Thalassa, or Proxima Centauri b: The Pale Green Dot

In the science fiction setting I am worldbuilding, Proxima Centauri b as in real life is a roughly Earth-sized planet with an orbital period of 11 days, and is the closest known exoplanet. In real life we don’t know much about Proxima Centauri b aside from these basic facts, so for my science fiction setting I have taken the liberty of filling in the unknown details in a way that makes for a great setting to tell stories. For all we know Proxima Centauri b might really be the way I describe it, but in real life it probably isn’t; nevertheless planets of the same type I describe might actually be out there somewhere.

In my setting Proxima Centauri b is an ocean planet, completely covered in water, which gives rise to its name in this setting after its oceanic nature is discovered by humans: Thalassa, named after the Greek primeval goddess of the sea. In this setting, which is also an alternate history, Thalassa is the first ocean planet to be directly imaged at high resolution in the 1990s, will be the first ocean planet or exoplanet to be reached by space probes in the 2030s, and is expected to be the first ocean planet or exoplanet reached by human colonists around 2060 or so.

A World drowning in Oxygen

Aside from being an ocean planet, Thalassa has another striking feature: a 98% oxygen atmosphere with 50 times Earth’s pressure at the surface. In a fascinating inversion of the norm in space, human habitation of the surface is rendered difficult not by a lack of breathable air but rather an excess of it. Because Earth’s atmosphere is only 21% oxygen, the partial pressure of oxygen on Thalassa is 250 times Earth’s level.

This oxygen-drenched atmosphere was not generated biologically, unlike the oxygen in Earth’s atmosphere; Thalassa went through the process described in recent studies and mentioned in a previous blog post on the topic of primordial water being split into oxygen (which is retained by the planet’s gravity) and hydrogen (which is blown off into space) early in the planet’s history. This process is speculated to be especially common in high-radiation environments, and the Proxima Centauri system was one of them.

Many planets might have been desiccated by having all their water blown off but Thalassa’s oceans were saved because it had a lot of water, possibly because it originally formed much further out from its current position and migrated inward shortly after its formation. The partial desiccation might have even helped life’s prospects, since the oceans, while many miles deeper than Earth’s (perhaps 30 miles, compare to the Marianas Trench’s 6 miles), became shallow enough not to turn into hot ice at the bottom of the sea due to the pressure, permitting ample mixing of minerals from the rocky layers of the planet.

Life under the Tempests of a Flare Star

Thalassa’s abysses fostered primitive life early in its history, the deep ocean completely shielding the earliest life-forms from Proxima Centauri’s solar radiation, which at the surface is harsh enough to kill almost any Earth life form during solar flares and storms. The native life evolved in the deep sea much as it did on Earth, eventually adapting to the higher levels of radiation in shallower waters via the same mechanisms seen in radiation-resistant microbes on Earth.

Proxima Centauri being a flare star means storms will spew out radiation much higher than normal and solar luminosity may greatly increase. Microbes might die off during storms, casting out spores into the depths to grow and move toward the shallow waters after the storm has passed. Other strategies include simply moving into deeper waters or going into a dormant and more radiation-resistant state.

Having to develop this radiation resistance for complex life to crawl out of the seas or even occupy shallower waters means evolution may proceed somewhat slower until life is sufficiently radiation-resistant; if anything higher mutation rates may then accelerate evolution. After complex life gets going and especially breaches the surface of the ocean there might be a radiation of species greater than Earth’s Cambrian explosion.

A pale green Dot

Also because of the high radiation, recent studies suggest that Proxima Centauri b (Thalassa in this setting) likely has very strong auroras compared to Earth, likely reaching the luminosity of the full moon in the polar regions on a regular basis. This is sufficient for plankton on Earth to photosynthesize at low levels, so ecosystems may thrive as much on the night side in the polar regions just as they do in the polar regions on the day side. The green auroral glow would give the night side a distinct green tint as viewed from space, so Thalassa will look closer to a pale green dot than the Earth (a pale blue dot) does. Thalassa is likely tidally locked to its sun, though such a thick atmosphere would ensure any variation from bright pole to dark pole is very temperate. During solar storms global auroras would be triggered with luminosity routinely reaching twilight levels across the night side, triggering photosynthesis for plankton and even complex plants on the night side. There would likely be entire plant ecosystems that would grow during solar storms and remain dormant the remainder of the time on the night side.

Life by now will be well-adapted to the higher prevailing radiation levels, but solar storms still present a problem, since thick radiation-resistant skin and armor would be required to withstand the storms out in the open. Some creatures might use this strategy, but other strategies would likely be more common. One, most likely used by simpler airborne creatures, would be to mate when the solar storms begin and then send out the resulting spores or eggs into the deep sea to grow before the storm ends the lives of the parents. Another strategy used by higher flying animals, especially those that regularly dive underwater as part of their usual lifestyle, might be to simply dive underwater to a depth that attenuates the radiation to safe levels, only surfacing for brief periods when necessary to breathe; in this way the radiation exposure could be kept to a minimum.

The last strategy I can think of is to simply fly over to the night side of the planet when the storm begins, returning to the day side when the auroras on the night side die down; after all, the night side is completely protected from the radiation, and food is much richer on the night side during solar storms anyway due to the auroral illumination triggering a spring for the night-side ecosystem. This strategy will be preferred by creatures that have a cosmopolitan (worldwide) range anyway, being adapted for both day and night vision.

An Oxygen Supercharge for Thalassan Life

The evolution of complex animal life will likely follow much the same pattern as on Earth at first, evolving fish-like water-breathing creatures at first in Thalassa’s equivalent to Earth’s Cambrian explosion. Since Thalassa has no land, only water and possibly some sea ice on the dark pole (I haven’t decided on the ice yet, but I’m leaning toward no ice), the first animals to routinely breach the surface will be the native equivalent of flying fish. As on earth they will evolve lungs in addition to gills, and this is where Thalassan animal life will really take off. The returns from this evolution will be far higher than on Earth, since the thick oxygen-rich air (250 times as much as Earth!) makes flight much easier due to the increased pressure alone and the oxygen provides 250 times as much of an advantage to air-breathing life over water-breathing life than on Earth. Powered flight would almost certainly evolve among these fish to gain an advantage over predators and to prey upon other flying fish, something that never happened on Earth.

The immense power advantage from air-breathing likely generates a radiation of air-breathing aquatic creatures, which will quickly out-compete their water-breathing counterparts, likely leading to a sea ecosystem dominated by air-breathing animals. This already has happened on Earth to some extent but will be far more pronounced on Thalassa.

Flight requiring far less energy expenditure means the aerial ecosystem is far denser than on Earth, possibly more resembling Earth’s marine ecosystems in richness than Earth’s land ecologies. The fact that 50 times thicker air in pressure terms is more like being underwater also helps a lot. Many lineages, especially higher up the food chain, will surely evolve to be airborne the vast majority of the time. For these species they will at first need to birth their young by fish-style egg-laying in water but over time live birth will evolve in both water- and air-borne life, at least in many lineages, thus freeing flying animals from any dependence on the surface.

This live birth while airborne trend could easily lead to the babies of these species flying in front of their mothers right out of the womb, with their mothers’ bodies behind to protect them in case they lose their bearings while flying. Babies might also cling to their mothers and crawl up along their backs for the ride after they’re born, and only start routinely flying later in life. Another possibility is a marsupial-style pouch until the baby can start to spread its wings.

If I include sea ice in this world laying eggs on the ice is another possibility; much as Earth’s penguins do, eggs could be laid and hatched on the ice near the dark pole on the night side, and sustenance could come from animals that ultimately derive their diet from the plant life whenever it comes out of dormancy, i.e. during solar storms when the aurora shines bright.

Supersizing alien Creatures

One of the coolest aspects of Thalassa is that the higher energy levels derived from the thick oxygen means these alien birds could easily evolve to immense sizes, dwarfing even the largest pterodactyls from the Mesozoic. Dependency on thick oxygen, however, will limit larger creatures to lower altitudes, thus opening up niches at higher altitudes for smaller birds. These low-altitude creatures may evolve a filter feeding lifestyle much like whales in Earth’s oceans have; the aerial ecosystem will certainly be rich enough to support such animals.

Partial pressure of oxygen remains higher than Earth’s surface up to roughly 20 miles in altitude, so the vertical space for flyers is immense compared to Earth, opening up a lot more niches for strategies reminiscent of dive-bombing. If Earth’s penguins are any indication birds could certainly evolve to withstand the difference in pressure over this altitude range. Birds on Earth fly in air as thin as what exists at the 30 mile level on Thalassa, so this might be the upper altitude limit for Thalassan animal life’s range. The average size of birds will of course shrink with height, becoming Earth-like for birds that spend their life above the 20 mile level.

The same principles apply to insectoid life to an even greater degree than they apply to avian life; thick oxygen will allow air-breathing bugs in both water and air (but mostly air of course) to grow to sizes that dwarf the Earth’s giant bugs of the Carboniferous period, as with the birds shrinking with height to Earth-like size around 20 miles up.

How big will these creatures be? I think it is reasonable to suppose that body masses could expand by a factor of 250 times Earth’s current animals, or 170 times the mass of the Carboniferous animals (especially relevant for the insectoid life). This is, respectively, 6.25 and 5.5 times as long on each dimension. So an insect over 2 feet in length, as the Meganeura was during the Carboniferous, would be more like 11 feet in length on Thalassa, with local equivalents of Meganeura having perhaps a 15 foot wingspan. The heaviest insect today is 2.5 ounces, which would expand to 39 pounds on Thalassa. The largest known arthropods from the fossil record, the Eurypterids (a marine arthropod incidentally), could reach 8 feet in length; this expands to an incredible 50 feet on Thalassa. This is a world in which 50-foot insects could actually attack!

The barrier to an insectoid intelligence that is commonly supposed, the lack of ability for individual creatures to grow to large sizes, will obviously not apply on Thalassa, where arthropod and even insect species might easily be as large as a man. This opens the door to a sapient insectoid species, though I’m leaning against including any Thalassan insectoid intelligence in my setting; nevertheless such an intelligence might evolve on another planet of the same type, and you might see such a world appear in my setting in another solar system.

Intelligent or not, the prospects for eusocial insects on Thalassa are quite interesting, since any airborne varieties would need to keep their habitats permanently aloft; conveniently, any hive that is mostly made out of air would probably float in the lower atmosphere just from the body heat of the inhabitants. This is how bee hives keep warm on Earth, and Thalassa could easily even have a direct native equivalent to honeybees, though much larger of course. Hives could easily be several hundred feet tall and wide, perhaps making up a prominent part of the lower-altitude skyscape.

Floating Reefs in the Air and More

They won’t be alone in this skyscape; Thalassa would present an ideal environment for lighter-than-air floating lifeforms similar to jellyfish or even hot-air balloons, “gasbag” lifeforms that could use pockets of methane or even hydrogen for lift. Some of these life forms could be very large, and I imagine plants would be particularly likely to use this method as it has less energy expenditure. Plants and sessile animals form symbiotic relationships with each other on Thalassa and form airborne reefs similar to the coral reefs in Earth’s oceans. Together with insectoid hives, mobile gasbag lifeforms, and whale-sized flying filter-feeding animals, reef structures dominate many parts of the lower atmosphere on Thalassa.

Entire ecosystems are dependent on these floating reefs; the tops of these reefs provide a stable place for plants and sessile animals to tether themselves, providing the closest thing Thalassa has to land, even in many cases being covered in forests of tree-like plants, together with a forest-like ecosystem to go with them. These settings bear a resemblance to the floating islands commonly encountered in fantasy worlds.

I really like this idea of a reef-filled aerial ecosystem thick with gasbag plants and animals with flying filter feeders the size of whales trudging through while giant birds fly and giant insectoids buzz all around. The large aerial volume available opening up the door to dive-bombing style tactics together with the giant size of the creatures opens the door to a dragon-like animal, one that uses fiery breath to attack the targets of its dive bombing. This is not as scientifically as far-fetched as you might think, considering that the bombardier beetle uses a hot noxious chemical spray to defend itself on Earth. A chemical spray that actually ignites and catches on fire isn’t that much of a stretch. Another possibility I like is a dive-bombing animal that uses a sharp appendage at its front to impale its prey.

Proxima Centauri: the Place we make First Contact

The highlight of Thalassa for any human explorers when they arrive in the 21st century in my setting, however, will be a sapient species of medium-to-large-size (by Thalassan standards; somewhat larger than humans) pterodactyl-like flying creatures, with a carnivorous diet and a range extending across all altitudes and both the day and night side. These avian people, who humans will likely be called the Thalassans, will be the first extraterrestrial intelligence discovered by humans, barring the possibility I might include a deep-sea intelligent species on Enceladus in my setting. Even in that event they will be by far the most human-like; being air-breathing creatures with two wings and two legs they will even have the rough humanoid body plan as the pterosaurs did in the Mesozoic and birds do today. As I detailed in a previous post on worldbuilding avian intelligence, their lifestyle and social structure are also fairly similar to humans, being nomadic hunters who live in bands of a few dozen individuals, most of whom are at least distantly related.

There is no reason to suppose Thalassans wouldn’t have language just as humans do. Birds on Earth have a syrinx which is capable of making as many sounds as humans do with the larynx, and as talking birds demonstrate can even be used to speak human languages. Almost any animal could vocalize the same way a crocodile can, but I will have the Thalassans have a vocalization organ more similar to a syrinx, which will make sounds similar to Earth birds but due to the larger size the pitch will of course be lower.

The Prospects for Thalassan Technology

The Thalassans in my setting have never adopted agriculture for various reasons, but have a culture much more sophisticated than terrestrial hunter-gathers. They might have one or more domesticated or at least tamed animal species to use as hunting aids and as companions, much like how human hunter-gatherers domesticated the wolf. Thalassans have no ready access to metal or stone and only have lumber from the few trees on the airborne reefs; bone, skin, and leather from animals would be the primary materials of this culture.

The equivalent of paper for the visual arts and writing could easily be provided by animal skin. Human hunter-gatherers never developed writing, but commerce and science will advance far more on Thalassa due to the much greater travel speeds of avians enabling many more people to be reached and thus a greater division of labor to be achieved. This could easily stimulate the invention of writing; libraries, art galleries, and other repositories of information and resources could be floated by the gasbag plants or carried on large flying creatures’ backs. The gasbag creatures floating the libraries and galleries could even be drug along with the nomadic band by these avian beasts of burden. Such techniques could also be used to float or fly buildings in mid-air for doing work (e.g. scientific experiments), though most Thalassan economic activity could easily be done while flying or soaring, so Thalassan habitation of buildings will be quite limited.

Bone can be made into glass; though I haven’t found any resources on transparent bone glass, but since translucent bone glass exists I am assuming in this setting Thalassans are able to make glass. This is important since this enables Thalassans to build microscopes and telescopes, uncovering whole new realms of scientific knowledge. Thalassans, due to their unobstructed view of the sky and ability to write down their observations, will already have sophisticated knowledge of the night sky even before they have telescopes; among other things, they will be able to figure out their world is round much more easily than humans were due to being able to circumnavigate quickly from time immemorial.

Once they discover the telescope they will be able to resolve the discs of any other planets nearby in the Proxima Centauri system and might eventually be able to observe any planets in the nearby Alpha Centauri A and B systems; if nothing else the orbital motions of the two stars of Alpha Centauri will be easily observable. The microscope opens the door to discovering bacteria, including those that cause disease, earlier and might stimulate an early discovery of the germ theory of disease, hygiene, and sanitation.

In a Bone Age (Paleo-ossic rather than Paleolithic) culture like the Thalassans have wood will be more or less unavailable to use as fuel, but as the Inuit of Earth’s Arctic demonstrate animal fat, which on Thalassa is an abundant resource, can substitute. Thalassa’s gasbag lifeforms also have a lot of biologically-generated methane and hydrogen in them which is combustible fuel. These sources will constitute the Thalassan fuel supply. On the path they are on they will eventually filter the trace elements out from ocean water, discover uranium, and figure out how to build nuclear reactors in time, but my current plan for them involves humans making first contact with them before this development happens.

Contact will enable them to acquire nuclear and spaceflight technology from humans, but they might be surprisingly close to achieving spaceflight even before contact. Methane and hydrogen rockets can, after all, reach orbit, and the native life is already generating a lot of the stuff; if dragon breath can evolve biological rockets are perhaps the logical next step. If nothing else the Thalassans could make crude rockets out of biological materials. These may be no more sophisticated than early Chinese models, but the possibility of reaching space or even orbit either biologically or technologically using such primitive rocket methods cannot be ruled out altogether. Merely expanding on the gasbag principle can get them a good part of the way there; balloons routinely reach 10-20 miles in altitude today, the record being 33 miles; on Thalassa this would be 53 miles up, which is pretty high.


That wraps up the location of Thalassa and its flora, fauna, and people that I am building for my science fiction setting. In my world this is the first planet humans will encounter that has aliens on it they can talk to, the first planet that has an aerial as opposed to underground ocean ecosystem, and the first planet where they can walk outside without a spacesuit or pressure suit; while a diving suit is needed at the surface 20 miles higher up the partial pressure of oxygen is the same as on Earth. Alien birds abound, including much larger specimens visiting from below, some with fiery breath. Thalassa when the time comes in my setting’s history will be a fascinating place to explore and colonize, and the Thalassan people might prove a fast friend of humanity in our collective spread across the cosmos.

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