So far in my worldbuilding of our stellar neighborhood I’ve avoided Earth twins or Earth analogues for the most part…but zooming out, it’s just a bit too tempting to look at a solar analogue star (a star that’s very similar to our own sun, a star such as Lambda Aurigae, which I’m using for this project) and wonder if it might also host an Earth analogue…but of course, true to the spirit of my project, it would have a twist.
For a change, why not suppose that the planet was about the same mass as Earth, received about the same amount of sunlight in its orbit, and even had the same composition of gases…but, in a crucial way, it went horribly wrong. To wit, imagine a world with a 78% nitrogen, 21% oxygen atmosphere, but one that was not one Earth atmosphere thick, but multiple atmospheres thick…to the tune of hundreds of bars, maybe even over a thousand. That’s where I’m starting from, with worldbuilding Lambda Aurigae’s planet.
“Earth but a thousand times thicker air” sounds very weird, and I suppose it is, but let’s get down to fundamentals: where does a planetary atmosphere even come from? In our case, most of it is produced by a process called “degassing”, where elements and compounds that form atmospheric gas tend to leak out of the crust and the mantle. This is where our nitrogen content came from, by the way. An Earth-mass planet at an Earth-like orbital distance will have enough gravity and cool enough temperatures to retain nitrogen (in the form of N2), which is a heavier gas, but of course lighter gases like hydrogen and helium will tend to escape into space. Carbon dioxide (CO2) is another component produced from planetary interiors that leaks out and yet is retained. Thus, among the terrestrial planets, you see atmospheres dominated by carbon dioxide and nitrogen; even Titan has a primarily nitrogen atmosphere, due to the same sort of processes.
We can readily imagine that an Earth analogue could simply have degassed much more nitrogen for whatever reason; perhaps its geology works a bit different, or perhaps it was more enriched with nitrogen to begin with from its formation. Or both! Very plausible; heck, as it is Venus has 3.25 bars worth of nitrogen, which is over triple Earth’s amount in terms of bulk. And of course Venus has over 90 bars worth of carbon dioxide. So tremendous variations in degassing and atmospheric buildup among terrestrial planets are already known to be a scientific fact. 1000 bars takes it to a fantastic extreme, but such extremes should be expected well beyond our solar system…
What would such a world even be like? And where do I get “Earth’s evil twin” out of it? Well…the idea I had in mind is that perhaps Lambda Aurigae’s planet started out somewhat Earth-like. A thick nitrogen-dominated atmosphere, yes, perhaps already over 100 bars worth, but even under 100 bars of nitrogen, you could see fundamentally Earth-like temperate-to-warm climate, liquid water oceans could still be there, and our familiar biochemistry still works. The biosphere would be accustomed to higher pressure, but it works more or less how you’d expect an Earth analogue to evolve.
But then suppose that once the biosphere gets established, degassing continues and the nitrogen envelope thickens even further over geological time; nothing life couldn’t adapt to, but over a couple billion years another few hundred bars could be added. Now you’re dealing with a warmer climate that’s edging beyond the Earth-like range — even without greenhouse gases being increased, pressure alone will warm temperatures, albeit only modestly (but hundreds of bars add up!) — but pressure is helpful with keeping water liquid. Already at 100 bars, which is where we’re starting from, water only boils at 591 degrees Fahrenheit.
So even temperatures that we’d associate with “boiling!” would be just fine for native life in this environment; similar mechanisms are observed today in deep-sea hydrothermal vents, where hardy life thrives at temperatures that would boil and sterilize us, due to the effects of great pressure. It’s thought, by the way, that Earth’s oceans condensed earlier than you might ordinarily expect due to the same factor: our original atmosphere remains somewhat mysterious in the exact details, but it’s known to have been carbon dioxide dominated and it’s also known it must have been much thicker than today, perhaps 30 bars or so. So water stays liquid at higher temperatures than it would on present-day Earth.
So already we’re a couple billion years or so into Lambda Aurigae’s history and the local life is facing an environment more similar to hydrothermal vents or very early Earth than Earth at a comparable stage of evolution. But then a transition analogous to our great oxygenation event takes place, altering the atmosphere, but on Lambda Aurigae it’s the advent not of oxygen-producing photosynthesis, but rather denitrification. This is a known metabolism from Earth microbes, a process that liberates nitrogen from the crust and releases it into the atmosphere en masse. This doesn’t have a large effect on Earth because nitrogen-fixing bacteria tend to consume atmospheric nitrogen, locking it back up into the crust…but nitrogen fixation is (apparently) more dependent on rare materials to enable the reaction than denitrification is. What is Lambda Aurigae has just a slightly different geology where these materials are just not as accessible? Uh oh. In this case denitrifiers run wild, dumping huge quantities of nitrogen into the atmosphere. From a starting point already over a hundred times as much as Earth has, reaching 1000 bars becomes realistic.
And at this point, something strange happens to the oceans: as pressure and temperature rises, the oceans undergo a phase change from liquid to what’s called a “supercritical fluid”, which is a phase that has properties of both gas and liquid. Instead of liquid water, the surface now has supercritical water, once conditions pass water’s critical point, which is at a temperature of 705 degrees Fahrenheit and a pressure of 218 atmospheres. Life as we know it would retreat into higher altitudes, spending its entire cycle in the clouds, clinging to liquid water droplets, much like might have occurred on Venus.
We’re talking about nitrogen here, but oxygen is the other component of an Earth-like atmosphere, and to have 21% oxygen in a 1000 bar atmosphere would take 210 bars worth of oxygen. Which is a tall order, but if Lamba Aurigae evolved conventional photosynthesis a la Earth, this transition is plausible too. Heck, if Lambda Aurigae had extreme degassing, then it stands to reason that carbon dioxide was released en masse as well…which would ordinarily suggest it would turn into Venus. But if photosynthesis evolved early enough and was efficient enough, the carbon dioxide may well have been consumed, keeping greenhouse warming moderate. The net result? You have an atmosphere that’s Earth-like in percentage, but 1000 times thicker…to the point where you can’t even support an ocean in a true sense anymore, rather you just have a contiguous gradient of dense fluid that becomes more water-rich as you head down to those toasty rocks being kept at 1000 degrees Fahrenheit and 1000 atmospheres of pressure.
This would be a hostile environment for life as we know it…but consider that a fully vibrant Earth-like biosphere is up there still, floating in those clouds. And also consider that the atmosphere of Lambda Aurigae at this point is not a desert like ours or Venus’s would be: supercritical water is an excellent solvent, much more so than liquid water is, so the contents of this fluid will be far more enriched with minerals harvested from the crust than any ocean water would be. To wit, the minerals necessary for nitrogen fixation will at last be found in abundance, and the atmosphere stabilizes over geologic time…but in a state much thicker than our own. The minerals necessary for complex life to grow will also be in abundance, in the form of dust grains lofted up beyond the supercritical water layer. Moisture and so forth will also be lofted up. So compared to present-day Venus this is a very dynamic environment.
Is this a cloud city type world, where you have Earth but floating up in the sky? Well…partially. But consider the properties of supercritical water: it’s an excellent solvent, so could it be suitable for life? It turns out that water in its supercritical phase is too good a solvent for life as we know it, since the carbon-based materials our bodies are made of would decompose in such a medium. But there are advantages to creeping closer and closer to the critical line: nutrient content is greatly enriched for any microbe’s metabolism. On Earth we don’t see this much since supercritical water just occurs briefly and in small zones near hydrothermal vents, but on Lambda Aurigae supercritical water is everywhere. Evolutionary incentive to adapt: much greater. So could life adapt? It would seem the answer is yes.
And this is where it gets really fancy: carbon-based compounds have a tough time in supercritical water, but the heavier and more robust silicon analogues of these same compounds would have a much easier time withstanding the dissolving effects of supercritical water. Indeed, the direct silicon analogues of our familiar carbon-based backbones are too inert at our temperature and pressure and our phase of water to be chemically active, i.e. usable for biology, but in a supercritical water medium, they’d work much better than their carbon-based counterparts. And, uniquely, silicon can directly substitute for carbon; it’s directly “under” carbon in the periodic table, so it’s possible for it to play a very similar role as chemical building block, structurally speaking.
We see heavy-element substitution even today in the form of rubidium salts being directly uptaken by the human body instead of the usual potassium salts, because rubidium is in the same periodic table column, and so it’s analogous. This only works up to a certain point; when the salt content becomes more than 40% rubidium in rats’ bodies the rats died, in one study that was done. But could such a creature evolve more rubidium salt tolerance over time, if there was an environment where rubidium salt uptake provided an advantage? Absolutely. Eventually you have an organism that uses rubidium salts in its chemistry instead of the more familiar potassium. Might the same be true of silicon instead of carbon? Absolutely, if there were some compelling advantage for such an adaptation. In our world there really isn’t, since carbon-based chemistry works much better in liquid water at our temperature range than a “siliconized” substitute would…but in supercritical water, with carbon-based life trying to adapt to it to access a new environment with rich nutrients? It’s a whole different game.
Indeed, this is exactly the sort of environment where you might most likely see direct silicon analogues of our life-form on an Earth-like planet. Consider that once life adapts to it, supercritical water has compelling properties as a medium; the higher temperatures and pressures involved could enable new reactions and access to much greater energy for metabolism, and supercritical water is an excellent nutrient transporter. Once life evolves to the complex macroscopic scale in supercritical water, you could even see other substitutions; for instance our blood uses iron, but in an environment at this high a pressure and temperature its heavier analogue ruthenium might have compelling advantages (it can still transport oxygen but is more robust in a high-temperature environment), so life might make the switch. Rubidium and caesium salts might be taken up instead of potassium and sodium.
All of these metals are rare on Earth, but consider that tunicates concentrate vanadium from seawater into their own bodies at 10 million times the ambient concentration, so bioaccumulation of rare elements is not far-fetched if there’s some evolutionary advantage to doing so. And consider that supercritical water enriches the mineral content of the ambient fluid they’re using, so harvesting rare elements is just much easier.
The really fancy part here comes when you consider oxygen respiration for a life-form that is now based on silicon. Such a life form would presumably breathe oxygen, which is in abundance in the atmosphere (thank you, photosynthesizers!), and the waste product (analogous to our carbon dioxide) would be silicon dioxide, i.e. silica. This could be excreted, but consider that even on Earth silica is used by diatoms to build their shells, so it might simply be recycled to build exoskeletons and so forth. Such creatures might not put out much waste, actually; they might tend to retain all the material they consume within their own bodies and grow continuously. Consider also the possibility that in supercritical water, and with silicon-based oxygen respiration, metabolism becomes more efficient at higher temperatures, so evolution would presumably tend toward life-forms running “hotter”.
The fact the metabolism of these life-forms is also presumably more efficient than their conventional carbon-based counterparts is really intriguing, because it suggests that eventually these life-forms might colonize beyond the supercritical water layer, much like how our air-breathing animals colonized the seas. This would require maintaining high pressure and temperature internally while in an environment much colder and more depressurized, which in turn requires a strong structure…which silicon-based materials would readily provide them.
The real question is how such high temperatures could be maintained at a metabolically acceptable cost…but these creatures have an answer readily available in their supercritical water fluid: dissolved radioactive elements. Uranium is a heavy metal, but certain daughter products in the uranium series such as polonium (a liquid in this temperature regime!) and especially radon (which in this realm is a supercritical fluid, not a gas) are primarily alpha emitters, serving as potent heat sources, and are easily shielded with materials like silica that are accessible to biology (it helps too that even lead, the ultimate decay product here, is liquid in this temperature regime, so it could simply be reused as ballast; nothing need be wasted!).
So a creature that concentrates enough radon or polonium in its body to heat itself up would be readily able to passively maintain enough heat and pressure to rise up above and conquer the skies. The form factor would presumably be more spherical than our familiar forms of life, because of the need to maximize internal volume (versus surface area) for lifting gas and heat retention. Scale is also an enormous advantage in this context, so your average life-form that colonizes the realm of our familiar carbon-based life up above would tend to be very big.
Not only animals but also plant-like forms could arise. Imagine a symbiosis forming between conventional photosynthesizers up above where the sun shines brightly, and down below where abundant nutrients can be accessed. You could see something like floating trunks miles long extending down, with silicon-based biochemistry dominating, encompassing carbon-based plants and leaves within its upper layers in a symbiosis (mitochondria are thought to have originally been independent organisms that were then incorporated into our own structure, so it’s not far-fetched). Again, scale is an advantage. So you have huge branching cloud forests dominating the entire planet at this point, with trunks dominated by material that’s more mineral than wood. This is also handy because these silicon-based floating giant animals would need some kind of food source, and these trunks could provide it to them.
More conventional carbon-based life would presumably evolve in a direction where arboreal animals would arise that might seem familiar. After all, their environment imposes a situation where bilateral symmetry is favored, and that means cephalization. Four limbs still make sense for climbing trees, and a tail to balance the head is obvious, so the form of these creatures might be surprisingly familiar. If they are indeed arboreal, then over millions upon millions of years in the same environment they might become well adapted enough to sport prehensile tails, and an organ like an elephant’s trunk is another obvious (and cute) adaptation that might be standard in their world (as opposed to a curiosity in ours).
Where it gets really fancy is that while the floating giants are based on silicon, their biochemistry is, mostly, directly analogous to that of carbon-based life on the same planet, so if the silicon-silicon and silicon-carbon bonds could be broken down, the material their bodies are made out of would be very digestable. And lacking agility, they’d be, in principle, vulnerable to predation, despite their huge size. So you could picture creatures roaming the branches up in the sky not too dissimilar to monkeys or lemurs (though with elephantine trunks), that feed on these silicon-based creatures. Pack hunting is the obvious strategy, and this suggests a lifestyle similar to wolves or humans, but that’s too easy. To accentuate the “Earth evil twin” vibes, why not make them eusocial? Eusociality is famous among insects and seems to be especially common in the hymenoptera specifically (ants and wasps), but it even happens among some mammals too; for all we know perhaps there was an evolutionary bottleneck at some point and most extant animal life on this world evolved from a eusocial ancestor, and simply retained those traits. Plausible, yet very alien…and useful.
Consider that while there’s abundant raw digestable resources from these siliconized floating giants, their insides are at extreme pressures and temperatures, so the food would need to be processed. Cooled down by hundreds of degrees and managed so depressurization doesn’t cause the carcass to just explode. Which might be a bit much for a solitary individual, but for a eusocial species where a caste could simply evolve the faculty of being a food cooler, with its whole body dominated by coolant, served and protected by the rest of the colony? Absolutely it’s believable. We see a conceptually similar caste on Earth today in the form of the honeypot ants.
Mineralization (from the selfsame silicon they’re eating, perhaps…) would also help with withstanding biting into such a hot and pressurized creature, so you could see enrichment with silicon-based material in the bodies of even carbon-based life forms in the arboreal layer that has a much more Earth-like environment. Indeed, evolution might even favor the substitution of silicon-based material for things like the exteriors of these creatures. Diatoms are famous for this, but hard material forms the shells of many a creature in the mollusc family, for instance, on Earth. It need not be a shell, though; you could readily envisage thin sheets of silica forming a surface similar to scales, but more “mineral”. Indeed, silica could easily be used for the same purposes that mammals and archosaurs alike use keratin for (our hair and nails are keratin-based, which is a carbon-based protein, but as diatoms prove there’s no reason silica couldn’t work just as well).
And keep in mind silicon-based material need not be hard like a rock or a shell; silica gel is a porous form of silicon dioxide that resembles gelatin or even aerogel. Silica gel can even be “doped” with various (easily found!) substances that change color as water makes contact with them. Instead of pigments in the skin, fur, or feathers, our creatures with silica-based exteriors could use this method to change or maintain their color. And where it gets really fancy is if you have a creature that’s already making use of silica to build tissue, there might be advantages to using silica, instead of calcium-based material, to build bone…and consider that silica gel has certain properties that might be superior to conventional bone. It’s lighter and more flexible yet also stronger in certain ways, which would be of great relevance in an arboreal environment; a silica-gel-based “skeleton” would not be able to carry as heavy a load, but it would support longer limbs and more flexibility, so for all we know on “Earth’s evil twin” you’d see vertebrate analogues evolve away from having hard bone at all, instead rather having an internal scaffolding more resembling aerogels, and skin more resembling gelatin, if anything, all from the silica gel that might be ubiquitous on this world.
Even their blood might evolve toward using the same ruthenium metal that the silicon giants use, since they concentrate ruthenium at levels far above background, and iron presumably is not very common up there at high altitude; ruthenium, for them, might be more readily available.
And indeed, all this ruthenium and all the platinum-group metals being in abundance in the biosphere implies dust content enriched in heavier metals compared to a normal Earth-like planet, which would lead to vivid, dark, almost metallic hazes at sunset (as an aside, the higher pressure and dust haze would lead to a reddish dimmer sky which might push plant life toward a golden or even red color rather than green, hence the picture of this post)…the spectrum of this “Earth-like planet” would reveal an atmosphere that’s Earth-like but with great pressure broadening…as well as various bizarre metals found in abundance, which would at first be attributed to geology, but it would then be discovered that the levels rise and fall with the seasons in tandem with known biosignatures, suggesting an Earth-like biosphere that in some way “runs” on heavy metals. How is an explorer to resist?
And the best part of all? In an interstellar context, Capella is bright as Venus in the skies of Lambda Aurigae, a mere 4 light-years away…and it harbors a planet that is very much not an Earth twin with a biosphere, but in some ways is more of a paradise for human comfort than Earth itself.
But before you leave Lambda Aurigae, there might be a “twin” of Venus inward of “Earth’s evil twin” that might also harbor some extraordinary ecology. But I’m still brainstorming that out, so more on that later…in the meantime, enjoy what I’ve conjured up here as a setting for science fiction. Potentially it’s a rich realm indeed…