Thalassa this, Thalassa that…yes, my version of Proxima Centauri’s potentially habitable planet (speculative, but based on real science!) is a fascinating place, but what about the rest of the system? Proxima is just one part of a triple-star system…two of which are some of the most similar stars to our own sun you’ll find in our stellar neighborhood, and the Proxima Centauri system itself has other known planets besides Proxima b.
Yes, in my world Proxima Centauri b is an ocean planet drenched in oxygen and rich in life, the most complex biosphere ever seen (more so even than Earth’s!), so of course the first manned interstellar expedition would make for that planet like a homing pigeon. But when the scientists and pioneers are settled into Thalassa (or rather over it: the atmosphere is almost pure oxygen but it’s 50 times thicker than Earth’s at the surface, so breathable air only exists miles high in the sky, inspiring cloud-city-style colonies), then what? Where else might they go to explore and study?
What else is out there in Proxima Centauri? Science in the real world provides us with two other candidate exoplanets orbiting Proxima Centauri. The first one orbits inward of my Thalassa, and is believed to be 0.26 Earth masses, a small world, but still over twice Mars’s bulk. Habitable? Perhaps, but consider that the equilibrium temperature of the planet is 360 kelvin, or 188 degrees Fahrenheit. Ouch. That’s already getting to the edge of where water would boil off in the daytime or in the tropics, and that’s without any atmosphere providing a greenhouse effect. With that…you’re probably looking at something closer to a mini-me of Venus than of Earth. Or you could even be looking at a bulked-up version of Mercury: Proxima Centauri is a flare star with a violent history, and part of the background for Thalassa is much of its original water envelope was stripped (that’s where all that oxygen came from: the water was broken down into hydrogen, which escaped, and oxygen, which was retained). For Thalassa this actually enhanced habitability, since it ended up with water directly interacting with the core in a liquid state rather than being locked up in high-pressure ice or going supercritical, but for a world this small? And closer in than Thalassa was? It could easily have just been desiccated altogether.
For the local Thalassan aliens, who have access to telescopes, it might make an interesting object for study. But the really interesting object for them would be Proxima Centauri c, the other candidate exoplanet, which orbits far further away: Thalassa orbits at 0.05 AU and takes 11 days to complete a year, whereas this outer planet orbits at 1.489 AU and takes 1,928 days. Whereas Thalassa’s equilibrium temperature is -38 Fahrenheit, this world’s is a frigid -389 Fahrenheit: colder than Pluto’s. Nevertheless at just over 1 AU away (much less than the distance between, say, us and Jupiter), it would easily be resolvable as a disk through even primitive telescopes. What would they see?
At 7 Earth masses and this far out Proxima c is thought to be a mini-Neptune, meaning its composition is similar to ice giants like Neptune and Uranus, but the mass is smaller, so the end result is slightly more Earth-like: the atmosphere is hydrogen-rich, but volatiles like water and ammonia might take more Earth-like forms instead of being locked up in something like supercritical fluid under deep pressure. This has led to the hypothesis that these “mini-Neptunes” might be “hycean planets”, a portmanteau of hydrogen and ocean, because the atmosphere would be hydrogen-rich, but the water might take the form of a liquid ocean…perhaps one that has a distinct surface.
This far out, however, the “surface” of Proxima c would be perhaps similar to Neptune in temperature (Neptune and even Uranus to a lesser extent both put out more heat internally than they receive from their sun, and the same process is likely here), far too cold for water to exist as a liquid. If there is a liquid water layer, it would be deeper down where the temperatures are warmer…and the pressure is higher. How high? About 1000 atmospheres or so. Comparable in pressure to the deep ocean trenches of Earth, so something similar to our deep-sea creatures might find it hospitable. You’re nowhere near water going supercritical: it would be a liquid with a defined surface similar to Earth’s oceans…only this deep down it’s several hundred miles below the 1-atmosphere level, and it would likely be pitch-dark, without any sunlight from Proxima Centauri penetrating.
Now, if the hydrogen-dominated atmosphere is clear, virtually transparent, and free of haze, that helps a lot…and it might even be likely, considering at this temperature range it’s too cold for clouds to form, which is why Neptune and Uranus both have a blue color (hydrogen weakly scatters blue light, just like the nitrogen and oxygen in our atmosphere do, so if you look through enough of it it appears blue and clear to the human eye). Proxima d likely has a blue color as well, and if the hydrogen atmosphere is clarified (again, a la Neptune), it might be possible to see very deep down…but even in air like that you’re not going to get a clear picture of anything several hundred miles down. It’s just too far: too little light penetrates even the clearest atmosphere. So would our Thalassan astronomers just have to infer the presence of an ocean they can never see?
Not necessarily. Consider that planets are geologically dynamic: internal heating is convective, and local plumes could easily raise temperatures in specific regions of the planet (with corresponding cool regions in others). In these regions subjected to greater than average internal heating, temperatures would be warmer…and the pressure level at which liquid water exists would lower, thus sea level would rise higher into the atmosphere. How much higher?
A good guess is that a strong plume could raise sea level to the 500 atmosphere level…possibly even the 300 atmosphere level. Now we’re talking! But that still doesn’t get us to good visibility as seen from space. Even 300 atmospheres is still similar to the pressure found thousands of feet underwater on Earth. Again, pitch black.
But wait! We’re not done yet. There are additional sources of warming: if the planet were tilted on its side a la Uranus, polar heating could become extreme in the summer (because the sun circles near the zenith for months on end and never sets). Temperatures could be boosted by up to 40 kelvin this way, which might not sound like much, but on a world like this, it’s more than enough to bring an ocean so risen by a mantle plume from 300 atmospheres all the way up to the 100 atmosphere level.
Under water, that would still be pitch-dark: on Venus, whose surface pressure is similar, it’s very dim. But on this world, with a clear hydrogen-dominated atmosphere? It’s enough to permit sunlight to reach the ocean surface. It would be a dim blue twilight, even so, but it would be there, shining over a shimmering ocean surface of obsidian black.
To get this effect you would need a strongly layered atmosphere that is truly hydrogen-dominated, but scientifically it’s considered plausible. Compared even to Neptune the blue color would be exceptionally deep…and from space, during the summer, the ocean would glimmer, shimmer, and be faintly visible. To a Thalassan, it would just be a hint, but as denizens of an air world where they routinely view water from a distance in twilight, it would be plainly obvious that there’s water down there on that planet they observe.
Indeed, if they have myths about the primordial waters of the deep a la the Book of Genesis, these scientific observations would feel like confirmation: I had a line of thought where they might believe this world, where there’s a vast ocean in utter darkness that only peeks out seasonally, is the origin of their form of life. Stories describe how the gods created the first man and the first woman, found the world they put them in too dark, and so they were launched from this summertime bulge that catches the twilight, descending into the realm of the sunlit clear air they call home today, like birth from a cosmic womb (womb motifs would figure highly: they’re flyers and more similar to birds, but they give live birth as humans do).
Thalassans over time would have been able to loft balloons high into the atmosphere, all the better for making astronomical observations, and methane gas is abundant enough from local gasbag life-forms that making methane rockets would be trivially easy. Even solar sails might become accessible if the right biological material is available in their environment (think something like silk). They lack the scale of population needed to make a true industrial base, but a once in a generation spaceflight? For them, this would be quite doable! A person could be sent on a suborbital flight in a capsule, but they lack the scale needed to make anything bigger. Probes, however? Quite doable.
They are at a clockpunk level of technology, even if their knowledge of physics, chemistry, and dynamics is advanced, so they would lack digital computers (or possibly even analog computers), and certainly they lack anything like radio. However, sending out objects with simple feedback machinery and that sends back optical flashes is doable. Think a color filter that if it senses red light triggers a mechanism that sends out a specific code of optical flash (which could be detectable by telescopes based on the homeworld). That sort of thing. These sorts of mechanisms could be scaled up into some rather sophisticated machinery.
A probe could even be devised by these aliens that not only flies by, but impacts the air and lands in the water of Proxima c. They do possess phonograph-type technology, so when the probe lands in the ocean, the splash is recorded on a phonograph and launched on a (methane) rocket into space, transmitting its data: the sound of the womb of the gods. Perhaps complete with sonar from creatures in there!
Which might exist! Consider that twilight levels of illumination are already enough to support periodic blooms of photosynthetic life, including forms like bioluminescent algae and plankton, so it could be an eerily beautiful place down there. These blooms could in turn be fed upon by filter-feeder-type organisms such as whales, ranging up to very large sizes (remember: they live on a big planet with a big ocean…), perhaps echoing the forms seen in Earth’s hydrothermal vents. Not as developed as Thalassa as a biosphere, but perhaps even more fascinating.
The fact Thalassans know this much about the universe around them despite appearing so primitive at first glance would be an astonishing discover to human explorers who get to know them over the years. And not only are they sophisticated, I’m sure Thalassans are curious and when humans suggest they’re going to explore the outer planet, the Thalassans want to go…and a joint expedition is organized.
But that only covers the Proxima Centauri system. What about the binary system right next to them, that no doubt Thalassans would see resolve in their sky as twin stars more brilliant than any we see from Earth: Alpha Centauri, a mere 0.2 light-years away from them. What worlds orbit those two suns?
I have some ideas. Thalassa is considered a remarkable discovery for centuries to come, and is the primary target of human exploration in the early years, so there can’t be any planets that are all that life-bearing in Alpha Centauri, if we’re going by the logic of the stories I’m telling and the lore of the universe. Based on real science, what could be in there?
Very massive gas giants are ruled out by observations, but it’s my understanding is it’s very plausible for Saturn-mass or lower orbits to exist, especially in closer orbits, but ones that are still well outside the scorching hot-Jupiter range.
My concept is that Alpha Centauri B has a water-cloud jovian planet of about Saturn mass (in the habitable zone but far too massive to be rocky, instead being a gas giant), and another planet inward that’s more Neptune-mass, a clarified jovian (cloudless and blue, due to it being too hot for clouds to form, rather than too cold). The net effect being the dominant color palette is blue to white. The water cloud jovian at least has a retinue of rocky moons similar to our own moon: not too interesting as life-bearing environments. Perhaps a dusty dark ring system as well.
Alpha Centauri A, meanwhile, in my schema has two clarified jovians: Alpha Centauri A has a smaller zone of orbits that are stable around it than Alpha Centauri B does (they’re a widely separated binary pair, and so any orbits far out, beyond the “frost line” as our gas giants are, would be disrupted by the other star), and is brighter than B, so it’s more likely to host clarified as opposed to water-cloud jovians. The zone of stable orbits around Alpha Centauri B is less torrid than A’s.
Spicing it up a little, perhaps the inner planet around Alpha Centauri A is tidally locked, but in a Mercury-style 3:2 spin-orbit resonance, rather than the classic 1:1 lock, owing to an eccentric orbit. The outer planet is too far away to be tidally locked, but might have been tidally dragged down in terms of its rotation speed (so the days might be long).
A concept I’m working with is a giant impact event that left this outer planet with a high obliquity a la Uranus, a slow rotation, and a large moon comparable proportionally to Earth’s moon…only it’s more Venus-like owing to the orbital position making it far too hot for habitability. It might have been ice-giant-like in composition originally but its small size left it depleted of volatiles because of atmospheric stripping. It may well have more atmosphere than Venus, several hundred bars worth of carbon dioxide, leading to supercritical oceans, and potentially more Earth-like plate tectonics owing to the lubrication. But it would be a more extreme version of Venus, leading even this uniquely high-mass terrestrial planet an airworld for practical purposes: colonies are on the cloud decks, just like gas giants.
Indeed the theme is that Alpha Centauri’s planets as opposed to Proxima Centauri’s are of more industrial use case rather than heavily life-bearing. Hydrogen and helium from the gas giants, carbon and oxygen from the Venus-like moon, and abundant solar energy. Left over in its formation are asteroid belts around each star, outer belts outward of the outer planets, and inner belts with asteroidal objects similar in composition to Mercury (unlike the hypothetical “vulcanoid zone” around the sun these are stable owing to being further out). The inner belts could create zodiacal light that would be impressive from a planet nearby.
Again, highly raw and industrial, potentially spectacular for explorers and colonists who hitherto have seen only our own solar system…but not a seminal discovery like a life-bearing complex biosphere that hosts an alien intelligence would be. Hence why Alpha Centauri and Proxima Centauri are dynamic well beyond Thalassa, but why Thalassa dominates the conversation…and the imagination of mankind for centuries to come.
I have other ideas I’ve been kicking around (I’ve been on a tear of brainstorming about planets in my sci-fi universe lately), but that’s enough for one post, I think! All this hasn’t been featured in a story yet and is of course subject to revision, but I’m satisfied enough to feel “hey, I might actually like to put some characters in here and follow them around”, which is always a good place to be…
This post’s featured image is a rendition of Proxima Centauri’s dust belts by M. Kornmesser/ESO (CC-BY 4.0). 2017.