Work in Progress: Formulating a Metric of Planetary Complexity
Friday 09 September 2022
In my “Toward Universal Biology: An Observational Scientific Research Program in Origins of Life” presentation last month in Scotland, I observed that we have no established scientific classification for planets, which leaves contemporary planetary science with, essentially, a folk planetology that largely relies on comparing exoplanets to familiar planets of our own solar system. Thus when we observe a large gas giant close to its host star, we call it a “hot Jupiter,” and when we observe an ice that that isn’t quite so giant, we call it a “mini-Neptune.” This is fine for the time being, but no one could satisfied with this when classifying thousands of exoplanets, and eventually we will have to formulate a planetary taxonomy less dependent upon our parochial conception of types of planets. Whether or not anyone explicitly attempts a planetary taxonomy, one will emerge from the needs of other sciences, as was my point in relation to origins of life research.
I further discussed the absence of a planetary taxonomy in newsletter 198, in which I suggested the broad outlines of a planetary classification consisting of three elements: composition, size, and insolation. I have been thinking further on this sketch of a classification of planets, especially in light of some new findings. A recent paper addresses planetary classification in a way that looks like it will be quite useful: “Density, not radius, separates rocky and water-rich small planets orbiting M dwarf stars” by Rafael Luque and Enric Pallé. There is also a commentary on this paper, “Three types of planets orbit red dwarfs: Precise densities of red dwarf exoplanets help distinguish potential ‘water worlds’,” by Johanna Teske. (Despite my best efforts — and I am pretty good at digging — I have not been able to obtain copies of these two papers, so if there is anyone out there with a subscription to Science and who would like to send me a couple of PDFs, I would be much appreciative.)
The abstract of the latter paper says, “…small planets orbiting around red dwarf stars… fall into three main types: rocky, watery (including icy), and gassy.” Following this classification, we would want three classifications of planetary composition to reflect these findings. However, since I can’t yet get a hold of the papers above, I don’t know the extent to which they identify a dwarf planet close to its star as “gaseous” whether it is a small rocky core with a thick atmosphere, or if it is more like ice giants or gas giants. That is to say, do we need separate gaseous types for small rocky planets (presumably formed within the frost line) and larger planets (presumably formed beyond the frost line), or would this merely confuse matters? Following a similar line of thought, do we need one “icy” (or watery) classification for mostly icy terrestrial planets and another “icy” (or watery) classification for KBOs, or would this merely confuse matters? The confusion is particularly obvious if, “Rocky planets form within the snow line, whereas water-rich worlds form outside it and later migrate inward,” (Luque and Pallé abstract), which makes water-rich worlds in the inner solar system the kin of KBOs, as both form beyond the frost line; they share a common descent.
Thinking about the absence of a planetary classification has made me aware of another ellipsis in planetary science. Among several projects I have have been working on is an update to my “How many branches are there on the tree of life?” (my NoRCEL presentation from last year, which was mostly derived from my 2019 Milan presentation “Peer Complexity during the Stelliferous Era”), and this update will incorporate some of my subsequent work, including ideas from my Scottish presentation. So in the revision I will touch on the lack of a scientific planetary classification, but beyond that I have realized that we have no metric for measuring and comparing the relative complexity of planets. This will be another slide of the presentation, and perhaps a talking point of mine going forward.
In regard to planetary complexity, some years ago in one of my more popular answers on Quora, If an alien species decides to invade earth for a resource or material, what would it be? (19 August 2017), I wrote:
“There are minerals that form on Earth due to the geological complexity of our planet that do not form elsewhere in the solar system…”
One reader, Taiye Adeboye, in response to this, asked me this question:
“‘…due to the geological complexity of our planet…’ The statement above got me wondering. What exactly does that mean? Is earth more geologically complex than the other planets? If so, is it terms of natural geological structures or in terms of chemical composition? Please pardon my ignorance but can you be more explicit?”
I answered as follows:
Yes, Earth seems to be more geologically complex than other planets because of the many processes occurring on its surface (as well as in the mantle, etc.). Mars may have had water at one time, but Mars freeze dried itself, so processes based on water (the hydrological cycle) stopped. Also, Mars is not tectonically active, and has a very weak magnetic field compared to Earth. This is only one example of another planet, but other planets in our planetary system are likely to be even less geologically complex. However, we are certain to learn a lot of interesting things when we get to study the moons of the gas giant planets, where there are probably processes occurring that don’t occur on Earth.
The natural geological structures of Earth are complex because Earth is geologically active: it builds mountains, oceans grow and shrink depending upon the present location of the continents, and these things occur because of plate tectonics and the supercontinent cycle. And because of these processes (think of the cycling of rocks from igneous to sedimentary to metamorphic, and so on) minerals are produced that are not produced in less complex environments. So the geological complexity and the minerological complexity are linked.
To get a general background on the role of complexity in the universe, I recommend reading books on “big history” (like David Christian’s Maps of Time). It might also be helpful to read Peter Ward and Donald Brownlee’s book Rare Earth. More specifically on geological and minerological complexity, I cannot highly enough recommend the works of Robert Hazen, especially his book The Story of Earth: The First 4.5 Billion Years, from Stardust to Living Planet, which goes into some detail on what Hazen calls “mineral evolution.” Minerals evolve to a greater degree of complexity in more geologically complex environments. I also highly recommend Hazen’s Great Courses lectures The Origin and Evolution of Earth: From the Big Bang to the Future of Human Existence. Hazen expresses himself a little differently from the representatives of the school of big history, but there is a basic commonality in these approaches.
That’s from five years ago, so I was explicitly aware of the relative complexity of Earth as a planet at that time, but I didn’t previously note that we have no scale of measurement for planetary complexity, which latter observation is tied to our more general lack of a common metric for complexity — a problem that often dogs big history, with its reliance upon the idea of emergent complexity.
We might come at planetary complexity from a number of angles: number of geological processes that take place, number of minerals present, presence or absence of life, and thus biogeochemical cycles that affect the overall geological complexity of the planet, relative complexity of the formation and development of the planet, and so on. That’s just a few ideas off the top of my head. Obviously, different forms of planetary complexity might be mutually exclusive, so it can’t just be a linear scale of one form of complexity supervening upon some antecedent complexity. However, as long we converge upon a reasonably exhaustive survey of possible forms of planetary complexity, we can arrange a linear scale, though there will be peer forms of mutually exclusive forms of complexity that are judged to be of approximately equal levels of complexity. This kind of divergent peer complexity may be important to track, in addition to a simple linear scale, because some forms of planetary complexity may be conducive to further complexity (like life) while other forms of complexity may not be so conducive, or may be conducive to alternative forms of emergent complexity.
As implied above, the geological complexity of a planet can be distinguished from the biological complexity of a planet, even though the two are tightly intertwined so that the distinction is often merely a formal distinction — but formal distinctions are important in the construction of scientific knowledge, so we must be mindful of them, though they are abstractions that exclude much that is relevant in other contexts.
On biological complexity, an idea that has exercised a certain influence over me is René Heller’s conception of superhabitable worlds, i.e., worlds that are more habitable than Earth (cf. Better than Earth and Superhabitable Worlds). Last week I thought of another way to come at this idea that is a little different from Heller’s approach. Imagine, as a thought experiment, a world at the outside edge of the habitable zone that, like Mars, once had liquid water on its surface, but instead of freeze drying like Mars, the ocean freezes over and retains the bulk of this water under its ice surface. Further suppose that this planet is geologically active, with plate tectonics. While on Earth geological activity served to end “ice house” climate periods by injecting carbon into the atmosphere, I guess we must further suppose that either this is an ineffective mechanism for heating on this planet, or no major coal seams (or equivalent) have a volcano erupt through them. The point of this thought experiment is to get at a world that, while not “superhabitable” in Heller’s sense, is more geologically complex that Earth, with plate tectonics going on beneath its ocean, and its ice surface cracked and buckled and rearranging itself in dramatic ways (as continental plates buckle and rearrange themselves in plate tectonics).
The imaginary world I have tried to describe would have all (or most) of the geological complexity of Earth, along with all of the cryospheric complexity of Enceladus or Europa. One could also throw Titan into the mix, and posit an atmosphere over the frozen surface, for further complexity. If the geological processes of this planet pushed up a high mountain range, there would be interactions between the mountains and the cryosphere, as well as interactions with the mountains and the subsurface oceans and the atmosphere. Any magma would melt through the ice and create localized zones of thawing where liquid water was on the surface.
Heller’s superhabitable worlds, by contrast, are a little older than Earth, a little larger than Earth, and likely orbit a K class star. The large planet would have more surface area, which Heller thinks would contribute to superhabitability, as well as smaller, shallow seas (rather than large oceans) and a thicker atmosphere with a higher oxygen content. A thicker atmosphere could make flight easier (although bodies would be heavier), and that could mean that much of the superhabitable biosphere would be aerial. On such a world, we could divide an “air column” into ecological zones as we divide an “ocean column” in Earth’s oceans, identifying distinct ecologies and food webs at different altitudes, as there are distinct ecologies and food webs in the ocean at Epipelagic, Mesopelagic, Bathypelagic, Abyssopelagic, and Hadopelagic zones.
Thus Heller gets at superhabitability by varying terrestrial parameters in a way that might result in a world that is more benign for life than Earth. My thought experiment approaches planets from a standpoint of complexity, and seeks to formulate a conception of a planet that is more complex than Earth, thus more enriched in rare and exotic mineral species than Earth, and with more rare and exotic geological processes than Earth. I am assuming a coupling whereby greater geological complexity may issue in greater biological complexity that supervenes upon the geological complexity, and this, in turn, implicitly assumes that Earth has a biosphere in part because it passed a threshold of geological complexity that allowed life of evolve. There may, then, be further thresholds of geological complexity that allows a further threshold of biological complexity to evolve. Needless to say, the kind of parameters involved in Heller’s imagined superhabitability could well be expressed on a planet along with the kind of complexity parameters involved in geological complexity, and thus superhabitability supervening upon this geological complexity, as imagined in my thought experiment.
As we come to know more about exoplanets, as technologies and techniques of observation improve, I have no doubt that we eventually will be confronted with forms of geological complexity that we have not yet imagined, and that we will have to expand our conceptions of the possible range of planets, and any life than may exist upon them, in light of unimagined complexities. As I have expressed elsewhere (though I can’t remember exactly where at the moment), I expect that higher levels of complexity at lower levels on the ladder of complexity as we find it, for example, in big history, will result in alternative complexities arising higher up the ladder of complexity.
The many thought experiments that I have done in relation to what I call intelligence-rich biospheres are an example of an alternative emergent complexity that might be found on a superhabitable world, or on an alternatively complex world (i.e., a world of peer complexity on a scale of planetary complexity as described above). In the event of an intelligence-rich biosphere, I do not think we would see the particular suite of emergent complexities that have arisen on Earth (technology, art, civilization, etc.) with only one abstract, calculating intelligence represented. Since it is so difficult for us to imagine different worlds from our own, on the rare occasion when we have an idea that suggests a radically different kind of world, we ought to exploit this idea as fully as possible — constructing as many thought experiments upon it as possible — to push ourselves beyond the limits of the familiar. Here, too, there is a threshold, such that if we push ourselves hard enough for long enough, we have a chance to glimpse a truly counter-factual and counter-intuitive world. As Spinoza wrote in the last line of his Ethics, “…all things excellent are as difficult as they are rare,” and this is certainly true for the excellence of thought obtained through the transcendence of the parochial.
