by Dave Moore
In this essay I’ll be examining the meaning of the word ‘habitable’ when applied to planetary bodies. What do we mean when we talk about a habitable planet or a planet’s habitability? What assumptions do we make? The first part of this essay will look into this and address the implications that come with it. In part two, I’ll focus on human habitability, looking at the mechanisms that could produce a habitable planet for humans and what this would imply.
If you look at the Wikipedia entry on habitable planets, the author implies that “habitability” refers to the ability of a planetary body to sustain life, and this is by far the most frequent use of the term, particularly in the literature of popular science articles.
Europa has sulfate deposits on it, which indicates that its surface is oxidizing. If the hydrothermal vents in the moon’s subsurface ocean are like those on Earth, they would release reducing gases such as H2S, and Methane. A connection between the two would provide an electrochemical differential that life could exploit. So it’s quite plausible that Europa’s ocean could harbor life, and if it does, would this now make it a “habitable” moon? If we find subsurface Methanogens on Mars, does Mars become a habitable planet? Traces of Phosphine in Venusian clouds point to the possibility of life forms there. If that’s so, would Venus now be considered habitable?
Andrew LePage on his website is more careful in defining what a habitable planet is. On his Habitable Planet Reality Check postings, he has the following definition:
…the best we can hope to do at this time is to compare the known properties of extrasolar planets to our current understanding of planetary habitability to determine if an extrasolar planet is “potentially habitable.” And by “habitable,” I mean in an Earth-like sense where the surface conditions allow for the existence of liquid water – one of the presumed prerequisites for the development of life as we know it. While there may be other worlds that might possess environments that could support life, these would not be Earth-like habitable worlds of the sort being considered here.
By Andrew’s definition, a habitable planet is first a body that can give rise to life. He then narrows it by adding that the type of life is “life as we know it,” which is life that needs an aqueous medium to evolve. If life evolved in some other medium, say Ammonia, then this would be life as we don’t know it; and the planet would not be classified as habitable. But this is not the only definitional constraint he makes. The planet must also be Earth-like in a sense that its surface conditions allow for liquid water. Europa would be excluded even if it had life in its oceans as its surface conditions do not allow for liquid water. His definition also implies that the planet must be in the habitable zone as defined by Kopparapu, which is thought to be the zone of insolation that allows for surface water on “Earth-like” planets. Would an ocean world with an ocean full of life fit his definition of habitable? Would a Super-Earth with a deep Hydrogen atmosphere (sometimes called a Hycean world) outside the habitable zone but with both oceans and continents and a temperate surface at moderate temperature be habitable? I do note however that his definition does not include human survivability as a requirement because elsewhere in his post he talks about the factors that have kept Earth habitable over billions of years, and Earth’s atmosphere has only been breathable to humans over the last 500 million years.
I’m not picking on Andrew in particular here; he has put more thought into the matter of defining habitability than most. Why I am using him as an example is to show just how fraught defining habitability can be. It’s a word that is bandied about with a lot of unexamined assumptions.
This may seem picayune, but the study of life on other worlds has very little data to rely on, so hypotheses are made using logical inference and logical deduction. And if your definitions are inexact, sliding in meaning through your logical process, then you are likely to draw invalid conclusions. Also, if the definition of habitable is that of a planet that could have life evolve on it, why include this arbitrary set of exclusions?
The answer becomes obvious from reading articles in the popular press. A habitable planet is not just one that is life-bearing, but a planet in which life gives rise to conditions that may be habitable for humans.
The assumption that life leads to human habitability is strongly ingrained from our historical experience. By the early 19th century, it was known that oxygen was required to survive and plants produced oxygen, hence the idea of life and human habitability became intertwined. Also, our experience of exploring Earth strongly influenced our perception of other planets. We found parts of Earth hot, parts cold, others wet and others dry. Indigenous inhabitants were almost everywhere, and you could always breathe the air. And this mindset was carried over to our imaginings of planets. They would be like Earth, only different.
For instance, H. G. Wells, an author known for applying scientific rigor to his stories, in The First Men in the Moon (1901), postulates a thin but breathable atmosphere on the moon and its native inhabitants. This is despite the lack of atmosphere on the moon being known for over a 100 years prior. Such was our mindset about other planetary bodies. Pulp SF before WWII got away with swash-buckling adventures on pretty much every body in the solar system without the requirement for space suits. Post WWII, until the early sixties, both Venus and Mars were portrayed as having breathable atmospheres, Mars usually as a dying planet as per Bradbury, Venus as a tropical planet as for example in Heinlein’s Between Planets (1951.)
When the first results from Mariner 2 came back in 1962 showing the surface of Venus was hot enough to roast souls, there was considerable resistance in the scientific community to accepting this and much scrambling to come up with alternative explanations. In 1965 Mariner 4 flew by Mars showing us a planet that was a cratered approximation of the moon and erased our last hopes that the new frontiers in our solar system would be anything like the old frontier. Crushed by what our solar system had served up, we turned to the stars.
Our search for life is now two-pronged: the first part being a search for signals from technological civilizations, which we regard as a pretty good indication of life; the second being the search for biomarkers on exosolar planets. We’re searching for biomarkers because, in the near future, characterizing exosolar planets will be by mass, radius and atmospheric spectra. Buoyed by our knowledge of extremophiles, we continue to search the planets and moons of our solar system for signs of life, but now it is in places not remotely habitable by humans. If the parameters for the search for life touch on habitable conditions for humans, they are purely tangential. These two elements once fused together in our romantic past have now become separate.
This divergence has led to a change in goals to the search for life. We look now for the basic principles that govern the emergence of life and under what conditions can life evolve and/or allow for panspermia? This leads to the concept of planetary habitability being secondary. Life, once evolved, in its single-celled form, is tough and adaptable, so it is likely to continue until there’s a really major change in the state of a planet; habitability is a parameter of life’s continuity, not its origins. So when describing planets, terms like life-potential or life-bearing become more pertinent. This latter term is now starting to be used in preference to the description habitable.
If we now look at the other fork, the idea of habitability when applied to humans, we note that the term has been used in a loose sort of way since the 17th century. Even the idea of the habitable zone was first raised in the 19th century, but it was Stephen Dole with his report, Habitable Planets For Man, under the auspices of the Rand Corporation in 1964 that put a modern framework to it by precisely defining what a habitable planet was for humans. The book can be downloaded at the Rand site.
This report has held up well considering it was written at a time (1962) when Mercury’s mass had not been fully established and Venus’s atmosphere and surface temperature were unknown.
Image: PG note: Neither Dave nor I could find a better image of the cover of the original Dole volume than the one above, but Stephen Dole’s Planets for Man was a new version of the more technical Habitable Planets for Man, co-authored by Isaac Asimov and published in 1964. If you happen to have a copy of the earlier volume and could scan the cover at higher resolution, I would appreciate having the image in the Centauri Dreams files.
Dole first defines carefully what he means by habitability (material omitted for brevity):
“For present purposes, we shall enlarge on our definition of a habitable planet (a planet on which large numbers of people could live without needing excessive protection from the natural environment) to mean that the human population must be able to live there without dependence on materials bought from other planets. In other words, a planet that is habitable can supply all of the physical requirements of human beings and provide and environment in which people can live comfortably and enjoyably…”
You’ll note that Dole’s definition contains echoes of the experience of American settlement where initial settlement is exercised with minimal technology and living off the land. There is emphasis on self-sustainment. It’s the sort of place you’d send an ark ship to.
I take a view of habitability as more of a sliding scale on how much technology you need to survive and live comfortably. On some parts of Earth, the level of technology needed to survive is minimal: basic shelter, light clothes and a pair of flip-flops will do the job. Living at the South Pole is a different story. You must have a heated, insulated station to live in, and when you venture outside, you need heavily insulated clothing covering your entire body and goggles to prevent your eyeballs from freezing. Move to Mars and you need to add radiation protection and a pressurized, breathable atmosphere. The more hostile the environment the more technology you need. By stretching the definition, you could say that an O’Neill colony makes space itself habitable.
I contrast my definition to Dole’s to show that even when dealing with what makes a planet “habitable for humans” you can still get a significant variation on what this entails.
Dole does however itemize carefully the specific requirements necessary to meet his definition. They are:
Temperature: The planet must have substantial areas with mean annual temperatures between 32°F and 86°F. This is not only to meet human needs for comfort, but to allow the growing of crops and the raising of animals. Also seasonal temperatures cannot be too extreme.
Surface gravity: up to 1.5 g.
Atmospheric composition and pressure: For humans, the lower limit for Oxygen is a partial pressure of 100 millibars, below which hypoxia sets in. The upper limit is about 400 millibars at which you get Oxygen toxicity, resulting in things like blindness over time. For inert gasses, there is a partial pressure above which narcosis occurs. This is proportional to the molecular weight of the molecule. The most important of these to consider is Nitrogen, which becomes narcotic above a partial pressure of 2.3 bar. For CO2, the upper limit is a partial pressure of 10 millibars, above which acidosis leads to long term health problems and impaired performance. Most other gasses are poisonous at low or very low concentrations.
Image: Original illustration from Dole’s Report. You may notice the lower level of O2 set at 60 mm Hg. This is the blood level minimum not the atmospheric minimum. There is a 42 millibar drop in O2 partial pressure between the atm. and the blood.
Other factors he considered were having enough water for oceans but not enough to drown the planet, sufficient light, wind velocities that aren’t excessive or too much radioactivity, volcanic activity or meteorite in-fall.
Dole then went on to discuss general planetology and how stellar parameters would affect habitability—something we now know in much greater detail–and he finishes up by calculating the likelihood of a habitable planet around the nearest stars in a manner similar to the Drake equation.
You will notice that these requirements listed bear little resemblance to the parameters used when discussing habitability with regard to life. The two have gone their separate ways.
Using Dole’s report as a basis for examining the habitability of a planet, in Part II of this essay, I will note how our current state of knowledge has updated his conclusions. Then I will look at how you could produce a planet habitable for humans and the consequences of those mechanisms.
——–
Wikipedia Planetary Habitability Definition
https://en.wikipedia.org/wiki/Planetary_habitability
Andrew LePage: Habitable Planet Reality Check: TOI-700e
https://www.drewexmachina.com/2023/01/23/habitable-planet-reality-check-toi-700e-discovered-by-nasas-tess-mission/
Manasvi Lingam, A brief history of the term ‘habitable zone’ in the 19th century, International Journal of Astrobiology, Volume 20, Issue 5, October 2021, pp. 332 – 336.
Stephen Dole, Habitable Planets For Man, The Rand Corporation, R414-R
https://www.rand.org/content/dam/rand/pubs/reports/2005/R414.pdf
This is a very timely essay. As you point out, “habitability” is a slippery term, and is often conflated by the non-scientific media as meaning humans could live there.
I would argue that this is shaped by the European experience of colonialism. Even in. the 1960s, my British grammar school education had geography books centered around what a manager in a commercial company should know about the best places to grow crops, obtain minerals, and how to sail to those places using the trade winds! The European expansion into the Americas, especially North America, has shaped our views on what successful colonization looks like and in turn shaped the literature, especially science fiction. Bradbury’s “The Martian Chronicles” is an allegory of the colonization of the continental USA. Venus was envisaged as a younger, primeval, hot, tropical world beneath the clouds, and even a temperate terminator zone on Mercury was once considered habitable for humans. Having grown up with Eagle comic’s Dan Dare, almost every known planet and moon in our system was habitable by intelligent beings and Earthmen with suitable technology, even if with difficulty.
Our planet is rapidly becoming one where unprotected humans can only live in 2 zones above and below the equator, and outside the poles unless suitably protected. Humans would have found most of the pre-interstadial time when the poles were covered in forests impossible to live due to the heat. Technology does change things. Like Captain Nemo, we can live in the oceans, at least for short periods, and there are persistent dreams of floating cities in the temperate, high-altitude clouds of Venus. Asimov changed his mind about off-planet living when he endorsed Gerry O’Neill’s space habitats idea as. the most logical approach to colonization, and recognizing planetary chauvinism that made planetary surfaces the only living place. In O’Neill’s follow-up book, “2081: A Hopeful View of. the Human Future” he proposed that the space habitat idea could be adapted for building enclosed cities in non-temperate parts of the globe, especially the frozen North. (The way we are going, we will need them to maintain populations even at the equator.
Like a broken record, I harp on about surface water as being an insufficient condition for life. We also know that a frozen world can support extant life below the ice in the oceans, as well as on the surface of glaciers in tiny pockets surrounding dust that allows for liquid water to support single-cell organisms. We now also know that Earth has a crustal biosphere well below. the surface. Life will certainly be more ubiquitous than “intelligent” complex life, assuming Earth is not some unique, living planet in. the universe.
I very much look forward to reading part 2.
Habitability
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An excellent rumination on the subject of “habitability”, not to mention how the concept itself is biased by our own experience and ignorance. I look forward to reading future essays on this topic.
At the risk of encroaching on subjects Mr Moore may cover later, I am reminded of some other issues that may bear on the habitability of worlds, at least as related to human colonization, or even exploration.
For example, what if an otherwise hospitable Earth-like planet is inhabited by a life form which is toxic, or even hostile to human life. A particularly virulent microbe or dangerous beast, or a poisonous vegetation of some sort that is deadly to humans (or our crops and livestock) may live there. We may be able to control this hazard with technology, but at the risk of upsetting local ecological processes which may turn out to be even more dangerous for the local environment. It is conceivable that our efforts to make an exoplanet suitable for human life may make it intolerable for all the local species. We barely understand our own environment, an alien one is no doubt full of nasty surprises, no matter how superficially familiar and benign it may appear.
We may not want to eat the local flora and fauna, or vice versa, and the indigenous germs may find our metabolisms to be unappealing (again, and vice versa), but we, or they, may find the other to be highly allergenic, or the local environment to be subtly poisonous (high concentrations of heavy metals, or as Isaac Asimov once pointed out, beryllium).
As for the evolutionary possibilities of advanced life, we have done very little speculation, much less rigorous research. Would an aquatic world ever develop an electromagnetic technology, the smelting of metals, or laboratory glassware and optical lenses?. Would a permanently overcast planet ever develop astronomy or classical mechanics? It is possible to imagine planetary surfaces where fire, stone tools, weaving, pottery and other key technologies would never arise. Perhaps others we can’t even conceive would take their place, but they might lead that civilization in other directions, perhaps as advanced, but totally different from our own.
I’ve often speculated on potential alien technologies which would not even be recognizable as such to us; civilizations and cultures that would look like rain forests or coral reefs to us, and their advanced biotechnology easily mistaken for natural ecosystems or habitats. Even speculation on these possibilities is difficult for us because we are so technologically chauvinistic, so inextricably tangled up with the directions our own physics took us.
Civilizations like this may abound in the galaxy, but it is unlikely they would ever develop the means to visit or communicate with us, and if we went to them we might not even recognize them as fellow intelligences. Perhaps highly advanced and sophisticated communities abound in the cosmos, but we are the only one who can build space ships and telescopes.
“… in the popular press. A habitable planet is not just one that is life-bearing, but a planet in which life gives rise to conditions that may be habitable for humans.”
Life bearing planets pose a particular problem despite being obviously habitable in its most basic definition.
Extant life can be toxic to humans, and vice versa. The atmosphere, land and water will be full of creatures and microbes, and their waste products, that are toxic to humans. This is true even if microbes are biologically inactive in a human body and creatures find humans unappetizing or toxic.
Enclosed habitations and individual protection must be used to protect both ourselves and the native life. Doing more with suitable technology is possible but mostly at a high cost for the locals. Presumably we would find that repugnant.
A proper human habitable planet will be one that is sterile or as yet devoid of life. I have no more than quibbles with Dole’s points listed by Dave. There is no real need to “terraform” one of these planets so that we can exist in the open, only that the habitable spaces we carve out are sufficient for human health and happiness. What these may be could be very different to what we currently believe when we reach a point in the far future when these matters are actionable.
We don’t really have to daydream about a “Hycean” world – we have something better (or at least *bigger*) right here. Our Saturn provides virtually Earth-normal gravity, comfortable temperatures at 10 bar or so, and most fundamental elements of life. Yes, there are some things missing like oxygen, terra firma, and (it would seem) a wide range of trace elements. But why shouldn’t an engineered ecosystem include strong living platforms that tap the turbulence of Saturn’s winds to stay aloft, and organisms that directly tap these platforms’ electrical power to the mitochondrial level in a symbiotic relationship?
Our obsession with atmospheres is a consequence of our planet’s own violent history. We’ve evolved to rely on breathing a corrosive gas that still damages our bodies to this day. But why shouldn’t organisms of another ecosystem evolve to eat oxidizing agents in one place and reducing agents in another? Why shouldn’t they evolve photoheterotrophy without relying on volatile gasses at all? Even Earth life began under a reducing and relatively unreactive atmosphere, and “respired” using the ferrous iron that is no longer stable in our ocean.
With some degree of engineering, to make them corrosion-resistant as flamingoes, radiation-resistant as Deinococcus, vacuum-resistant as tardigrades, oxygen-independent as purple sulfur bacteria … even the children of men may turn up in unexpected places. Meanwhile, we oughtn’t forget that until a probe lands on a planet, we don’t really know what it will find.
Self-organization connects organic molecules to form life thus placing life in a continuum to the Laniakea Supercluster.
That is, “life as we know it”. Any alternate on the many forks in the road could lead to something different, or even unrecognizable. As might be the case of travelling much longer, or elsewhere: abdifferent time and a different place.
Thank you for your comments. I was thinking of doing some specific replies but a lot of your comments will be answered or at least cast in a different light in the second part of my essay.
What I was hoping was, for this part, to get your thoughts on how to define the word habitable. Should we for instance scrap it as a relic of our colonial past and go with the terms life-bearing and Intelligent life bearing? What do you think of Andrew LePage’s definition?
Habitability has become a very flexible term. Even the concept of the Habitable Zone (HZ) has been modified to be constrained by the adjective “continuous” to cover the period of increasing luminescence while teh star was in the main sequence. In teh other direction, Stapleton had his 18th Men living on Neptune when teh sun became a red giant.
You raised the suggestion that we could live in space in habitats and therefore does that make space habitable?
The Earth has been life-bearing for nearly 4 bn years, but aerobic, complex life for less than 1 bn, and arguably only successfully with the Cambrian “explosion” just over 1/2 bn years ago. Since then, the mass extinctions show that the planet became a poor place to life for some events. Humans could not survive on the surface for most of prehistory, it was too hot, except perhaps near the poles.
When we look at tidally locked worlds, humans could only live on the surface in a zone between the hot pole facing the star and icy conditions at the other pole.
My thoughts would be to just use different terms.
For places humans could live, perhaps with no more than stone age technology (fire and some simple tools) call this teh human habitable zone. That may include icy areas where thick clothing and shelter could allow sustained living, but not hot areas that require cooling to survive. At this time, there is no other place outside of Earth in our solar system that is human-habitable. Given that we rely on a host of “ecosystem services” to live, I would argue that without good terraforming, there is no place in the universe that is human-habitable without a lot of technology to transform a planet. Space habitats may prove the only places humans can survive off-planet, and even that may be a stretch.
Life (as we know it) OTOH, clearly can survive in a far greater range of habitats. As long as there is liquid water, even just transiently, microbial life can flourish. Just which metabolisms can flourish depends on the available suitable environments. Early Earth could not support aerobic life. Now it supports both aerobes and anaerobes. There may be worlds that only support aquatic complex life. On other worlds, only life adapted to very dry desert conditions. Some of those worlds may be icy moons with subsurface oceans. They may not have had a biogenesis, but if life is introduced, it may well flourish, which, IMO, makes that world potentially “life-bearing”. This may apply to almost any rocky world with internal heat and primordial water.
What if we terraform a world? Does a sterile world become “life-bearing” and eventually “human habitable”? If the world was not sterile but was life-bearing but with incompatible biology, does terraforming it make it Earth-life-bearing and eventually human-habitable?
If modifying a world only provides temporary suitable conditions, e.g. 10K-1000K years, does that count as life-bearing/human-habitable, or should this only apply if the world is stable for at least some minimal period? If transience is acceptable, does this mean that Mars might have been at least life-bearing during its early watery period and during periodic flooding?
My personal feeling is that human-habitable worlds in their native state are nowhere to be found if putting stone-age technology humans on its surface indicates no long-term survival. OTOH, even if life-bearing worlds of any type also prove extremely rare, or even absent, they most likely could be made to support terrestrial life of various types. We could green the galaxy and beyond.
Finally, if we can get past our bio-chauvinism, then artificial life and intelligence might exist almost everywhere there is energy and materials to be accessible. How would we classify places that are “machine-friendly?
As you mention in the essay, even deep space could be described as intelligent life bearing. It is potentially much too broad or poorly confined. As well, if we are trying to avoid anthropocentrism, the term risks defining intelligence as human level intelligence. Earth-like, though not perfect, may be a better option. Liquid water and exposed land masses are likely both required for a complex ecosystem that can birth an intelligent species.
When it comes to describing a planet or moon capable of supporting life, the term life-bearing is bullet proof.
Hi Dave. I really enjoyed Part I of your 2-part presentation. I grew up at a similar time to you I suspect. My three SF “gods” were Clarke, Heinlein, and Asimov in no particular order. Such a wonderful time to get to know works by true masters of the genre. I think it will take some time to finally arrive at an accurate description of planetary habitability. I tend to want to move away from homocentric ideas, and I certainly don’t think our “happiness” has anything to do with it. We could spend years debating what human happiness is and I think it is entirely irrelevant to the subject at hand. The bare bones must include liquid water but not necessarily on the surface of the body in question. Life anywhere in or around a planet or moon etc. should apply. I look forward to our first discovery of life under the ice of a moon of Jupiter or Saturn or in the clouds of Venus. In fact now that I look at it, we should stick to life bearing or intelligent life bearing. The intelligent life bearing description will be much harder to pin down I think. Will we even recognize some forms of intelligent life? We definitely need some data to chew on at this point. Bring on the next fleet of probes to the planets nearby.
I agree with the idea of moving away from homocentric terms. If the therapod dinosaurs had eventually gained a higher level of intelligence, their preferences for environmental conditions would be different from ours. Intelligent cetaceans even more so.
But in the SETI, I think we should consider abandoning notions of biological habitability.
The famous and funny short story by Terry Bisson They’re Made Out of Meat is a conversation between 2 artificial entities that meet whiles scouring the galaxy for intelligent beings. The ending might even be an answer to the Fermi Question. It was so successful that it has been adapted for plays and even a short video.
Just as we can speculate on discovering alien probes on Earth, so too one could imagine finding kilometers down in some terrestrial rocky crevice on Earth microbes with a different molecular cell machinery, different nucleotides and their triplet codes, alien aminoacids and a diffenent chirality. Perhaps with a decade or century life-cycle.
For those “in the know” that would be as amazing and paradigm–shifting as finding an alien probe.
But more prosaic leftover bits can be quite as interesting: molecular machinery of chloroplast relicts persisting in the malarial parasite from its dinoflagellate ancestors is no longer capable of photosynthesis since perhaps the time of the dinosaurs.
Not knowing what is required for a planet or moon to be life-bearing is an obstacle. Wouldn’t our efforts to define the term habitable be rendered mute when we add the necessary qualifiers’ “according to our limited knowledge”? Eventually, we will have a classification system similar to our system for star types or Star treks planetary class system.
I think that multiple definitions and refinements might indeed be the direction to go. We describe different biomes and ecosystems to characterize what type of life and their lifestyles are present. Subsurface oceanic environments in icy worlds are closer to our terrestrial deep ocean environments than the terrestrial land surfaces primarily exposed to the air. In this regard, classifying a whole planet as being in some “habitable” category might be less informative that more granular regional classifications. Earth isn’t exactly habitable for humans at the poles apart from gravity, breathable air, and water availability, albeit frozen. Deserts are similarly marginally habitable, and the tropics may not be habitable for humans again. The oceans are only habitable with surface vessels or platforms, and not yet habitable for long periods below the surface. Life is far more adaptable with greater ranges of environmental conditions. If life could thrive in conditions simulating the subsurface oceans of Europa, would that make Europa “habitable”?
If we do eventually get enough data to classify exoplanets in more detail, we may well start to refine the general “habitable” classification with more granularity. Star Trek “Class M” planets are presumably ready for human settlement without requiring much adaptation. For fun: Star Trek – Planetary Classification.
Clearly, ammonia or chlorine breathers would require a different planetary class.
What may be habitable for humans may even change, at least at the margins. KSR’s Mars trilogy has the inhabitants genetically modified to be able to live in the thinner Martian air after terraforming. Poul Anderson’s story “Call Me Joe” has humans reengineered to live on the surface of Jupiter (when it was thought that Jupiter had a rocky surface), and Clarke’s “A Meeting with Medusa” has the cyborged Howard Falcon explore the upper atmosphere of Jupiter.
Since my work was cited in this article, I thought I would weigh in. The definition for a “habitable planet”, in the Earth-like sense, that is quoted here was not an attempt to come up with a universal definition of where life of any sort can be found. Instead, given the lack of a widely accepted definition, I provide a definition centered on Earth-like habitability (at least in terms of how it had been used for decades following Dole’s seminal work) to establish the framework needed to assess the potential habitability of exoplanets in my articles. Without such a definition, any meaningful analysis would not be possible.
The definition I choose to use in much of my work does not include the possibility of “biocompatible” environments (a term that Martyn Fogg used in his work some three decades ago, which can support life but are not Earth-like) that could be found on Mars today or below the surfaces of the Solar System’s icy moons nor does it include the needs of life “as we don’t know it” (an impossible task because we have no idea what the required conditions may be). Personally, I’d like to see the term “habitable” reserved for worlds with Earth-like conditions (in keeping with the historical use of the term) and employ a term like “biocompatible” for non-Earth-like worlds which have environments which can support life as we know it.
But which Earth-like conditions? All of them from the Archaean eon onwards? The Proterozoic eon? The Cenozoic era? The Quaternary period? The current Holocene conditions of the last 20,000 years when humans flourished and dominated the planet?
A verdant planet Earth with all types of plants including flowering plants, grasses, trees, etc. where humans can traipse about breathing the air, is how our short history depicts Earth-like conditions in stories of space travel to other worlds. At the extreme, this would be a planet from the late Cretaceous to today. Earlier and there are no flowering plants. We cannot include the Devonian as there would be almost no plants on land, nor land vertebrates.
Going back further and teh planet just has bare rock surfaces, perhaps covered in ice, and the O2 levels would not support us.
If we recognize Earth-like is a world that could be mistaken for Earth without too close an inspection, then it assumes evolution has proceeded rather convergently with that of Earth, with vertebrates living on the land in predator-prey ecosystems.
What we might think of as Earth-like has been shaped by life. There are no lifeless Earth-like worlds. Oxygenic photosynthesis must have evolved, as well as complex life that fills many of the niches terrestrial life occupies. A world of bacteria would not be Earth-like to my mind, although it would be life-bearing, as you say.
I would argue we are in the planetary equivalent of a pre-Linnean period. Astrobiology will eventually classify worlds as we do for living organisms, with the appropriate levels of granularity from Kingdom down to species and sub-species.
“What we might think of as Earth-like has been shaped by life.”
I think that’s key. Consider that had life failed to arise on Earth, what would it look like today? If ET were to spy such an Earth, would they see it as habitable, and why? After several billion years lying fallow, I suspect that Earth would have evolved into a condition that it would appear not to be habitable, despite lying in the HZ. We will encounter exo-planets of this kind and I wonder how we will classify them.
> But which Earth-like conditions?
I’ve already addressed that in my articles (and is repeated here): “And by ‘habitable’, I mean in an Earth-like sense where the surface conditions allow for the existence of liquid water – one of the presumed prerequisites for the development of life as we know it.” So this covers a WIDE range of surface conditions including those which have existed on the Earth since the Archean when life arose on this planet. This is opposed to other potentially biocompatible (I’m liking this word the more I use it) but non-Earth-like worlds which can potentially support life like Mars, or some of the icy moons of the outer planets, or even the clouds of Venus.
Andrew, thank you for replying. I very much wanted to get more of your thinking behind your definitions. I really like the term Biocompatible. I was wondering though, could you tell me the difference between a habitable planet and an Earth-like planet.
Thanks