If we ever thought it would be easy to tell whether a planet was ‘habitable’ or not, Stephen Dole quickly put the idea to rest when he considered all the factors involved in his study Habitable Planets for Man (1964). In this second part of his essay on habitability, Dave Moore returns to Dole’s work and weighs these factors in light of our present knowledge. What I particularly appreciate about this essay in addition to Dave’s numerous insights is the fact that he has brought Dole’s work back into focus. The original Habitable Planets for Man was a key factor in firing my interest in writing about interstellar issues. And Centauri Dreams reader Mark Olson has just let me know that Dole appears as a major character in a novel by Harry Turtledove called Three Miles Down. It’s now in my reading stack.
by Dave Moore
In Part I of this essay, I listed the requirements for human habitability in Stephen Dole’s report, Habitable Planets for Man. Now I’ll go over what we’ve subsequently learned and see how this has changed our perspective.
Dole, in calculating the likelihood of a star having a habitable planet, produced his own ‘Drake equation.’
Image: Dole’s ‘Drake Equation.’
Dole assigns the following probabilities to his equation: PHP=Nsub>S Pp Pi PD PM Pe PB PR PA PL:
Pp = 1.0, Pi = 0.81, PM = 0.19, Pe = 0.94, PR = 0.9, PL = 1.0, PB = 0.95 for a star taken at random, 1.0 if there is no interference with the other star in a binary system. He calculates that for stars around solar mass there is a 5.4% chance of having a habitable planet.
I’ll only summarize his calculations as this is not the primary thrust of this essay. Some of his estimates such as Pp = 1.0, the number of stars with planets, have held up well. Others need adjusting, but by far the biggest factors that determine the likelihood of a planet being habitable for humans are those he didn’t consider in depth.
Since Dole’s report, we’ve learned a lot more about the carbonate-silicate cycle and atmospheric circulation. The carbonate-silicate cycle provides a stronger negative feedback loop over a wider range of insolation than thought at the time of his report. Atmospheric and oceanic heat transport have been shown to work more efficiently also. This leads to a more positive assessment to the range of habitability. Planets with high axial tilts and eccentricities, which Dole had excluded, are now considered potentially habitable; and more importantly, there’s the possibility that tidally-locked planets around M-dwarf stars may be habitable. M-dwarf stars being the most common in the galaxy, this makes a big difference to the number of potentially habitable planets. Nsub>S, the mass range of stars, is now opened up. Pi, the range of inclination, is probably 1.0, and PD, the probability that there is a planet in the habitable zone, which he gave as 0.63 and is still a good estimate, is now extended to M dwarfs. And given that tidally locked planets are no longer excluded, PR, the rate of rotation is not a limiting factor.
On PM, Dole’s assumptions for the size of a habitable Earth-like world have held up well. His calculations on atmospheric retention and escape conclude that planets between 0.4 Earth mass and 2.35 Earth mass could be Earth-like. Planets below 0.4 Earth mass would lose their atmospheres. Planets above 2.35 Earth mass would retain their primordial Hydrogen and Helium atmospheres and become what we now call Hycean planets or Super-Earths.
This gives a range of surface gravities, assuming a composition similar to Earth’s, of between 0.68 and 1.5 G, which would mean from a gravitational perspective most of the range is within what humanity could handle. Dole puts the upper limit at 1.25 G based on mobility measurements made in centrifuges from that time. I would agree with him even though there are a lot of people walking around today with one and a half times their ideal weight. The limiting factor for high G is heart failure at an early age, a condition extremely tall people here on Earth suffer from. If you are a six-foot person on a 1.5 G world, your heart is pumping blood equivalent to that of a nine-foot person. In this case, people of short stature have a distinct advantage. A five-foot person would have the blood pressure equivalent of being seven foot six on a 1.5 G world and six foot three on a 1.25 G world.
However, when it comes to the frequency of Earth-sized worlds in the habitable zone, Dole’s guess at PM = 0.19 is probably too high even when we now include tidally-locked planets around red-dwarf stars. He, like the rest of us until recently, had no clue that sub-Neptunes and super-Earths would be the most frequently-sized planets in the habitable zone of a roughly Sol-mass star.
From our observations, Dole’s guess on orbital eccentricity, Pe, looks like it’s in the ballpark, again due to the inclusion of red-dwarf stars with their tidally circularized orbits. With a lot of these factors, though, slight changes in probability do not make a big difference in the frequency of habitable planets. The big differences come from those he didn’t consider.
Dole noted that water coverage on a planet could determine its habitability. He did not go over this in any detail, however, mainly I suspect because he had no information to go on. He didn’t include a term for it in his calculations. But, we do know from density determinations of transiting Earth-sized planets that there’s a significant possibility that a large percentage of them may be excluded due to being covered by deep oceans. This would mean, even if they had breathable atmospheres, they would not meet Dole’s criteria for habitability.
While Dole went carefully over the range of breathable atmospheres humans could tolerate, he essentially assigned a probability of 1.0 to the formation of this atmosphere once life appears on the planet, PL, and sufficient time has passed, PA, to which he arbitrarily assigns a period of 3 billion years. He made no consideration of how likely it would be for this process to go off the rails.
Yet, if you consider the range of possible atmospheric compositions and pressures on Earth-like planets, those that meet the requirements of human habitability are narrow. This is the one factor that is most likely to winnow the field with the possible exception of average water composition.
When considering what percentage of Earth-like planets could have a breathable atmosphere: Oxygen between 100 and 400 millibars, Nitrogen less than 2.3 bar, CO2 less than 10 millibars, and no poisonous gasses, we are helped by a natural connection of these parameters. Oxygen destroys most poisonous gasses. The Carbonate-Silicate cycle will draw down CO2 to low levels. With Nitrogen we note that Venus has 3 bars of Nitrogen. Earth has a similar stock, but most of it is either dissolved in the oceans or mineralized as nitrates. Mars still has a 2.6% by volume trace of its primordial Nitrogen atmosphere. This points to a certain consistency for terrestrial planets with regard to their Nitrogen stock; however, Oxygen to Nitrogen ratios do vary from star to star. Getting the level of Oxygen within breathable parameters is more problematic, though. It’s a reactive gas that disappears with time. I can see two possible pathways that can lead to a breathable atmosphere, one abiotic and one biotic.
On the abiotic front, there’s a robust mechanism available for generating Oxygen. If the planet is warm enough to have significant quantities of water vapor in the upper atmosphere or has a steam atmosphere, then photolysis and subsequent Hydrogen escape will result in the build-up of Oxygen.
Planets less massive than the Earth-like range lose their atmospheres. Planets more massive retain their primordial Hydrogen, which means any Oxygen resulting from photolysis will recombine to form water. Intermediate-sized planets, however, can build up Oxygen via Hydrogen escape.
How much it builds up depends on the balance of production and removal. The amount produced depends on stratospheric water vapor and UV levels. The rate of removal is determined by three main processes: Oxygen escape, which is dependent on planetary mass, magnetic field strength and the strength of plasma wind from its primary; chemical reaction with reducing gasses, which is proportional to the level of volcanic emissions; and the oxidation of exposed regolith due to volcanism and weathering, the first being proportional to the level of volcanism and the second being proportional to the planet’s temperature.
Abiotic Oxygen atmospheres are probably transitory in nature over geological time periods, but I do see sufficient Oxygen being generated at various stages in an Earth-like planet’s history. The first is from the time when a planet’s red-dwarf primary is sliding down its Hayashi track towards its position on the main sequence. Due to the star’s greater luminosity at this time, an Earth-like planet destined for the habitable zone will spend 100 million to a billion years with a steam atmosphere. Models of this process indicate it could lose up to several Earth oceans of water through photolysis and Hydrogen loss. The loss of an Earth ocean translates into roughly 300 bar of Oxygen, most of which, as with Venus, will finish up oxidizing the crust. If, however, the various factors balance out, so that when the planet’s steam atmosphere condenses as the star arrives at its main sequence position, the water fraction is sufficient to provide both oceans and continents, and the Oxygen production and removal hove balanced out to produce a breathable but non-toxic level of Oxygen, then we should get a habitable planet, albeit one with a highly oxidizing surface chemistry like Mars.
If this all sounds highly unlikely, you are probably right, but there are a lot of red dwarf stars in our galaxy.
Image: Artist’s impression of the ultracool dwarf star TRAPPIST-1 from the surface of one of its planets. We’re beginning to learn whether the inner worlds here have atmospheres, but will we find that any of the seven are habitable? Credit: ESO.
Oxygen generation through photolysis occurs anytime an Earth-like planet has a high level of water loss. Mars is thought to have lost an ocean of water corresponding to 1.4% of Earth’s ocean early in its history, which translates into a total partial pressure of 4.2 bar of Oxygen (under 1 G.) This Oxygen generation would have occurred over a long period, so the partial pressure at any given time was probably low; but you’ll notice that the mineralogy of Mars from around 4 billion years ago is highly oxidizing whereas Earth’s surface didn’t become oxidizing until 2.2 billion years ago.
Also an Earth-like planet suffering from runaway greenhouse such as Venus did two billion odd years ago would also experience a build-up in Oxygen.
If the presence of life in the galaxy is sparse, then this mechanism may result in more planets having Oxygen in their atmospheres than those that get it through biotic means, so Oxygen lines in the spectra of a planet’s atmosphere would not be a good indication that it harbors life.
We are familiar through descriptions of the history of life on how the biotic process leads to a breathable atmosphere. This has implications, however. To frame this, I’ll use a model in which planets become habitable at the rate of one per million stars starting nine billion years ago. (The figure I selected is arbitrary. You are welcome to adjust it and see what sort of results you get.) Given that star formation in our galaxy is about one star per year (star formation rates have varied over time but an average of one per year will suffice for this model), this will result in the total of 9000 planets that will be habitable to humans at some point in their lifetime. There may well be many more life-bearing planets than this, but this model is only interested in the ones that become habitable to humans.
If we assume these planets have a similar evolutionary track to Earth, then the youngest 5% of these will be at the prebiotic stage. Until about 2.2 billion years ago Earth was dominated by anaerobic life, so the next 20% will have anaerobic atmospheres full of toxic gasses. Hydrogen Sulfide in particular is lethal, killing at 1000 ppm. Intrepid explorers will have to live in sealed habitats with airlocks and go around on the surface in spacesuits. Does this meet your definition of habitable?
About 2.2 billion years ago on Earth, photosynthetic aerobes got the upper hand in Earth’s chemistry and the surface became oxidized with an atmosphere of 1-2% of oxygen. If their timeline is similar to Earth’s, then 20% of these planets would fit this condition.
These planets would be a far more pleasant place to explore. Toxic gasses would be removed by the Oxygen. You could probably go around with just an oxygen concentrator on your back feeding a tube to your nose. Habitats wouldn’t need airlocks; double doors would do. How would you classify these planets?
Then 500 million years ago Earth became fully habitable when the Oxygen concentration crossed 15% and the air became breathable. This period represents 5% of the sample. However, there’s a side effect to this. Oxygen is not very soluble in water and O2 concentrations fall off rapidly with distance. This is why the macroscopic lifeforms from the Pre-Cambrian age (>500 mya) were either flat leaf-like shapes or sponges, both of which give short diffusion distances throughout the organism. Once the oxygen concentration rose, however, lifeforms could develop thickness, and with thickness, they could develop organs such as hearts and circulatory systems, which could then circulate an oxygenated fluid throughout their bodies. A breathable atmosphere allows for the development of complex macroscopic life.
And, over time, complex macroscopic life gives rise to the second side effect of breathable Oxygen levels – sapience. This has often been considered a rare possibility, a fortuitous combination of circumstance, and in the Drake equation it is assigned a low fractional value, but the idea that intelligent life is rare and unique derives from our historical and religious concept that mankind is something unique and apart from the animal kingdom. However, studies show a steady increase in encephalization over time and its widespread occurrence in different phyla and classes: octopi in the mollusks, parrots and corvids in the birds, and dolphins, elephants and apes in the mammals.
Varying levels of communication signaling have been found in numerous species. Just recently, a troop of Chimpanzees has been found to have a 390-word vocabulary constructed by combining grunts and chirps in various sequences. It therefore seems that our ability with language is merely a development of existing trends rather than something that came out of nowhere. And language is the abstract representation of an object or action, so the manipulation of language leads to abstract reasoning.
Encephalization is a tradeoff between the energy consumption of neurons and the benefits they produce in reproductive fitness. Increasing the number of neurons in an organism is easy. A simple mutation in the precursor cells allowing them to divide one more time will do this; however, organizing those extra neurons into something useful enough to justify their extra metabolic cost is a lot more difficult. But increases in neural complexity can lead to more complex behaviors, which can increase fitness or allow the creature to colonize new niches. In addition, neurons, over time, have evolved to become more efficient. Moore’s law operates, but with a doubling time on the order of 100 million years. Parrots’ neurons are both smaller and three times more energy efficient than human ones. So, not only does encephalization increase with time, but the tradeoff moves in its favor. However, like any increases in biological complexity and sophistication, this does take time.
This points to the conclusion that on planets habitable to humans, the evolution of sentience is not so much a case of if, but when.
An atmosphere breathable to humans is also flammable over most of its range, so a good proportion of these sapients would have access to fire allowing smelting technology to develop. What the model I used implies is that 50% of habitable planets will by now have had intelligent life forms evolve on them, a majority of which could develop technology.
I would support this argument by applying the Law of Universality that states that no matter where you are in the universe the laws of nature operate in the same way. This means that a planet like Earth would produce intelligent life forms. There is a certain contingent element in evolution, so the timing and the resulting life forms would not be identical; however, the broad driving forces of evolution would produce something similar. This can be seen in the many cases of convergent evolution that have occurred on Earth. How different from Earth a planet has to be before it stops producing intelligent life forms is a matter of conjecture, but if these changes cripple the evolution of intelligent lifeforms, there’s a good chance they cripple the formation of a breathable atmosphere.
What these intelligent life forms would do to their planet over the eons is a matter of speculation, but if for some reason intelligent life did not arise, then complex life could thrive and the planet would be habitable for another billion years or more – depending on the star’s spectral type – before the star’s increasing luminosity sets off a runaway greenhouse. This means that of the planets that are habitable for humans at some stage in their life approximately 15-25% will be habitable at any given time. (The upper bound assumes that there are a high proportion of them around lower mass stars with longer lifetimes.)
If, however, intelligent life develops on planets as a matter of course, then the model indicates that for every habitable planet we have now (5% of the total) approximately ten planets had intelligent lifeforms at some stage in their history (50% of the total.) And if intelligent life is a side effect of habitability, then there will be a correlation between the number of habitable planets and the number of exosolar technological civilizations in our galaxy. So, in an inversion of the usual order of things, we can estimate the number of planets habitable for humans from the number of alien civilizations in the galaxy. The model I’ve been using points to them being within an order of magnitude of each other.
Adding in the fact that we have no information on the evolution of intelligent life on non-habitable planets, then calculating the number of habitable planets from evidence of alien civilizations is an upper bound. On the other side of the scales, there’s the number of planets that are habitable through abiotic means. Planetary atmospheric spectra within the next couple of decades may give us some indication of this. If, however, we use Hanson’s estimate where he deduces that from the lack of evidence of alien civilizations in our galaxy that the number of technological life forms is just one – us – then this would also point to the number of habitable planets in our galaxy being just one: Earth.
As a final point I would like to add that while I have not done a full literature search, I have read widely in this field and have not come across as rigorous consideration as Dole’s work on defining habitability for humans and considering the likelihood of finding planets that match that criterion. The field’s general mindset seems to focus on finding the conditions upon which life arises; then it just assumes evolution will automatically lead to a habitable planet for humans. We have learned a lot since Dole wrote his paper, but there does not seem to have been much reexamination of the topic. It is perhaps time we applied our minds to it.
References
Stephen Dole, Habitable Planets For Man, The Rand Corporation, R414-R
https://www.rand.org/content/dam/rand/pubs/reports/2005/R414.pdf
Dave Moore, “’If Loud Aliens Explain Human Earliness, Quiet Aliens Are Also Rare’: A review”
https://centauri-dreams.org/2022/05/20/if-loud-aliens-explain-human-earliness-quiet-aliens-are-also-rare-a-review/
Robin Hanson, Daniel Martin, Calvin McCarter, Jonathan Paulson, “If Loud Aliens Explain Human Earliness, Quiet Aliens Are Also Rare,” The Astrophysical Journal, 922, (2) (2021)
Such equations embody top-down thinking. If we make the assumption that all these conditional probabilities are statistically independent — not because it is plausible, but because it is easy — then we have something to write down which gives the illusion of progress toward understanding the chance of alien life. All that’s left is for some technicians to tidy up pesky small details like the value of P sub L. I think of students teaching classes or tending to cancer patients while their betters earn vastly more flying to conferences to do career networking. Top-down certainly sells to those few with the money to buy.
But I doubt life will really be understood this way, because life emerges from the details. Oxygen poisoned the atmosphere and oceans, and life became more energetic. Earth froze over, and animals diversified. Catastrophic events destroyed most synapsids, then most sauropsids, and each time the ecosystem came back stronger. An Ice Age destabilized the planet, and sentience evolved. From an administrative standpoint, the emergence of humanity resulted from a long series of events that should have been prevented to help life flourish.
If a planet is the wrong distance from the star, synchronous rotation might allow life to form anyway. If the mass is too high it might develop some other sort of life, or have moons that can evade tidal locking. Maybe if the eccentricity is high it could retain an atmosphere longer and host life forms better able to survive by prolonged hibernation. If a planet flunks several of these tests, I’m skeptical whether anyone really knows if it is less or more likely to pass the final exam.
When I wrote up a review of Prof. Terrell’s work on simulating randomness on planets and how that could result in very few planets remaining habitable, what was evident was that the large perturbations and runaway heating/freezing conditions were those that caused the problem. Small events and negative feedback loops did not seem to cause planets to become permanently uninhabitable.
In essence, I think you are suggesting that perturbations large enough to cause mass extinctions, but insufficient to kill all life, maybe a necessary ingredient to drive evolution off fitness plateaux and onto the next peak/plateau.
However, while this is an interesting idea that has been shown in simulated genetic algorithms, is this key for early terrestrial life?
For example, as we don’t know how or where the successful abiogenesis occurred, we don’t know what conditions allowed it. Similarly, the “accident” that created eukaryotes from prokaryotes seems to have been a singular [!] for the incorporation of cyanobacteria, followed by a similar one that created mitochondria. During the Cambrian, the emergence of the vertebrate body plan and its survival could easily not have happened and this would have ended the whole major line of vertebrate evolution that eventually gave rise to us today. Was there some environmental change that ensured the vertebrate line succeeded and radiate, despite the very successful invertebrate phyla?
While catastrophes and mass extinctions eventually produced humans, we may be looking at evolution from the perspective of survivorship bias. Rerunning evolution might never recreate vertebrates (or analogs). Could invertebrates, ever gain high intelligence? IDK.
Going back to the major early events preceding complex life, were these extraordinarily improbable events unlikely to be repeated, or were they inevitable once the improbable event had been attempted a vast number of times until the probability of eventual success was 1.0?
OTOH, the mass extinction events had to be large enough to remove who species and genera, yet not enough to remove all life, and certainly not all vertebrates. If the Permian extinction had killed off all vertebrates, that would have been game over for that route to intelligence.
Finally, as for human intelligence and why it seems to have rapidly developed, there are a lot of explanations that seem like rather sophisticated “just so” stories. One would think that our own cultural impacts would stimulate evolution too, with greater complexity driving intelligence gains, albeit offset by technologies to offload that cognitive effort. Physically, larger brains and skulls would have to develop post-birth, or only be possible in artificial wombs. Have we reached an intelligence plateau that can only be exceeded by artificial means?
You’re right that there is so much we don’t know. I can’t say whether Earth’s random walk of adversity was more likely to have ended up as badly as Venus. We can’t even know that Earth isn’t an event that happens once in a googol of universes, conjured from the aether by the strong anthropic principle. Maybe tomorrow one of the Voyagers will pierce some holographic curtain, and we’ll learn that our local nature preserve has been carefully managed to promote eco-tourism the whole time.
It does seem that life depends on adversity even in a ten-year evolutionary scale: https://phys.org/news/2023-07-safe-havens-major-drawback.html Animals can quickly become, well, “dodoized” in the absence of predators. Whether there is some higher order analogue of this applying to mass extinctions, I have no idea.
I don’t think human intelligence has reached any fundamental limit. People made that assumption in the past, assuming that brain size would strictly limit the intelligence of women and certain minorities – but today we live atop a mountain of evidence that it was never true. Novelty genetic surveillance apps can tell you which finger you’ll put on top when you clasp your hands together, but none of them can tell you whether you will pass your calculus class. I expect the explanation for big heads and excruciating childbirths is just that the human species is newly evolved. Fast-evolving genes like ARHGAP11B look like true Goldschmidtian “hopeful monsters”, loaded up with pleiotropic implications and a generous helping of buyer’s remorse.
{In essence, I think you are suggesting that perturbations large enough to cause mass extinctions, but insufficient to kill all life, maybe a necessary ingredient to drive evolution off fitness plateaux and onto the next peak/plateau.}
Evolution can be, even in macroscopic animals, very fast—major changes in the order of tens of thousands of years if there are open ecological niches to spread into—however, in a stable ecosystem, species reach a point where any divergence from the current form is less optimal and so the forces of evolution keep the species static. So some disruption is required.
To reach the level of complexity of humans, there is probably some theoretical minimum time. How close we are to that is an interesting question.
{Similarly, the “accident” that created eukaryotes from prokaryotes seems to have been a singular [!] for the incorporation of cyanobacteria, followed by a similar one that created mitochondria.}
I don’t think this was a unique occurrence. From a Scientific American article on algae, I learned there are some species that have double and triple lipid layers around their chloroplasts. This means that a first species engulfed the chloroplast (layer 1), then another organism engulfed the first organism incorporated its genes and eventually reduced it down to its chloroplast (layer 2). The finally, another organism engulfed that organism (layer 3). Also, during algae blooms, certain species of algae suck the chloroplasts out of other species and incorporate them. The chloroplasts only work for a couple of weeks, but that provides a handy boost to the species at a time when rapid growth is needed.
{During the Cambrian, the emergence of the vertebrate body plan and its survival could easily not have happened and this would have ended the whole major line of vertebrate evolution that eventually gave rise to us today.}
I’m pretty sure that if the vertebrates had not colonized the land, the Mollusks would have. They are an extraordinarily diverse phyla ranging from filter feeders (clams) to fast predators (squid) who still give fish a run for their money. And they wouldn’t have to evolve manipulative appendages like we did.
An interesting paper describing many examples of endosymbiosis.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2817226/
Endosymbiosis looks like a general tool available to life rather than a fluke.
Just learned something new today.
“Parrots’ neurons are both smaller and three times more energy efficient than human ones”
In which case the next leg up the human intelligence plateau, could be genetic engineering with parrot DNA. We won’t ever be able to use “bird rain” as an insult ever again!
While astronomers use the term habitable to mean that given the right conditions, surface water can exist and with it, life is possible, Dole, as per the title of his report, means habitable for humans.
The report, therefore, is looking to determine “Earth 2.0”, the sort of planet found in science fiction and very much in our galaxy written about in Asimov’s stories, and still the hope for escape to like some secular heaven after we have trashed this world.
Human habitable planets are clearly a subset of habitable planets and were acknowledged in sci-fi once the idea of terraforming was introduced to expand the subset.
The idea of sterile planets with O2 atmospheres due to water photolysis and hydrogen escape as being suitable for human habitation is likely wishful thinking for those of us with great concern about a “Prime Directive”. It might be convenient to have a sterile planet in some Goldilocks” condition of atmospheric pressure with a safe N2/O2 atmosphere, but it will require reworking with terrestrial life to make it truly habitable for humans, i.e. terraforming will be needed, which probably cannot be created by some Star Trek Genesis Device.
Andrew Page commented in the last post about “biocompatible” as another issue. Terrestrial life is only poisonous to humans because of metabolic pathways producing toxins. The organisms themselves rid of these toxins are entirely edible, as blowfish connoisseurs are aware. But on exoplanets, there is no obvious requirement that the core biology will be compatible with ours. Different nucleic acid bases, and different amino acids, could make such organisms problematic. While this may result in the separation of our respective biospheres, it would mean a constant competition for resources that would be needed by our transplanted agriculture, effectively requiring local or eventually global terraforming.
Depressing as it is, your conclusion may not be far wrong, (although I discount the issue of technological civilization as evidence), that there may be only one human habitable planet in the galaxy – Earth (and therefore no Earth 2.0). This supports my view that machine civilizations would be the dominant form of “life” in the galaxy. But that is another story.
From a colonisation point of view, planets that are “Earth like” but are sterile or have only a primitive ecology sound like much better candidates for terraforming than Mars.
Imagine an Earth like planet a bit further from its sun. Its snowball Earth phase might have lasted hundreds of millions or billions of years. Only to start melting as its sun got brighter towards the end of its life. Even if that planet had only a few tens of millions of years left as a habitable planet, it could be an ideal candidate to make “Earth 2.0”
Thanks for bringing back the Dole analysis for further examination. Perhaps as we look at his initial survey it will help to get our current bearings. The Drake equation has served as a slide rule of a sort – but it looks to me like it has often been re-tasked or re-calibrated.
In the majority of cases where I have encountered the invocation of the Drake equation, it would appear in the context of interstellar communication and the
likelihood of a “civilization” that would employ some sort of E-M signal to conduct the operation – or contact us. Consequently, when the various P factors were identified, they were improvised to culminate in an expression that might validate continued vigilance with radio telescopes – such as the Green Bank.
This is not necessarily a bad idea or a good one – just a way of making a case.
But when one examines the P factors provided by Stephen Dole or others – there is a great deal of room for artistic license or adaptation for the point that one wants to make. Sometimes the factors introduced reflect more on our own thinking than conditions around the galaxy. After all, when Dole provided this formula, the number of known exoplanets was zero.
In principle and in practice, the Drake equation has been used for estimating the likelihood of an off-world civilization capable of interstellar communication in our neighborhood – and the probability of life as the broader use with fewer P factors. That is, a series of factors is truncated before you get to communication likelihood.
Truncation depends on whether it suffices to predict civilization, intelligence, life… Additional breakdowns are possible. How many factors should be erased between communication and life is hard to say. Depending on definition, they could comprise many additional “subfactors” dependent on our ability to distinguish developments. Many environmental or cultural probability issues not included could be complete filters (e.g., not developing a language or symbol system for communication but still “intelligent” – interstellar dolphins for lack of a better example).
On the other hand, Dole’s inclusion of definite planetary eccentricity or obliquity limits on life or intelligence strikes me as hasty. In addition, Dole’s write off of all M dwarfs could be considered the same, unless we assume Trappist-1 b is an adequate sampling. ( In that instance, stellar mass was about 0.1 solar; above he included everything below 0.35).
It’s not that Dole’s survey or limits made all the mistakes and we’ve corrected them. It’s more like he rushed to judgment with limited data and we are still in the same position. Our main difference is that we can confirm the existence of
planets all over the place. Yet we can ascertain very few surface conditions.
When we start to get some atmospheric data on nominally habitable zone planets, we should be able to qualify or quantify habitability better.
Of late, I would like to note that planetary Bond albedo is something to which we should give some consideration. It’s not the full indicator of surface temperature ( e.g., for Venus), but reflectance into space can vary over a planet’s history more than the star’s radiation itself.
I notice that some of the literature coming out assumes HZ exoplanets have Earth’s current Bond albedo. Should their surfaces be all ice, all liquid, mostly land or have a lot of clouds, the result will be different. And it could change over time if those relations change. I would recommend starting from stellar effective temperature at distance ( 400 K for us) and examine the envelope.
Or, to give another illustration, the four Galilean satellites are the same distance from the sun, but have different albedos and different surface temperatures. One cannot simply say, “No fair. Because Jupiter had an influence on them.” The influences of exoplanet environments we are still trying to determine.
What relation does the Bond albedo to (BA) regular (geometric?) albedo (A) have on surface temperature? Naively, it would seem a BA > A would mean a cooler than calculated surface, and hotter if BA < A. How different are the calculated values?
Bond albedo .
Hi, A. T.
Had been thinking about how to present this…
Some of the information I had typed out.
In some cases Bond albedo is adequate to model a planet’s surface temperature. In others, such as Venus, where Greenhouse effects are significant – it’s very misleading. But to assume all planets have the same
Bond albedo as the Earth – or other reflectance measures – that is misleading too. So here is first cut table:
For solar T_STAR = 5800 K at surface ( r = 700,000 km),
T at 1 AU =400 K
Bond Albedo based
T_PL eq = TSOLAR at 1 AU = 400 K * (1/2)0.5 ( 1 – A_B)^1/4
Half of spherical surface illuminated ( hence the 1/2 factor)
A_B T_PL
0 282
0.1 275.5
0.2 267.5
0.3 258.7 (nominal Earth value = 0.306)
0.4 248.9
0.5 237.8
0.6 224.9
0.7 209.3
0.8 189.1
0.9 159.1
0.95 133.7
===
Solar System Body Examples:
Bond Visual Geometric
Albedo Albedo
Mercury 0.088 0.142
Venus 0.76 0.689
Earth 0.306 0.434
Moon 0.11 0.12
Mars 0.25 0.17
Jupiter 0.503 0.538
Saturn 0.342 0.499 ( & RINGS?)
Enceladus ( & Europa?) 0.81
Uranus 0.300 0.488
Neptune 0.290 0.422
Pluto 0.41 0.51
Charon 0.29 0.41
Kuiper Belt Objects
Humea 0.33 0.66
MakeMake 0.74 0.82
Eris 0.99 0.96
Galiean Satellite Estimates
Bond Albedo – Galilean Moons
Io 0.63 +/- 0.02
Europa 0.68 +/- 0.05 (2007)
Ganymede 0.43 +/- 0.02
Callisto 0.22 (geometric)
As can be seen with the planetary Bond albedos, they are not all alike.
And if one were to apply that of Venus to derive surface temperature, it would provide a temperature below the terrrestrial value. But in effect,
Bond albedo just undershoots the meteorological values for the Earth.
Both Earth and Venus have Greenhouse atmospheres, but the latter case is much more effective in trapping heat.
What reflectance Earth has is based on clouds, ice caps and snow, oceans, lakes and the ground surfaces varying from woodlands, deserts and pavements… All of this quite changing in nature over seasons, not to mention geological periods of time.
Curiously, it was difficult to find Bond albedos for the Galilean satellites.
Hence the visual geometric values will be a little off – but still illustrative of
varying reflectance at distance from sun.
So, Nature’s blessing or not, I think that fixing exoplanets with stations in HZs based on Earth’s Bond albedo is misleading and a mistake. Almost all of them so far are detected indirectly ( radial velocity, transit decreases in stellar luminosity and here and there an astrometric effect). Unless it is hot out of the oven, we don’t know how much light they are radiating, reflecting, or in Greenhouse cases, storing.
The following has nothing to do with your main position concerning BA and planetary surface conditions.
Looking at Earth and adding a GHG value to the T for Bond Albedo to adjust for ground truth. Now consider the Snowball Earth condition when the planet was [nearly] completely covered in snow and ice. Using the BA of 0.6, a value closer to Europa, the BA estimate for Earth is now 34C lower. If the average Earth’s temperature is 15C [288K] that puts the Earth in a very frozen state of -19C.
Apart from very brief periods, the Earth’s average temp was 10C than today. So assuming 25C vs 15C today with higher GHGs in te atmosphere, the Earth would still be well below freezing.
That would require a lot more GHG emissions than paleo analyses suggest to push the Earth’s temperature back above freezing.
Obviously, if the Earth’s BA was never doubled, then this invalidates the above BoE calculation, perhaps putting a constraint on the possible BA that is well below that of an icy moon which is certainly possible given below about the albedo of snow:
Wikipedia entry for the albedo of snow: “Snow albedo is highly variable, ranging from as high as 0.9 for freshly fallen snow, to about 0.4 for melting snow, and as low as 0.2 for dirty snow.”
Maybe vulcanism dropped ash on the snow, reducing its albedo and therefore reducing its BA to allow for warming without massive excess GHG emissions.
{Maybe vulcanism dropped ash on the snow, reducing its albedo and therefore reducing its BA to allow for warming without massive excess GHG emissions.}
One a planet goes snowball, there is no more precipitation, but the glaciers will still flow downhill leaving the mountain tops exposed and their fine ground up rock (loess). Wind erosion will spread this dust and, will in certain places, erode the ice exposing the bedrock underneath. From you albedo figures, I can see this would make a strong negative feedback loop.
A. T.,
While mulling A_B this last week or two, I was drawn to accounts of the ice-ball Earth. To summarize, it looked like there were several and two of the most recent were between 700 and 600 million years ago. These preceded the Cambrian period – though I hesitate to say how the Edicaran flora and fauna fit into the story. Probably some overlap.
In any case, the Wiki articles related the concern of many geologists that the ice-ball Earth’s could have locked the planet into that state indefinitely.
It could have been a long time before the ice-sheets became less reflective and the planet warmed up. Mega years anyway. But in the case, of the Earth ( allowing that Ice Ball Earth is not simply a myth or a mix up reading rocky entrails) volcanic emissions ( including CO2, CH4 and other Greenhouse gases) could have come to save the day – as well as a number of biological agents during the Edicaran. And that story gets confusing, considering that simple plants and animals’ contributions to warming up the Earth would vary. But it would appear that it happened a couple of times.
The tables above for Bond albedo ( 0.6 to 0.7) would reduce Earth temperature to 225 to 209 K correspondingly, vs. the 254 K or so we assume. Or, one can say, assumed by using the charts in the Nature article.
But once again, my contention is that placing an exoplanet in a position around a star corresponding to the Earth’s designated 254 K value – the temperature could be quite different based on reflectance. Using that of Mars, it would be significantly warmer, for example.
And then observing that, for both the Earth and Mars, regional surface temperatures would vary.
That kind of begs the question of whether Mars at Earth’s orbit with the same Bond albedo would be more habitable or not. But maybe it’s someone else’s turn.
{When we start to get some atmospheric data on nominally habitable zone planets, we should be able to qualify or quantify habitability better}
I agree, and I’m eagerly looking forward to it.
Habitability does not grant appropriate evelution. Stephen Jay Gould pointed out that replaying evolution’s tape a million times on Earth would not result in another instance of Homo sapiens.
Oxygen-carrying molecules (hemoglobin, hemocyanin) that bind to and dissociate from oxygen at different partial pressures together with a circulatory system to get then to regions of the organism at a remove from the surface allowed expansion in organism size; better gas exchange systems (lungs) augumented this to the point where the largest living vertebrates ever are present-day cetaceans.
Prerequisites for human intelligence include binocular vision and three axes of movement in shoulders, enabling depth perception and accurate use of projectile weapons (spears, arrows) that enabled killing of prey at a distance, the only poor approximation to which is the spitting cobra. Both binocular vision and three-axes shoulder movement arose through brachiation which in turn needed continuous overhead canopy provided by angiosperms (not available with gymnosperms or cycads).
Another important basis of intelligence is speech. Much as they may try to speak, apes and other animals do not have an appropriate vocal apparatus. Even birds that produce intelligible phrases have a distinct artificiality to their vocalizations.
Two major contributors to human vocalization were the control of fire and upright stance. The former occurred between 800,000 and 1,400,000 years ago and led to the cooking of food, rendering it softer, followed by the shrinkage of dentition and the oral and pharyngeal cavities, this improved modulation of sound; upright stance induced an angulation between the oral and pharyngeal cavities that allowed for more varied control of air flow in the posterior oral cavity (soft palate and tongue) and resonance in the nasopharynx.
Spread to a wide variety of different environments across the planet brought social, cultural, anatomical and physiological adaptations. Strict application of correctives for lapses from accepted norms over millennia is now associated with highest IQs among Mongolians and then East Asians. Perhaps Genghis Khan may have had an influence on it.
Octupuses and cetaceans are quite intelligent, but are hampered by a lack of many features found in humans. Yet on other worlds there may be other avenues to intelligence beyond our speculation.
{Another important basis of intelligence is speech. Much as they may try to speak, apes and other animals do not have an appropriate vocal apparatus. Even birds that produce intelligible phrases have a distinct artificiality to their vocalizations.}
This is anthropocentric. Recently they have found a troop of Chimps, that have a 390 word vocabulary mage by combining a limited number of chirps and grunts together. Chimps have great difficulty in speaking human languages and we would have great difficulty speaking this language. Languages are abstract representations of objects and actions. Any arrangement will do. Take sign language.
Behavior patterns in creatures are acquired through the following ways:
Innate: Genetically programmed in.
Learned: There are various types of learning where the creature associates one thing with another (I’m a bit rusty on these.)
Imitation from parent: This is how chimps learn. They copy what their parents do.
Instruction: This is the big advance humans made. Children learn to associate certain abstract collections of sounds with actions and objects. These collections of sounds are then used to impart knowledge to the children. This leads to a massive increase in acquired knowledge, further helped by writing and storage technologies like clay tablets.
What I’m saying here its that human’s abilities, the ones that produced a technological civilization are not unique. They exist in nascent form on other creatures. It’s just that they are scaled up in humans sometimes by immense amounts, and when those scaled abilities are combined with other abilities, it produces a paradigm shift.
While I think the evolution of humans is happenstance, the evolution of intelligence on Earth is not. If we had not arisen, I’m pretty sure that if you came back in 100 million years, you would have found that there was intelligent life on Earth.
Maybe, maybe not. The dinosaurs had far longer than 100 my and they did not have anything like human intelligence. Their avian descendants despite their head start failed to achieve anything like human-level intelligence after 65 million years when their non-avian cousins went extinct.
The apes did not achieve human intelligence. Our lineage did so for some reason, perhaps driven by environmental pressures. There was also possibly strong sex-selection that may have driven the evolution of large brains for cultural reasons. Interestingly, brain-to-body size has declined with civilization, suggesting perhaps that our raw intelligence potential may have declined, offset by our technological cognitive prostheses.
Our rise from hunter-gatherers to “high civilization” has been remarkably quick, a mere cosmic eyeblink, yet we have no idea whether it is a long-term survival trait. We hope it is, but it may not.
It may have been too early. The thing about encephalization is that it is a gradual arms race. Certain predators (not all) use increases in intelligence to improve their ability to catch prey. This puts selection pressure on the prey to become more intelligent to evade capture. This results in a general but gradual selection for increased intelligence over time. This leads to the opening up of heretofore unexploited niches that require a certain level of intelligence. It is interesting that after the KT extinction, mammals, not been predated on by smart dinosaurs got dumber, but then smarter mamalian predators evolved and the average level of intelligence has steadily increased ever since.
If the KT extinction hadn’t occurred, I think that some of the smaller feathered dinosaurs would have reached a high level of intelligence, sufficient for one of them to break out like humans did. It just takes time.
Part of the reason birds are stymied is their small size. The larger a creature is, the easier it is to become intelligent.
A counterfactual: Terror birds
I think they were plenty large enough. Also, the extinct Moa was larger than a human, as was the Elephant Bird. Flightless birds do not need to limit their weight to fly, so if intelligence for a predatory lifestyle was useful for increasing intelligence, they had plenty of time to acquire it, and large enough to house a larger brain. They didn’t, indicating to me, that the rapid rise in human intelligence is a result of other factors, and probably not due to our ancestry, other than social behavior.
The Homo lineage rapidly increased brain size while our cousins, the apes did not. We are tribal as are the apes. Other animals like wolves live in packs and organize “hunts”, and have some form of communication. Birds live in colonies but I don’t think they maintain a social hierarchy.
Something, perhaps a fortuitous feedback loop, drove the homo lineage to increase brain size and intelligence. It was aided along the way by the use of fire (which aided the energy extracted by digestion), tool-making, etc, but this must have started in a much simpler lifestyle that was more reminiscent of apes today, and then the lineages diverged from the common ancestor.
Speculation is endless, but what seems clear to me is that other animal lineages had many advantages yet failed to have an IQ takeoff, despite far more time to evolve bigger brains and gain higher IQs. It is possible that our lineage just got lucky with a gene mutation that other species did not get. [Or we got some sort of external intervention. Got to look for that monolith or those Martian “grasshoppers” spacecraft”. ;-) ]
A nice theory about IQ takeoff is the connection with Pleistocene glaciation. After it’s onset, climate destabilized so much that forests and savannas started to replace each other much faster than evolutionary adaptation time. There were permanently forested areas where chimps and some other apes took refuge; but in the areal of direct human ancestors, habitats of both types often shrank to islands and then disappeared completely. This put strong evolutional pressure for survival of the smartest – the ones who could, through tool usage and social cooperation, survive both in the woods and in the open. After this level-up, human ancestors became masters of both types of habitat, and gained much more living space than competing species.
So, for generalized intelligence take-off, two things are needed. First are the pre-requisites – high basic intelligence, free appendages and signal system not more than one evolutional step from tool-making and complex signalling, respectively. The other is evolutionary pressure of right “get smart or extinct” kind and intensity that excludes any adaptation except for intelligence step-up.
From this POV, human emergence appears to be not a common thing. Planets which exhibit glacial cycles are dangerously close to snowball state. Earth had maybe four major glacial periods, one or likely two of which ended up in snowball state (Huronian and Cryogenian). Slightly more seneral insolation, and no glaciations at all, slightly less, and a snowball state annihilates the nascent intelligence together with all advanced lifeforms. (Or, maybe we’re here to prevent it? =] )
If this was indeed the case, together with failure of dinosaurs to produce intelligence, it may mean that intelligence does not evolve unless it’s absolutely necessary. Some kind of “evolutionary lasiness” hypothesis. But what type of other factors may present such kind of pressure? Maybe strong obliquity cycles seen on Mars and some exoplanets, or any other periodic, but not permanent action which forces sub-intelligent species to migrate over entire planet with all kind of terrains. I’d bet there could be something else other than glacial cycles.
Competition between predator and prey can select for so many other traits besides intelligence. It seems so much easier for competition to leverage a bodily trait such as stamina and speed or a mental trait not associated with higher IQ such as patience or wariness. I am not saying it is impossible just not very effective at increasing IQ.
The argument may be better applied to omnivores with diverse diets. Every food source added to the diet and every change in a food source could leverage an increase in IQ. The smartest animals on Earth are also social. Since scientific advancement looks like a social enterprise, my money is on social intelligence being required for high IQ traits. Theory of mind makes a great entry into abstract thinking.
Chimpanzees produce diverse vocal sequences with ordered and recombinatorial properties
Molluscs have been around for a long time, but only the marine squid and octopi have become large enough to gain intelligence. Both never evolved lungs for land living, unlike the land snails.
It is possible that they are unsuited to land living as their size and lack of skeleton (internal or external) makes land living very suboptimal. If they were to acquire much higher intelligence, there would need to be some evolutionary push to do so.
As for communication, for short distances, using their chromatophores to communicate with light patterns might become the basis of a language. Or they could develop a sign language with their 8-10 tentacles.
But unless they can find a way to live on land, I don’t see how they can become a technological civilization.
Interestingly, cetaceans seem to have a pretty decent intelligence level. They evolved from land-living mammals, so it is possible they could evolve a branch of land descendants again and become a technological civilization.
If humans disappeared, I would bet on rodents like rats evolving into the next, intelligent, technological species. Their ancestors did it in 65 million years after the non-avian dinosaurs went extinct.
Did two pulses of evolution supercharge human cognition?
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Basically the vertebrates, having got there first, prevented any other phyla from occupying the large land animal niche. Consider our fish ancestors and how much evolution they had to undergo to produce us. If you were an alien and you were presented with our ancestral fish and an octopus, which one would you bet on to colonize land and produce an intelligent species?
Hard to imagine upfront how evolution will play out. However, there are no large land mollusks despite 100’s of my of evolution. Tentacled mollusks predated bony fish yet none made even basic forays onto land. The arthropods captured that niche, as their hard exoskeletons allowed them to become adapted to the land. The higher O2 levels during the carboniferous allowed them to grow to huge size before becoming smaller again as the O2 levels fell back towards 20%.
Let’s not forget the genome reduplication prior to osteichthyes: this allowed one set of chromosomes to carry the original essential genes, while the other three sets could experiment with substitutions without incurring the disadvantages or even lethality of adverse genes.
I’m enjoying reading your posts Dave, this one was interesting too.
I can remember reading Habitable Planets for Man in the mid 90s. My Uncle suggested I find and read the book at the University Library. Who would have thought that I would have ended up working at the same university too in the future.
I’ll look forward to your future posts.
Thanks Edwin
I wonder if life could be deposited into low mass stars:
https://www.universetoday.com/162426/your-oven-gets-hotter-than-this-star/
What could you do with a cool Star?
New ways to beam heat from one place to another?
Ansible with a plug adapter?
Narrowly defining intelligence as something only humans have is less productive than defining habitable as places were humans can live on the surface. Once defined as a set of traits, it is obvious there are many animals on Earth with an intelligence quotient. Humans just have a very high IQ.
As long as we confine ourselves to planets with life as we know it, the claim that intelligence is a signal that a planet is hospitable is solid only because life passed through earlier phases that already made the planet hospitable. I don’t see how it increases the population of hospitable planets. The claim that we can use the Great Silence as evidence that there are no other hospitable planets leans into the narrow definition of intelligence.
The claim that only time is required for high IQ, paradigm shifting, space faring animals to appear is equivalent to claiming only time is required for animals to move faster than the speed of sound. Time is a necessary component but evolution is a path dependent process and not all paths lead to high IQ. Birds evolved dense neurons and efficient brains because flight selected for low mass. This selection could permanently block the route to evolving a high enough IQ or, since a planet’s hospitality is finite, take them on a too circuitous route.
The large population of animals on Earth with measurable IQ’s does increase the odds that intelligence and high IQ beings can evolve on other planets. The many examples of endosymbiosis increases the odds that life on other planets can evolve past simple, single cells. We just don’t know if space faring intelligence is the equivalent of drawing a royal straight flush or just a straight flush or if multicellular life is a full house or four of a kind.
On the topic of human hospitality…
Would proteins with the same bases as Earth versions but with a different structure pose a significant risk of causing auto-immune responses, allergic responses or prion diseases?
Are you asking about proteins with the same 20 amino acids but with different sequences? Then the answer is yes.
Are you asking is proteins can use just the 4 nucleic acid bases instead of amino acids, I don’t think that is possible.
Are you asking is the amino acids were the same, but of the opposite chirality, then that is an interesting idea. I suspect that they can cause immune issues as the shapes are what is important. They would however, cause digestion issues and would not be useful as food.
It would be an interesting experiment to try designing the protein sequence to fold in a way that mimics a known allergen and see if it can induce a reaction using a skin patch test. Alternatively, synthesize random protein sequences with these AAs and see if any cause reactions on experimental mice.
Hello and thanks for the response. I should have been clearer. I am asking about the effect protein shape, even when all amino acid bases are the same, could have on human habitability. The dangers of an alien ecosystem are often minimized because toxins, viruses and parasites evolved their effectiveness symbiotically over millions of years. Could an ecosystem that uses the same amino acids but different secondary, tertiary, quaternary structure be broadly inhospitable to humans? Our metabolism uses a lot of amino acid keys to bind proteins.
Transaminases can and do transfer amino groups between molecules, from an aminoacid to an organic acid and might do so with “alien” molecules. The essential aminoacids for humans are ones that humans cannot synthesize (other species may): L³T²PMV (leucine, isoleucine, lysine, tyrosine, threonine, phenylalanine, mehtionine and valine, IIRC biochemistry in 1967).
Superearths may be habitable even if in deep space. Geothermal and volcanic activity and thick atmosphere with warm oceans…
Liquid Water on Rocky Planets Could be 100 Times More Likely.
https://www.universetoday.com/162402/liquid-water-on-rocky-planets-could-be-100-times-more-likely/
This could be the most common habitable rogue world. These rogue superearths could be less then 1/4 lightyear from earth.
New Study Reveals NASA’s Roman Could Find 400 Earth-Mass Rogue Planets.
https://www.nasa.gov/feature/goddard/2023/new-study-reveals-nasa-s-roman-could-find-400-rogue-earths
They may have colonized the whole galaxy, but only on rogue superearths. The Fermi Paradox is that they are our nearby neighbors!