Where is the nearest Earth twin? It’s a crucial question for my sci-fi fictional universe, that I try to base on real scientific information about exoplanets but also keep interesting. Habitable planets abound in the closest star systems, but none are quite Earth-like. Proxima Centauri’s world is all ocean and drenched in oxygen 50 times thicker than Earth’s at sea level: breathable miles up, but not a true Earth twin. Sirius’s planet is breathable at the surface, the closest world to Earth that answers to that description, but its stark majesty makes it the opposite of lush or verdant. So where is the closest world where you’d see not only breathable air and swimmable oceans, but lush green hills, forests, trees, and even sandy beaches with driftwood fit for a bonfire?
Well…I’ve basically decided it will be Mu Cassiopeiae; at a distance of 25 light-years from Earth, it’s close in cosmic terms, but is a challenge to reach in an era dominated by nuclear pulses and solar sails: only the assistance of a laser beam or antimatter propulsion would enable mankind to reach it in a reasonable stretch of time.
When images stream back to the mother planet, it’ll seem extraordinary: the closest thing humanity has found yet to an Earth twin…only Mu Cassiopeiae’s garden world is a planetary-mass moon of a larger body, and is far older than Earth, at 12 billion years old.
It’s a world dominated by deep liquid water oceans and low platforms of rugged continents dominated by giant trees, in form not unlike the redwoods of Earth, but stretching miles tall into the air, their roots stretching into aquifers miles deep into the ground. Massive upwelling occurs seasonally, making the climate mild and extremely foggy. Northern Californians might feel it familiar…only it would be heightened, uncanny, like their homeworld filtered through the vision of dream logic.
The continents of this world stretch from north to south, pole to pole, ringing the planet around the terminator — that is, the the limb of the moon as seen from its parent planet — due to gravitational stresses on the moon and the geological dynamo that is the core of the planet having long since frozen out and run down to zero as far as tectonic activity is concerned. This reflects real dynamics we see in real moons, by the way; Iapetus is similar. It’s just that we’re not used to seeing this in an Earth-like body.
If you’re wondering if I’m setting this up to be some sort of dream-world version of Northern California’s Redwood Empire…well, you’re not wrong! In my universe, narratively, this is man’s first real reward for stretching our powers of exploration beyond the very closest stars, and into the wider universe. A mere Earth twin would be too uncreative, so what I’d like to depict is a place that’s like Earth but filtered through a dream. Everything heightened, even subtly uncanny, yet not sinister.
Underlining that feeling is how the air is thick compared to Earth; we have 0.78 bar of nitrogen in our atmosphere, but this planet will have perhaps 1-2 bars worth of nitrogen. Enough for the pressure to be noticeably higher, but below the level that causes nitrogen narcosis in humans. Not instantly breathable coming off a conventional spaceship, but after a period of adaptation it would be of no concern (deep-sea divers use weirder atmospheres than that sometimes…). The oxygen is also thick compared to Earth, being perhaps 0.5 bars worth (compare to Earth’s 0.21 bars); high, but breathable. The usual trace gases are also found, perhaps in different proportion to Earth standard, but nothing irritating to a shirt-sleeved visitor.
The oceans? Salty, but unlike in Sirius’s world they’re not briny, and unlike some worlds that might be out there they’re not mixed with ammonia or some such; denizens of the Redwood Empire would find them eminently swimmable, even familiar.
The entire environment, while alien, is surprisingly, even eerily, Earth-like…at least until you look up into the sky. Up in the sky will loom an oceanic planet 5 Earth masses, appearing nearly motionless (the planetary moon is of course tidally locked to its primary), looking like a deep blue watery disk suffused in clouds.
Since the continents cluster around the limb of the planet, its primary as viewed from the land surfaces always appears near the horizon, big and low. Call it Nereus, after the old god of the sea, with the moon being Thetis, his daughter.
These low and old continents are dominated by these miles-tall redwood-like trees, with Earth-like beaches — mild, fogged-in, and with large waves for surging — abound where these forests meet the sea. Amped up due to the thick air…and the low gravity: this moon has only half the gravity of Earth, and perhaps weighs in at a quarter of an Earth mass.
The biosphere is so old — 12 billion years, if it’s of comparable age to its planet — that there has been ample time for specialized niches and even for intelligence to form, yet there is no sign of intelligent life apparent to the explorers, no matter how hard they look…aside from the fossil layers of radioisotopes generated by long-vanished civilizations billions of years in the past, which otherwise by now have left no trace.
Despite it seeming “dumb” at first glance, however, there is something extraordinary on Mu Cassiopeiae: these trees are neural nodes of a kind of biological Internet that spans the planet. Starting with something like the mycelial fungi that use chemical signals in between trees on Earth, this connection grew in sophistication over millions and billions of years, evolving to utilize bioluminescence, and eventually culminating in something like biological fiber optics, which evolve to use shorter and shorter wavelengths, up to soft X-rays, leading to the trees, by the time humans arrive, glowing with the tell-tale blue of Cherenkov radiation in their internal fluids and saps, as well as the mechanical stresses of these signals inducing low-frequency sounds like a hum. Very alive and eerie.
This global-spanning forest even takes advantage of neutrino signaling. If the solar system spent time deep in the galactic core, as it probably did at some point, it could have been enriched with exotic super-heavy elements that form equally exotic crystals that can block or modulate neutrinos (note that this is very speculative, but is not inconsistent with real physics).
Plants could extend their roots into these crystals deep underground, taking advantage of their neutrino-sensing properties to feel when a surge of radiation was about to happen. Nowadays this would all be rather esoteric (why would plants evolve such an ability!?)…but when Mu Cassiopiae was young, the Milky Way was itself an active quasar galaxy, so especially near the galactic core this sort of sense would have been very important!
Over time as part of the development of this biological Internet that spans the planet, these crystals were not only used for sensing neutrinos but for actively modulating neutrino flux, by manipulating them to block the streams of neutrinos that come in via solar sources, sending signals through the planet’s interior directly on a straight line faster than they could ever have been sent across the curved surface. Now, in our quiescent era, they function as shutters for solar neutrinos, but this ability evolved out of an environment of a world older than our galactic disk itself sensing the screams of the supermassive black hole at the center of our galaxy.
That’s already freaky enough, but where it gets not only freaky but fancy is when you consider that over time, DNA could be repurposed as a storage medium for this planetary-scale computer (as has been proposed in real life for our own technology), and like the proverbial elephant, never forgetting would be an advantage…
Over time this communication system evolves a memory, encoded in these trees in their very genetic structure, so there’s the possibility that the screams of the galactic quasar from 12 billion years ago are encoded in these trees’ DNA, perhaps as a base layer that is shared by all life on this planet, part of its genetic code that functions as the most basic layer of this global-scale biological internet. Still recorded for anyone smart enough to unlock its secrets…
Something like tree rings that become fossilized…and later revealed near the surface on the oldest cratons, a la what we see with rocks in South Africa, but with said rocks being 12 billion years old instead of “only” 2 billion years, could reveal annual growth patterns characteristic of radiation storms, that match neutrino fluxes sent out today at the most basic-layer common level, and then human scientists could piece two and two together. Galactic memory from a time closer to the Big Bang than to us…and all accessible a mere 25 light-years away, instead of some exotic location in the deep core or some such like one might expect. Huh.
It’s worth noting also that such a biological Internet that can “feel” across a whole planet would by necessity become aware of gravitational waves. Which sounds insane, but how gravitational waves work in the first place is that they flex and bend the planet by an amount that’s measurable at these scales…but, alas, this wouldn’t be particularly relevant, since the planet couldn’t use this for communication, nor might it be aware of the true significance of the waves it feels pulsing through it.
But it would have some degree of awareness, if only animalistic…which goes from ordinary to fascinating once you contemplate what it would be like on a planetary scale. Mu Cassiopeiae’s world is a fascinating place: it’s by no means sapient, but it is sentient.
Between these flashes, the fog, the big waves, the low gravity, the thick air, and no doubt numerous flying creatures, not to mention oxygen-breathing aquatic creatures that evolve to be far larger than our whales (that thick air gives them so much energy…), and the looming bright big blue primary near the horizon — oh, and not to mention there’s even a red-dwarf companion star orbiting far out (!) — Mu Cassiopeiae would very much feel like Earth, the pale blue dot, but it’s the pale blue dot filtered through a dream.
Maybe it’s not the biggest or the baddest world; maybe it doesn’t occupy many superlatives that a computer would assemble onto a list. But from a human point of view, Mu Cassiopeiae may well be the one world that’s the most memorable. And in our journey to the stars, those are the worlds worth living for…
This place feels so familiar yet utterly alien, like stepping into a dream where every detail is heightened but nothing feels threatening. The idea of these trees as neural nodes is mind-blowing—their bioluminescence and Cherenkov radiation create this eerie, living atmosphere that’s both beautiful and deeply strange. The way the environment is thick with nitrogen and oxygen, making it almost instantly breathable after adaptation, adds to the sense of a world that’s almost an extension of Earth. But the truly unsettling part is the global biological internet, with its memory of a galactic quasar embedded in DNA. It’s as if the planet itself is a living relic of a time long past, and the trees are its conscious witnesses. It’s a reminder of how much we might be missing in our own search for life—life that could be right here, in our own backyard, but on a scale we can barely comprehend.