An Exoplanet Menagerie for SciFi Writers

What would it be like to stand on an alien planet? Here's a quick tour of recent discoveries.

It’s the golden age of planetary astronomy. We’re drinking from a firehose of new data about what other stellar systems are like, but it’s still possible, as a non-expert, to follow along.

One of the easiest (and most fun) ways to do this is to occasionally drop by Arxiv and browse the titles, or do a search on, say, “Trappist-1” or “Proxima-b.” Sure, there’ll be math, and 90% of the terminology will be over your head; but even as a non-expert, you can glean some amazing stuff from just this one source. One example from last week is this paper: “Properties of Free Floating Planets Ejected Through Planet-Planet Scattering.” It’s dense stuff, and based on simulations of orbital evolution rather than direct observation—but the authors come to a startling conclusion: about half the planets in the galaxy are orphan worlds that do not orbit a star. They wander forever through interstellar space, invisible, undetectable.

My mind works by connecting the dots between such facts. I remember a previous paper, from several years back, that pointed out that orphaned super-earth or hycean worlds (heavy ocean worlds) could retain habitable conditions on their surfaces for billions of years, even without heat from a star. Put these two ideas together and we have the following worldbuilding premise:

There is a vast, galaxy-spanning civilization that none of the star-centered empires are aware of. It is spread across tens of billions of orphan super-earths, who communicate directly with one another by laser, and whose inhabitants travel only among these worlds. What happens when a human ship accidentally stumbles upon this reclusive, paranoid civilization?

At Play With the Real

If you treat science fiction as a form of fantasy, you can imagine other worlds however you want, and ignore the wonders we’re discovering in the golden age of planetary astronomy. And that’s fine—it’s perfectly valid for you to focus on important qualities such as traditional storytelling values, character, and theme. There are many mansions in the house of SF.

The traditional values of space opera and ‘planetary romance’ don’t float context-free, though. They reflect the culture that spawned them. So, characteristically Western narratives about other worlds are freighted with Western assumptions and values, which influence more than just the characters and plots of the story. In the case of science fiction, they also affect the worldbuilding.

If you make an extreme postmodernist assumption that ‘reality’ is a social construction, then science fiction as a literature that explores (at least some) objective reality, doesn’t and can’t exist. It can only act as a mirror to the social and cultural milieus in which it was written. If this is true, then we’re not obligated to follow current astronomy, because that’s just another cultural construction. We could go further and treat it as a form of scientism, and therefore as yet another colonialist narrative.

On the other hand, if even after it’s been thoroughly massaged by Kuhn, Feyerabend, Latour, and Barad, we find that science can still make universally true statements, then it could be a (perhaps the) culturally neutral narrative. As my character Gennady Malianov puts it when describing plutonium, “Some things are real in any world.” From this perspective, the most scientifically accurate stories carry the least cultural baggage. Even if you accept that a large amount of what we consider ‘scientific fact’ is cultural construction, the residue that is not would seem to give science fiction the potential to be the least colonialist literature.

This is ironic, considering how right-wing and triumphalist I find most ‘classic’ hard SF. (For example, the argument about genetic determinism at the core of The Mote in God’s Eye could easily be seen as a dog-whistle for racist politics.) One of the defining features of space opera is the idea of the interstellar or galactic empire, which everybody from Isaac Asimov to Ann Leckie uses. Manifest destiny of and for human beings seems to be the driving philosophy of space opera—and, by association, with much of ‘hard’ science fiction. It would be too bad if people have come to associate science-driven SF with Musk-like libertarian, white-male expansionism. That interpretation would be wrong—Le Guin wrote hard SF—and it might ultimately make it harder rather than easier for cultures to meet. Because science, for all its subjectivity and other faults, is the closest thing we’ve got to a common ground.

The Majority of Worlds

Assuming you’re interested in what real stars and colonizable planets are like, then our vision of the universe shifts significantly from the one we inherited from space operas written before the planetary Renaissance.

You might remember from an earlier post that the term ‘habitable’ that scientists use does not necessarily mean colonizable. A hycean planet with a crushing three g’s of gravity, a hydrogen/methane atmosphere of 10 bar, and a planet-wide ocean with a surface temperature of 90C is perfectly habitable, because life could exist there. But it would not be colonizable by humans; we couldn’t live on it. I use the term colonizable to refer to worlds that might not be able to sustain life on their own, but that could host human colonies. Mars, for example.

If we start by investigating planetary abundance, we see first of all that, yes, about half the planets out there don’t even orbit a star. According to the simulations in the above-cited paper, most will be small, rocky worlds, because of how scattering happens in young star systems. So our starting point is a galaxy full of tiny, frozen, isolated, and invisible Mars analogs.

When we turn our attention to stars, we find that about 70% of them are red dwarfs. These are tiny compared with our sun, and they only light and warm worlds that are very close to them. Their surface temperature is about the same as the filament in a 50-watt halogen bulb; consequently their light is golden, further reddened if you’re looking up through an Earth-like atmosphere. The sky of such a world would not be blue, but lavender, and the brightest it would get would be about equivalent to ordinary indoor lighting. Some theoretical studies suggested that red dwarfs wouldn’t emit enough light in the right frequencies for earth-like photosynthesis to work—but then somebody actually tested whether plants could survive under such conditions, and they grow just fine. Chlorophyl’s not optimized for the spectrum, but we know of other molecules that are, and they are likely to result in purple and brown grass and trees.

Despite the dim golden quality, this sunlight could feel warm because red dwarfs radiate more infrared than sunlike stars. You could also end up with a nasty sunburn if you spent much time outside, because paradoxically, these stars also emit more ultraviolet than our sun. They even erupt random x-ray bursts from frequent flares. Plants and animals could shield themselves from the UV, but the x-rays are a bit harder; they might result in most plants residing a meter or more under the surface of lakes and oceans. Or, they may evolve more efficient DNA repair mechanisms or use a more robust genetic molecule.

The weirdest characteristic of such planets (most of which will be super-earths, 1.3 to 2 Earth-radii in size and with 1.2 to 3 gees of gravity) is that they’re usually tidally locked, keeping one face eternally pointed at their sun, the way the moon faces the Earth. On such worlds, earthly plants’ strategy of sprouting leaves in all directions to catch light at different times of the day wouldn’t make sense. Instead, there would be competition to get in front of your neighbour and directly intercept light from a predictable, unchanging direction. Think solar panels, not leaves. This could result in bizarre forests where every possible chink of light from the sun has something covering it, and eternal darkness reigns on the forest floor.

Cities in the safe twilight zone of a tidally locked red dwarf planet

Cold tidally-locked planets could be “eyeball worlds,” shrouded entirely in ice except for a circular open ocean at the sunward pole. Eternal storms would rage around this.

Sometimes the tidal lock breaks; every now and then, a planet could go from millions of years of stable orientation to a chaotic regime where the side facing the sun switches, maybe once a century for a while, then once in a thousand years, then twice in one year. The aliens in The Three-Body Problem evolve on such a planet. Chaotic flips like this don’t require a binary star, they can perfectly well occur for worlds orbiting a single sun.

Astronomers have tried to deduce what kind of atmospheres red dwarf planets have—or whether they could have any at all. Most studies suggest that all the air would be stripped off Earthlike worlds in the habitable zone of red dwarfs by their tremendously powerful stellar winds. But, most such studies have assumed planets without strong magnetic fields or ozone layers, and also that ‘secondary atmospheres’ such as Earth has today would not be replenished by outgassing caused by internal heat. Our instruments aren’t strong enough to resolve the question—the James Webb telescope didn’t find an extended atmosphere on either Trappist-1b or Trappist-1c (two red dwarf planetary superstars) but the detection doesn’t rule out a thin nitrogen or oxygen atmosphere on either.

Even if these planets, or Proxima-b, the closest Earth-sized exoplanet, are not habitable, they could easily be colonizable. In fact, regardless of its atmospheric conditions, there are almost certainly areas on Proxima-b that would be easier to settle than Mars. On its dark side or twilight areas, for instance. Its gravity is nearly the same as Earth’s, and that fact alone makes it more welcoming than the Red Planet. An orbital civilization is harder to maintain, because of the fierce radiation and solar winds.

There can be no large moons around large terrestrial red-dwarf planets, because instabilities in the orbit caused by the closeness of the star destabilize them. There might be rings, though, and if you live on the permanently dark side of such a planet, their reflected sunlight may be the only sunlight you see. Auroras can be expected to be much stronger if the planet has a magnetic field, so picture a colony on Trappist-1e’s dark side, its dark lavender skies lit by a golden ring and pulsing auroras, and the wandering light of six nearby planets, each bigger and brighter than the moon is in our sky.

Just as red dwarfs are the most common star, super-earths seem to be the most common type of planet. Which is odd, because our solar system doesn’t contain one. Habitable super-earths are not colonizable, partly because humans would find it hard to adapt to weighing twice what they do at home; and partly because after a certain cutoff point, around 1.5 gravities, it becomes impossible to launch anything into orbit using chemical rockets. Only hideously poisonous nuclear technologies could get you off a super-earth. Visiting one of these worlds would likely be a one-way trip.

One implication of this is that technological civilizations might be common on this most typical kind of planet, but spacefaring cultures originating on them would be vanishingly rare. Most starfaring aliens likely come from Earth-sized worlds.

Hycean and Ocean Worlds

Many exoplanets have densities less than Earth’s. The Trappist-1 planets are like this, which suggests they are mostly water worlds. Such planets can have oceans hundreds of kilometers deep, with high-pressure ices at the bottom that cover the mineral-based mantle. Unless there are volcanoes or other ways that minerals can break through this ice barrier, these ocean planets will be barren—their 100% pure water won’t have enough carbon and other elements for life to develop, even if they’re comfortably warm and have oxy-nitrogen atmospheres.

However, if volcanic sources can break through the ice layers, aquatic life could flourish on them.

The weirdest ocean worlds are the hycean planets, which are much bigger than Earth, and shrouded in hydrogen atmospheres. Hydrogen Ocean—hence, Hycean. You’d be much heavier on a hycean world than on Earth, but if you spend all your time in the water, maybe that would work out? Typically, they’ll have very thick atmospheres, and they’re vast—their horizons would be far more distant than we experience here. Not really colonizable, but hycean worlds are expected to be common—maybe as common as Earth-sized terrestrial worlds. This makes them significant for our alternate vision of space opera. If I were writing about one, I’d have my human explorers in orbit, using telepresence bodies (like in Avatar) to explore the oceans.

While life on hycean worlds could use photosynthesis, their oceans are likely shrouded by thick clouds and such planets have to be much farther from their stars than Earth is to the sun to have Earthly surface temperatures. Hydrogen’s a greenhouse gas.

So picture endless, stormy dark seas, under brooding gray clouds at the peak of noon, otherwise black as night the rest of the time. It sounds bleak—but life in these waters could use a very powerful methane-hydrogen reaction as good or better for metabolism than oxygen. If they have enough other elements dissolved in them, dark hycean waters could be absolutely packed with life.

The galaxy is full of hycean worlds, and since they’re at least as likely to have life on them as terrestrial planets, they’re natural settings for adventure and exploration.

Fallow Worlds and Super-Venuses

When the powerful stellar winds of a young star strip the atmosphere off a new planet, they’ll often take the water with it. When this happens, the water gets broken into its components, oxygen and hydrogen, and the lighter part (the hydrogen) is blown off into space. What’s left is a deep well of oxgen.

For science fiction, this process gives us two really interesting kinds of world to play with. The first is what I call a super-Venus. These are like our Venus, but instead of being shrouded in hundreds of atmospheres of crushing carbon dioxide, their surfaces endure hundreds of atmospheres of pure oxygen at temperatures that would melt lead. If you thought Venus was a hostile environment, it’s positively placid compared with these places. Essentially anything on them that can be oxidized is, meaning explorers have very few materials to use in building probes or landers. Everything will burn on its way down, even titanium. If you want to hide something somewhere nobody can get at it, drop it on a super-Venus.

If there wasn’t too much water to begin with, you’ll get a thinner atmosphere. If it’s thin enough, it could be breathable (pure oxygen is used sometimes for astronauts, although the Apollo 1 tragedy shows how dangerous it can be). A planet with a breathable oxygen atmosphere, but no life, I call a fallow world. Such worlds will be rare, but they are the ultimate prize in interstellar colonization, because you can land on one and step onto its surface without a pressure suit. If there’s no local life to kill you, you can immediately introduce earthly plants and animals (if there’s still some water, and for plants, if there’s a trace of CO2, which there almost certainly will be). These are planets that don’t need to be terraformed; just scatter seeds and they will become gardens without further intervention.

Twenty years ago nobody imagine you could have an oxygen atmosphere that wasn’t produced by life. Now, it looks like they’re common as heck—the only question is how thick they are. For us as writers, this provides a way back to the classic space opera planet that’s just landed on and settled.

The only irony is that such planets are more likely around flare stars such as red dwarfs. You’re unlikely to find one in an earthly orbit around a G-type sun, because our sun doesn’t broadcast the kind of energy that strips oceans. So a fallow world is also likely to be a tidally-locked, UV and x-ray blasted eyeball.

But that’s about as safe and welcoming as exoplanets get.

More to Explore

This is just a hint of the exoplanet discoveries made in recent years. I encourage you to follow the Arxiv literature, because new findings come in daily. Climate modeling of these aliens worlds is particularly interesting; I’m reading about permanent storms and global heat transfers that can result in an unlivable sunlit side, and a balmy and comfortable dark one. New forms and alternatives to photosynthesis are being discussed, and the crustal dynamics of super-earths is a hot topic. All of these can drive spectacular worldbuilding for new science fiction that steps past the colonialist fantasies of 1900s space opera and imagines what life on real worlds might be like, and what settling them might involve.

Next week I’ll return to the topic of foresight with some more tools to help you think about time. In the meantime, enjoy, and don’t forget to look to the skies now and then.

—K

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