A conceptual praeparatio astrobiologica

The View from Oregon – 368: Friday 21 November 2025

For several years now when I’ve been writing and thinking about astrobiology related problems I’ve been struggling with the limitations of the term “biosphere,” which has, to date, topped out our sequence of ecological concepts of increasing comprehensiveness. This sequence of ecological concepts is uncertain at points, in part because ecology has never received the theoretical treatment is deserves; in this respect it lags far behind biology. There is widespread agreement that “biosphere” means the envelope of life taking a spherical form around the entirety of a planet, but even here there is some ambiguity because “biosphere” could mean only the living matter near the surface of a planet, or it could mean the living matter as well as the inanimate matter with which the living organisms immediately interact. There is also widespread agreement that “biome” designates a large region, less than the entire biosphere, but larger than a single lake or meadow, for example. Biomes have been classified in some degree of detail (there’s more than one system of biome classification—Köppen, Köppen-Geiger, Köppen-Trewartha, and maybe others as well), being used in the identification of climates, and as a result these uses have been regularized.

At the other end of the scale, there’s widespread agreement on what is meant by “cell,” “individual organism,” and “population,” as these are concepts ecology shares with biology, so the effort that has gone into clarifying these as biological concepts can be directly borrowed by ecology. But between individual organism at the lower end of the scale (i.e., a scale of comprehensiveness), and biome at the upper end of the scale, there’s some ambiguity about how terms are used. Other concepts include “community,” “ecosystem,” “bioregion,” and “biotope.” No doubt there are some precise definitions of these concepts out there, but I haven’t found definitions that consistently eliminate all the ambiguities as to how comprehensive an ecosystem is in relation to a community, etc.

Because these terms remain ambiguous, they are ripe for being hijacked for other uses, and I have another ecological use for which I need a term. I said above that I run into problems with “biosphere” as the most comprehensive term, and this is what I had in mind, i.e., something more general than biosphere, but I’ve just realized that at least two additional concepts are needed—a more comprehensive concept than that of the biosphere, and a more general concept that I’ll describe below. The more comprehensive concept is needed for the possibility of life that spans several worlds and therefore is more than a biosphere. There could be a single origins of life event that is subsequently distributed to worlds other than that upon which life originated. If two or more worlds have life derived from the same origins of life event, then that distribution of life belongs to a more comprehensive concept, and two biospheres fall under that more comprehensive concept. There has been a great deal of speculation that this has happened with our own solar system. It has been proposed that life began on Mars or on Venus, and was subsequently distributed to Earth, or that life began on Earth and was distributed to Mars. Even if unlikely, we can’t yet rule out these scenarios, and we have no ecological term with which to refer to such a state-of-affairs. We have the concept of panspermia for the mechanism of distribution, but no concept for the biological structure that results from the mechanism.

Shortly after the TRAPPIST-1 planetary system was discovered, with its many small, rocky worlds quite closely spaced together around a red dwarf star, at least two papers appeared that argued for the possibility of rapid lithopanspermia at TRAPPIST-1. Since we know that matter is exchanged among the inner planets of our own solar system (e.g., the famous, or now notorious, Martian meteorite ALH 84001), with so many planets even more closely spaced than the inner planets of our solar system, as with the planets at TRAPPIST-1, it’s likely that there is a significant exchange of material among these worlds, and on a shorter time scale than the exchange of matter among planets in our solar system. If life evolved on one planet in a planetary system like TRAPPIST-1, it would have a reasonably good chance of being distributed to the other worlds of that planetary system.

We can go further yet with this. I’ve mentioned in past newsletters the concept of “urability,” which has been introduced to distinguish planets on which life can originate, in contradistinction to “habitability,” where life, once it originates, can continue. We assume Earth is both urable (life began here) and habitable (life continues here over cosmological scales of time). However, in the event of one of the scenarios I mentioned above, where life originates on Mars or Venus and was subsequently distributed to Earth, it is entirely possible that there’s much that we don’t know about the boundary conditions of the origins of life (i.e, urability), so that these conditions may have obtained on Mars or Venus in the early history of the solar system even while these conditions did not obtain on Earth, so that Mars or Venus was the urable planet and Earth the habitable planet. If either of these scenarios occurred, then Mars or Venus were urable, but have since ceased to be, but they were not habitable, while Earth proved to be eminently habitable. Again, this is all speculative, but it can’t be ruled out (which gives a sense of how little we know about these fundamental astrobiological questions).

There’s an interesting detail that just occurred to me, and which might eventually be a useful idea, so I’ll spell it out here before I forget it. One of the problems that origins of life research faces is that ongoing geological and biological processes on Earth have wiped away the evidence of the earliest life on Earth (not to mention proto-life before life proper got going), so we see only the late results of the origins of life, billions of years after the fact, and no traces of the origins itself or of transitional forms of life immediately following upon the origins of life. If there were a planetary system with two or more planets, one of which was urable but not habitable, and the other of which was habitable but not urable, then the urable world would have a greater chance of retaining evidence of the origins of life and the immediate transitional forms, since life would originate on this world, but which not endure here even as that life is distributed to the habitable planet where it does endure. Discovering a planetary system with this structure (if it exists, which we certainly don’t know to be the case, though again we can’t rule it out) would be the best place in the universe to research the origins of life. Further, the urable world could continue to produce additional origins of life events, so that a planetary system that had an urable world might be populated with multiple biospheres with different forms of life derived from different origins of life events, all generated by the same urable world. Just as conditions on habitable worlds change over cosmological scales of life, conditions on urable worlds would also change over cosmological scales of time, and this could give rise to changing forms of life produced in sequential origins of life events on the urable world.

Further yet, natural selection would come into play in a distinctive way with these more comprehensive ecological concepts than the biosphere. Firstly, life originating on the urable world would be selected for distribution to habitable worlds based on its survivability under conditions of lithopanspermia. Only that life that could survive the trip from the urable world to the habitable world would be successfully distributed to the habitable world. If an urable world were positioned between hotter worlds closer to the star and cooler worlds farther from the star, life tolerant of higher temperatures (thermophiles) would successfully be distributed to hotter worlds, while life tolerant of lower temperatures (psycrophiles or cryophiles, depending upon your preferred term) would be successfully distributed to the cooler worlds. Given sufficient time, both the hotter worlds and the cooler worlds could provide an environment for life to evolve toward greater extremes, so that this life could then be distributed further outward to even hotter or even colder planets in the same planetary system. Thus we see that we also need a concept more comprehensive than the concept of the biosphere to describe processes like natural selection on a planetary scale that could occur given the right boundary conditions.

Peter Ward in his book Life As We Do Not Know It: The NASA Search for (and Synthesis of) Alien Life, suggests the novel taxonomic classifications of Dominion (“Life containing two-stranded DNA and utilizing it as its genome/information storage molecule; proteins made up of twenty amino acids for specific amino acids, which are coded for by three-letter nucleotide sequences for specific code; with a phospholipid membrane and water used as solvent in the cell”), and Arborea (“a taxonomic category that is above the level of dominion and is made up of all life coming from one independent genesis of life on a planet or moon”). This is helpful, but Ward’s focus is the life itself, and not the environment which constitutes the boundary conditions of life’s urability and habitability. Ward’s Dominion and Arborea add to the taxonomical hierarchy of species, genus, family, order, class, phylum, and kingdom, and not the ecological hierarchy of individual, species, population, community, biome, and biosphere.

The other concept that’s needed, besides a concept of life more comprehensive than the biosphere, is a general term that can apply to any isolated (or, if you prefer, independent) self-sufficient and self-sustaining living environment. Recently I’ve been favoring the term “biotope” as a concept with sufficient generality that it could be used in this way. Biotope already has a specific meaning in ecology and vague meanings in ordinary language, but it’s not so familiar that it couldn’t be pressed into service for other uses, such as what I have in mind. We need an astrobiological concept like this to cover life should we find it on a sub-planetary scale. For example, it has been suggested that life is possible in deep craters on Mercury, where, despite the heat, the sun never reaches the bottom of the crater. And it’s been suggested that Ceres doesn’t have a global subsurface ocean like Europa or Enceladus, but “brine pockets,” which could be what remains of a former global subsurface ocean. These brine pockets wouldn’t be spherical, so if they were inhabited, it wouldn’t constitute a biosphere, but it would be planetary-scale life. If life were found in an unlikely environment like these examples of Mercury and Ceres, neither would be biospheres, but we could call them biotopes. However, it may be necessary to distinguish urable biotopes from habitable biotopes, but this will require some additional clarification.

My copy of the Dictionary of Environment & Ecology defines biotope as follows: “…a small area with uniform biological conditions such as climate, soil, or altitude.” This would seem to exclude its use as I have suggested above, except that on the scale of astrobiology I could make the case that the two examples I’ve cited—life in craters on Mercury or in the brine pockets of Ceres—are in fact “small” areas (they’re smaller than a proper biosphere) and that conditions within them would be relatively unified. Here “relatively” is carrying a lot of weight, but, again, on astrobiological scales our measure of what’s unified is going to be different than if we’re studying freshwater limnology. I could argue that, on a planetary scale, the terrestrial biosphere is unified, since all life in the biosphere belongs to the same tree of life, and it’s possible that a world (like a superearth) could be larger and offer even more diverse conditions than Earth, perhaps even being the home for life from distinct origins of life events.

This newsletter has been something of a praeparatio astrobiologica, as there’s a thought experiment I want to formulate in a future newsletter, but in order to do so I need to clarify some of the definitional issues I find in astrobiology. However, in writing this newsletter I’ve already uncovered some additional conceptual problems I hadn’t previously seen (which is why it’s always a good idea to write one’s thoughts down in detail, since the process of making one’s thoughts explicit often brings much to light that we don’t otherwise realize), so I need to think more about these problems, and see whether I can converge on a conceptual framework adequate to the thought experiment as well as being true to astrobiology (even if the concepts don’t yet exist in astrobiology).

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