and you can devise your own experiments within the limits of the model approach. Or you can write your own model in the code of your choice.

    The point of presenting this model was not to suggest this was a realistic climate model, but rather to provide a different approach to thinking about life on exoplanets and to think outside the box from the usual course of such discussions. Using more realistic, sophisticated, but compute-intensive climate models, the same sort of analysis could be done with suitable resources. That may provide a better answer for the temperature stability of exoplanets. However, as we know, the climate is not the extent of life’s survival. There will be arguments about cosmic events like a nearby supernova, the ever-perennial big Moon hypothesis, and any myriad of other factors that may impact life. You may recall Dr. Ramirez’s post of the features that restricted surface-living complex life. Life might be a lot rarer or common, than we believe.

    • AlexTru June 1, 2021, 4:07

      you can write your own model in the code of your choice.

      our scientific knowledges about habitable planets is limited by only one sample (the Earth) , this limited data source does not allow us to build any model of habitable worlds diversity that science can trust…
      Meanwhile , with our current knowledge every variant of ETL that human imagination can invent is possible – but this state is area of religion or Sci-Fi literature, but not science.

      • Alex Tolley June 1, 2021, 15:06

        I partially agree with this. However, even with terrestrial life, it doesn’t seem as if every phenotype imaginable can appear. Whether this is entirely due to contingent paths taken, or something else we cannot yet say. What we can say is that all the organisms evolution has created retain the body plans of those phyla that appeared in the Cambrian. No 3-legged Puppeteers of Niven’s Known Space universe, or any of the other imagined forms that could not be mapped onto any terrestrial phyla. Whether such phenotypes could ever exist is, of course, unknown. However, organisms must be fit to survive and that fitness must adhere to physics constraints.

        • AlexTru June 2, 2021, 2:53

          …even with terrestrial life, it doesn’t seem as if every phenotype imaginable can appear…

          Probably for the Earth specific conditions “3-legged Puppeteers” are impossible due to initial condition limits, but for some other virtual planet that has somehow different initial conditions a “3-legged Puppeteers” are only possible variant :-)
          Meanwhile science does not have plausible answer the question: “what is the life, why/how it is different from “no life” and how it is emerged on our planet”.
          I know there is one widespread variant of the answer to this question – panspermia, but in reality it is not answer – it is the way to “make poker face” and hide absence of knowledge…

          • Alex Tolley June 2, 2021, 9:35

            Tripods. Consider for a moment 2 factors that make such an organism unlikely.

            1. Development. Is it radial or bilateral symmetry? If radial, then the often depicted bilateral symmetry of the head is at variance with the legs. If bilateral, then one leg must be a fusion of 2 legs, yet the 3 legs are usually shown as the same.

            2. Fitness. Is a 3-legged organism mobile enough compared to animals with pairs of legs? What is the walking/running/hopping pattern? Would such a creature evolve other defensive measures, like ejecting moxious gases to deter a predator?

            The symmetry issue and embryonic development is a natural division in terrestrial organisms. Are there other symmetries elsewhere, and what would they be? What would be the evolutionary path that eventually gets an organism, perhaps marine initially, to become a 3 legged, land-living animal?

            We can imagine all sorts of alien life, but they must meet some criteria for evolutionary success to exist. On Earth, any 3rd leg would likely atrophy as the animal became bipedal, or become a balance organ like a tail, breaking the symmetrical 3-leg form.

            • AlexTru June 2, 2021, 13:01

              On the planet Earth some results (mutations) of evolution hard to explain.
              We have insects, octopuses here – very far from bipedal.
              By the way tripod – is best and simplest method to keep vertical balance :-) Let imagine that there is somewhere planet that high height and vertical balance – significant feature…
              I agree that my speculations are sci-fi… but I am sure none can scientifically prove it is impossible.

  • kb May 29, 2021, 21:05

    Earth’s atmosphere is already substantially opaque in the CO2 absorption bands, at only 400ppm by volume CO2. Adding more CO2 can have no sensible effect on the planet’s heat balance. All it does is trap heat closer to the surface so the surface temperature rises. But it won’t runaway like Venus. It simply reaches a new equilibrium with the surface slightly warmer.

  • Geoffrey Hillend May 30, 2021, 17:11

    I’m an advocate for thought experiments and theories which attempt to think outside the box. It’s a common and scientifically uneducated opinion that we don’t know Earth’s climate history is which is random, and always changing so it is unpredictable. I thought that is what this paper was implying. We do know what Earth’s climate history is and it should apply to a similar planetary system, but not Earth like exoplanets without Moons. In theory, it does apply to exoplanets without Moon as far as temperature and distance from the Sun are concerned. When we include astrophysics, geology and solar wind stripping which might change that. I will admit this is still an unknown and hopefully the JWST and other extremely large land based telescopes which shed some light on it. I think we can generalize that all Earth size exoplanets the same size as our Earth with a similar sized Moon should have a high probability of producing life forms which are similar to ours. Also the probability of life is the highest with the exact Earth like conditions I have mentioned. If we star to remove those, then the probability of life decreases the more of them we remove. This of course is only a theory which starts which the most requirements for life to survive. I don’t think it is a conservative theory since limitations are part of the principles of physics and physical reality.

    In my opinion, the idea that life began as the result of totally random events is mutually exclusive with the anthropic principle and also a with a philosophical and teleological view. I will admit that we don’t know the climate histories yet of any Earth sized exoplanet’s yet, and we can make generalizations about different histories, but I think one has to be careful the application different climate history’s if we limit them only to temperature because they may not be universal. For example: The idea that Earth could have an alternate climate history where life could become extinct because the temperature becomes to high is not a possibility in Earth’s past history based on looking at the whole system, or all of the science involved. Some climate paths might be imaginary and have to be ruled out based on first principles. If our climate included such a possibility, then the idea of a Goldilocks zone or life belt would have no applicability and we could not predict anything if we have to contradict the principles of physics, the planetary science.

    Also what has not been acknowledged is there is some difference in temperature histories in Earth sized exoplanets in the life belt around different class stars. An exoplanet in the life belt around an M dwarf does have a different long term temperature history than Earth because the exoplanet has to be tidally locked. The brightness of the M dwarf remains on the main sequence hydrogen burning much longer than a G class star like our Sun which means the temperature from starlight on the surface of the planet stays the same over deep time.

    • Alex Tolley May 30, 2021, 21:50

      Let’s just think what conditions might cause a very deep glaciation that might be difficult to recover from. One state is increased weathering that draws down the atmospheric CO2. A few posts back there was a suggestion that the Snowball Earth was caused by the position of the continents around the equator, allowing the weathering to continue even as the glaciers covered the rest of the globe. Another state might be the planet orbiting closer to the outer edge of the HZ, requiring more CO2 pressure to maintain a habitable temperature. Combined the 2 states might cause a deep freeze. A coup de gras might be that the planet orbits a star that has flares, and a large series blows off most of the atmosphere, resulting in a very deep freeze. The only recovery is volcanic emissions to pump up the CO2 again, and sufficiently high to overcome the high albedo helping to cool the planet. Perhaps the various conditions are so positively reinforcing that any CO2 now freezes out as it can on Mars, preventing a recovery, at least until the star’s luminosity increases sufficiently, perhaps a billion or so years later.

      We can think of all sorts of planetary conditions that might disrupt the stability of the temperature. On Earth we have 100,000 year Milankovitch cycles. Could a planet with a more eccentric orbit and tilt have much longer cycles? What about a planet that initially can rotate, but later becomes tidally locked? What about a planet with less radiogenic heating whose plate tectonics wind down? What about a planet bombarded more frequently by large asteroids, resulting in more frequent recurring, higher temperature pulses? All these various factors might result in a planet that can only keep its surface habitable for a much shorter time than the referenced 3 Gy.

      If we can accept that such varied conditions might push the surface temperature outside the habitable range, then it might make sense to assume more varied planetary states, including feedbacks, that could make these planets less stable regarding life.

      IMO, the most common theme in planetary climate modeling seems to be to find conditions that might make a planet habitable depending on its orbit. The Tyrrell paper is more like the opposite, finding conditions that make a planet’s surface temperature unstable and tip into uninhabitable bounds. I think we can consider both Mars and Venus as candidates in this regard as both are believed to have had inhabitable surfaces in the past, but both lost them for various reasons. In the case of Venus, its orbit and perhaps the increasing solar luminosity were the causes of its runaway heating.

  • wdk May 31, 2021, 11:33

    In the midst of masses of data, models and timeline responses, I lost my bearings on the question of whether the Earth itself was considered exempt from wandering out of the region of habitability.

    It might have been a “given” that the Earth had remained, and yet we speak of about half a dozen major extinction events in its geological history, the most recent of which we are appear to be in the midst of.
    If the Upper Cretaceous boundary event was bad, what about these earlier ones? Just how clean was the table wiped?

    In the most recent crisis we assume that we are much to blame, but it is possible that previous extinctions on Earth were due to internal conditions as well: the outcome of un-curbed growth of biota or their waste products.

    In recent editions of the journal Science it has been noted that ocean depths above the Earth’s crust were affected by the internal heating of the mantle. This was not necessarily a moon first order effect. The heat of formation and the nuclear decay rates internally have leaked out; so early in the geological history when the sun was relatively cool, the internal heat was higher. The point that the Science article makes with some application to other terrestrial planets ( solar and beyond) is that vast quantities of water once above the crust has sunk back into the the now cooler mantle. Earth for a while was a water world. And likely a very steamy one during some episodes ( as well as icy).

    …As a matter of fact, we could consider the Galilean satellites and their water distributions for a moment in this light. What do you suppose Io, Europa, Ganymede and Callisto have to say about the ice, crust and mantles of systems like Trappist 1? A significant difference in Ganymede and Callisto, both with significant water ice, is that the inner moon has something akin to plate tectonics and convection surrounding a core, and Callisto appears to be internally relatively undifferentiated.
    For Trappist bodies with similar water budgets and internal structures, the opportunities, risks and histories for early life would differ. Some of these effects are due to harmonic forcing functions in near circular orbits, but also to the relative collision energies as these bodies formed.
    The radio-isotopes might have been similar, but if they were not concentrated at the core, the heat gradients and transfers would be different.

    An addendum here:
    Earlier I provided some material on the Ediacaran flora and fauna.
    I have continued my search, because I still have not found some of the more spectacular claims about its vanished life forms ( or illustrations of the more exotic specimens). But here is another abstract:

    Bobrovskiy et al., Science 361, 1246–1249 (2018) 21 September 2018

    Ancient steroids establish the Ediacaran fossil Dickinsonia as one of the earliest animals
    Ilya Bobrovskiy1*, Janet M. Hope1, Andrey Ivantsov2, Benjamin J. Nettersheim3,
    Christian Hallmann3,4, Jochen J. Brocks1*

    The enigmatic Ediacara biota (571 million to 541 million years ago) represents the first macroscopic complex organisms in the geological record and may hold the key to our understanding of the origin of animals. Ediacaran macrofossils are as “strange as life on
    another planet” and have evaded taxonomic classification, with interpretations ranging from marine animals or giant single-celled protists to terrestrial lichens. Here, we show that lipid biomarkers extracted from organically preserved Ediacaran macrofossils unambiguously clarify their phylogeny. Dickinsonia and its relatives solely produced cholesteroids, a hallmark of animals. Our results make these iconic members of the Ediacara biota the oldest confirmed macroscopic animals in the rock record, indicating that the appearance of the Ediacara biota was indeed a prelude to the Cambrian explosion of animal life.

    The authors “indeed” connect the Edicaran with the Cambrian, but since they are considered two different geological epochs, one still has to wonder what kind of devastation divided them and how the recovery proceeded. Evidently on a very different path.

    • Robin Datta May 31, 2021, 22:54

      A quick look-see at the Ediacaan fauna.

      • wdk June 2, 2021, 10:40

        Thanks! The result is reminiscent of a search for the works of surrealist Max Ernst. Maybe appropriate in this context.
        Searching around on this, one result I came to: that the trilobite was one of the few surviving fauna from the Ediacaran and “radiated” remarkably in the Cambrian, hanging around for 300 million years or so until the Permian. Many details unclear, but a couple of Ediacaran possible progenitors. Decades ago in Nature, I saw a picture of a Swedish trilobite fossil which left clear evidence of its demise: struck by a meteorite 200 million years ago. Insufficient armor.

  • Geoffrey Hillend May 31, 2021, 17:08

    I don’t think we need to have the continents near the equator to get the cooling since the snowball Earth period is between supercontinents so the continents were spread apart giving them more coastline then before and more mountains widely spaced or more distributed around the globe. The first supercontinents was Rodina between 1,130 and 750 million years. The next supercontinent was between 633 Ma and 573 Ma. The snowball Earth period was between 710 and 640 Ma. The supercontinent periods are theorized to be warmer than when the continents are apart. The photosynthesis of micro organisms is what removed the carbon dioxide and volcanism is what returned the carbon dioxide to the atmosphere.

    I also thought about whether or not the Earth might have frozen carbon dioxide during the snowball Earth period, but I don’t think it did because Earth has a much thicker atmosphere than Mars. A thicker atmosphere has a bigger greenhouse effect and is the result of a stronger gravity so I don’t think Mars can be used as an example due to Jeans escape, i.e., we have to stick to the idea that the exoplanet has to be a Earth sized planet since the temperature and atmosphere are affected by the size of the body as in Jeans escape.

    Mars can be used for it’s eccentricity and an Earth sized planet with the same eccentricity as Mars, the eccentricity is the orbital distance variance from the star. A big difference in aphelion and perihelion or large variance in distance from the star, but the exoplanet is still in the life belt would cause large changes in the short term weather. I still don’t think there would be a model where all life becomes extinct unless the planet moves out of the life belt.

    I agree the orbit and eccentricity is crucial for the right temperature.
    Both Venus and Earth were in the life belt. Venus has moved out of the life belt so its oceans evaporated. As far as temperature is concerned, Venus had Earthlike temperatures four billion years ago.

    A planet might rotate and become tidally locked afterwards. Venus is considered to be tidally locked with a slow rotation. An M dwarf exoplanet will experience the solar flares. Speaking of M dwarfs, we can assume that Proxima Centauri b probably has tidal forces which keep it volcanically active, so we can expect there to be an atmosphere which might even be as thick as Earths. We can predict what those gases might be based on Earth and Venus since the JWST will be hopefully be available soon. Mostly Carbon dioxide (CO2), Sulfur dioxide(SO2), hydrogen sulfide (H2S) and water vapor(H2O). No oxygen* *I am hoping there will be no oxygen found in the spectra of Proxima Centauri b because I am making the a priori, intuitive assumption that there will be no false positives of oxygen because there won’t be enough of it due to photo lysis from extra solar radiation from Proxima Centauri and what even small amounts of abiotic oxygen won’t hang around because it will be solar wind stripped or removed from other process of atmospheric loss, Jeans escape, sputtering, ion escape, etc. This is only a speculative theory, but it removes the ambiguity of oxygen false positives. If we do find oxygen, how strong is the spectra?

    If there is oxygen in the spectra of Proxima Centauri b, then oxygen will probably be found in most Earth sized exoplanets and how are we going to know if it is biotic or abiotic oxygen? We won’t know unless we can detect other biosignature gases like methane and nitrogen. If there is no oxygen, then life is less hardy, and we will have to look at a large number of stars and exoplanets before we can find oxygen. With this unambiguous theory, when we do find life, beyond a shadow of a doubt, we will know there must be life on the exoplanet with oxygen spectra, but it could be argued that I have unrealistic expectations for unambiguity, and I intuit will be easy to tell if life is on an exoplanet or not and we will just have to look at a lot of exoplanets.

  • Geoffrey Hillend May 31, 2021, 17:16

    The temperature does matter for Proxima Centauri b because if there is an ocean and a lot of evaporation, then there will be a lot of water vapor in the atmosphere, photo lysis of water vapor and abiotic oxygen? Will it’s oceans have been lost two solar wind stripping and atmospheric loss? They might not be lost. The lifebelt of an M dwarf is stationary for a long time with Proxima Centauri. Hopefully, will we get the spectra of Proxima Centauri in the next two years and then we can begin to make an model of atmospheres of exoplanets in the life belt around M dwarfs.

    • wdk June 2, 2021, 12:20

      Reviewing photolysis, I notice that the process is very dependent on
      UV radiation. And when you compare M dwarfs with G2V stars, the origin of the UV < 320 nm probably arrives in different forms:
      For the M dwarf in bursts of flares that depart widely from the star's
      presumed blackbody spectrum vs. the smaller and more sporadic outbursts of a G star which also has significantly more UV in its
      black body spectral profile. Probably worth pondering further…

  • Spaceman May 31, 2021, 17:37


    1. How much more common, based on your estimates, might continuously habitable planets be compared to Tyrrel’s numbers?

    2. Do you think the lack of evidence thus far for a shadow biosphere on Earth suggests that abiogenesis is an unlikely process?

    3. If abiogenesis occurs fairly commonly on earth-like planets, do you think the biochemical basis of that life is likely to be DNA/RNA/proteins with an identical genetic code to Earth? Or, do you think abiogenesis on another earth-like planet will be more likely to use another information polymer with proteins albeit using a different set of amino acids from life on earth? I see this as a battle of two likelihood’s—what is more likely, chemical evolution converging on the exact same biochemistry on different worlds based on some hypothetical “thermodynamic preference”, or, given that there are potentially millions of DNA-like polymers, is it more likely that one of these would be “picked” on another world?

    • Alex Tolley May 31, 2021, 20:17

      All interesting questions. However, I cannot answer any of them. What we need is data, and it looks like we can only get it through observation of exoplanets. I hope we discover an unambiguous biosignature in my lifetime, but it is possible we may not, either due to lack of technology or that none show unambiguous signs of life.

      Our best chance to find other DNAs is by finding life within reach of our probes. Life in the subsurface of Mars, or the icy moons of the gas giants, perhaps in an interstellar object entering our system. It isn’t just the DNA, but the whole biology that would be interesting – information storage, metabolism, biochemistry, etc, etc, etc. Any alien biology would be a treasure trove for study and inform us of possibilities of life elsewhere. Just one example would ensure that we know terrestrial abiogenesis was not unique. If we found alien life on any other body in the solar system, this would radically shift the odds in favor of life being common in the universe. If the life proved identical to terrestrial biology, then that might increase the odds for panspermia, and again increase the odds for life being common. Conversely, if the solar system is sterile apart from Earth, it tells us little about life elsewhere.

      Just as Douglas Adam’s “Deep Thought” computer allowed philosophers to argue over the meaning of life, the universe, and everything for millions of years, any lack of detection of alien life will allow humans to debate the Fermi Question until we do. ;)

      • Patrick King June 2, 2021, 21:46

        I think that we can take an educated guess at point 3. Given the large space of possible genetic codes, it’s difficult to imagine that an alien tree of life would evolve to have a code identical to ours. The genetic code is almost universal on Earth, but we also have lots of examples of small variations, including the use of amino acids outside of the common 20 (https://en.wikipedia.org/wiki/List_of_genetic_codes). If each of these variants can happen without destroying that organism’s evolutionary fitness, why couldn’t we discover a genetic code that swaps all of the terrestrial amino acids around to different codons, or that includes other amino acids with similar chemical properties to do similar jobs to ours?

        Given an alien biology built on DNA/RNA/amino acids, it seems like it would be an astonishing coincidence if it was identical to ours in its details. But whether a different basic biochemistry with different polymers in each of those roles is possible or likely, I agree that it’s anyone’s guess.

  • Robin Datta May 31, 2021, 22:31

    The right materiel at the right place (with the right environment) at the right time may be what’s needed:
    Clay templates in abiogenesis of RNA and DNA – Google Search.

    Though a long way from a Shakespeare, an Einstein, a Christ or a Buddha, it might lead to a creature weirder than human imagination might conjure up, and with an intelligence as far removed from us as we are from fruit flies.

    • Alex Tolley June 1, 2021, 9:39

      Clay minerals as a template for abiogenesis is now an old idea first suggested by Graham Cairns-Smith. A pop-sci article by the BBC: (with a misleading title) The idea that life began as a clay crystals is 50 years old..
      There remains interest in the idea that pre-cellular life may have started on mineral surfaces. That there are so many hypotheses shows how much in the dark we are concerning abiogenesis.

  • Gary Wilson June 1, 2021, 11:42

    It’s a brilliant topic to discuss on here. Thank you for the work it took to produce it Alex (and the work done by Dr. Tyrrel). We have a long way to go but that’s how all science progresses. If I was a betting man I would bet life is abundant in the universe at the single celled level and more and more rare as complexity increases.

  • Gary Wilson June 4, 2021, 17:01

    As an aside I have been continuing to watch the coverage of the UFO (or UAP) footage released recently by the US. Several people who seem very intelligent and highly trained have stated that they do not believe this is alien hardware. But when asked what the pictures are of they say they have no idea. So why rule out one possibility if you have no idea? I think there would be a list of possibilities that we are capable of coming up with including alien devices of some kind. Surely leave the question as open as possible?

    • Alex Tolley June 5, 2021, 7:23

      Surely leave the question as open as possible?

      As long as the probabilities are clearly understood and not assumed to be a false equivalent:

      1. Natural phenomenon, sensor error, misperception, or human technology – likelihood: extremely high.
      2. Alien craft or technology -likelihood: extremely low (p<<<0.01?).

      The Initiative and Institute for Interstellar Studies' magazine "Principium" (#32) has an article criticizing Loeb over his claim that it is an interstellar craft. The article includes a statement excoriating Loeb:

      A more destructive criticism of Loeb and his intercession here is by Etan Siegel in Science, Jan 28 2021 [1]. He sees Loeb on a personal publicity campaign. “For the better part of the past four years, Harvard astronomer Avi Loeb has appeared all over the media to gather public support for an idea that absolutely defies the scientific evidence.” Siegel points out that Loeb is connected to the Breakthrough Starshot project,
      and complains that Loeb has been bombarding journalists for the last 4 years with attempts to get his ideas circulated, while claiming he has no personal wish for fame or recognition. Siegel sums his view up with:
      “Loeb…has written papers with his postdocs and students insisting that ‘Oumuamua is just as likely to be an alien spacecraft (that looks suspiciously like a light-sail) as it is to be one of the expected
      ~10^25 naturally occurring objects in our own galaxy. Despite the fact that the spectral signatures of the object — its color, reflectivity, size, etc. — are consistent with a natural origin, Loeb offers only loud,
      immodest speculation about aliens and diatribes about community groupthink.”

      The article finishes with:

      Of course there may be no bad thing about a bit of speculation in science and society but the question is how much are we promulgating bad science by producing unfounded speculation where there is a fairly prosaic explanation for something. If we see a corn circle in a field do we still consider it might be made by aliens even after the people who made it tell us they did it? If we can explain the ticks on a roll of chart paper by the signals from a pulsar should we still be saying “of course you never know – it may be Little Green Men”?

      As for the US Navy videos on UAPs, and the non-judgment over what has been shown, it is recommended to read debunker Mike West’s analysis of the published famous videos. Start by Googling “Mike West debunks UAP”.

  • Gary Wilson June 7, 2021, 16:04

    Thanks for the information Alex. I definitely would not try to assume anything about assigning any particular probability for any of the possible causes (and there are probably many more possibles). I just assume I don’t know what they are but it’s worth thinking about what the UAP’s could be. I tried looking up Mike West but none of his comments seemed to be available without payment.

  • Geoffrey Hillend June 7, 2021, 16:17

    Alex Tolley, I looked at Mike West debunks UAP and anyone with any common sense can easily debunk his attempt to debunk the Navy night vision triangle UAP film. Mike West claims that one can use a camera with a triangular iris or tape the front lens of the camera in the shape of a triangle, so when we see stars they look triangular. The problem with that he made the mistake of attempting to debunk the Navy film before the radar data became available to the general public. Also can we see stars that bright through the fog? The objects appear to be moving, completely, self luminous triangles. The whole triangle flashes light. A jet airliner does not look like a luminous triangle under any atmospheric conditions. It does not make any sense for the NAVY or any of it’s officers or even anyone with only an OR-1 rank on active duty to fake a UAP sighting as if anyone found out, there would be disciplinary action.

    Quote by Alex Tolley: “1. Natural phenomenon, sensor error, misperception, or human technology – likelihood: extremely high.”

    Senor error must be false. The triangular objects were only hundreds of feet above the observer’s head and radar and infra red are infallible at that range. Radar can have phantom blips or echo’s from the side lobe effect, but these only matter when an object very far away and visibly unobservable. Phantom echo’s are also very predictable and calculatable based on radar wavelength or frequency which is EMR subject to the inverse square law of energy loss decreasing over distance. Technology like Radar and infra red is pretty much infallible so there is no “noise” at that range.

    UFO or UAP’s should also be attempted to be bunked only case by case which is how it was done in Project Blue Book which is not what Mike West article does and it is unscientific.

    • Ron S. June 8, 2021, 9:41

      I find comments about radar like those above to be a little disturbing. I wonder is those who make such facile statements about radar know anything about the matter. I know something. As a matter of course I have had to pore through the raw data (after RF down conversion) from radar radio systems, as part of doing the signals analysis (DSP) and interpretation (Bayes, pattern matching) of the data to glean actionable results.

      There is a simplistic understanding out there that radar somehow almost magically delivers range and direction of real objects. Not so. These results need to dragged kicking and screaming out of ambiguous, noisy, overlapping signals and more to build something useful.

      For example, the raw data is confounded by switching transients, exogenous RF, FM due to acceleration (Bernoulli functions, anyone?), RF images due to down conversion, sampling and other processing, near field reflections and antenna pattern complexities, FFT and windowing “noise”, RF noise, ADC noise, more noise, noise, noise, and more.

      Radar is not simple, and it is far from infallible. Professional and military users of radar know this, usually. However there can be a lot of bureaucratic blindness to its intricacies when it is convenient to those involved. Law courts, for a different example, have had fun just dealing with police traffic radar, which is even more fraught due to its greater simplicity and often poor handling.

      How this applies to the present UAP situation? I don’t really know, of course. All I can say is to be very careful before making sweeping statements and using words like “infallible”. With that said, I will again move to the sidelines on this topic.

    • Alex Tolley June 8, 2021, 10:31

      This explanation for the “pyramid” craft near LA seems pretty convincing to me. Certainly more convincing than that it is some sort of advanced technology (i.e. alien) craft.

      “Pyramid UFO” – NEW FOOTAGE. It’s Just Bokeh, not a Pyramid

      I think it is also worth watching David Kipping’s interview with Mick West. West provides good explanations of the various video footages. If the explanations are wrong, the US Navy could easily explain why, but AFAIK, they have not.

      As expected, the NY Times’ early view of the “much anticipated” Pentagon report indicates that the meme “might be the craft of unknown origin” is still in play, although why IDK (it couldn’t possibly be for more funding, could it?).

      I also recommend picking up and reading the odd Skeptic and Skeptical Inquirer magazines. The articles help sharpen one’s own critical thinking skills.

  • Geoffrey Hillend June 8, 2021, 17:05

    I am not sure who first said those Navy triangle UAP’s were pyramids. I never thought they were. They are clearly triangular. The problem I have is believing what Mike West says about using a triangular iris or triangular shaped front lens will make stars look like triangles in a photograph or digital video film. Stars are only points of light that remain the same even with a strong zoom because stars are so far away. The whole idea is phony and wrong. Why make up some unscientific idea like that? It looks like he used a computer graphics program to make the stars look like triangles. If the Navy wanted to fake a video they would use state of the art graphics or even photoshop which would be a much more believable attempt to debunk UAP’s the triangle shaped, lens and iris idea. There is no reason for them to do that as they are responsible for our defense and security.

    Military ship radar use thousands of watts of power, so there will be no inaccuracy if an object or UAP is only 500 feet above ones head. Even a hand held police radar gun can easily pick up something at that distance and I recall reading they are only at maximum 100 milliwatts power output. I don’t see any way to argue that. My point with the radar lobes and ghost echo’s do not matter when the object is literally right in front of the radar dish, so 500 feet is unambiguous range for a powerful navy radar and one can easily see the target with the naked eye as well as get a radar return. The signal strength and time the radar takes to return give the location. See Radar signal characteristics Wiki and side lobe Wiki.

Leave a Comment