The Horizon System: Horizon, Lost in Paradise
Emp
Horizon (Temperate Marine Terra, Planet)
System - Horizon-Actinophrys
Mass - 0.515 Earths
Radius - 5,588 kilometers (0.872 Earths)
Global Average Temperature - 37.81°C
Day Length - 12h 3m 56.2s
Year Length - ~2.950 years
Number of Satellites - 2
ESI - 0.901
Etymology - From the concept of a horizon; a teleological line in the distance to which all elements of the setting converge.
Overview
Though it is the birthplace for a form of life which crosses the stars, Horizon is nothing at all like our own Earth. With just half the mass and two-thirds the gravity, yet a choking carbon dioxide atmosphere three times as thick, Horizon is perhaps more like a habitable version of Mars than any vision of Earth. Trapped in a co-orbital resonance with the much larger Panthalassa, Horizon circles its sun in just under 3 Earth years - though the length of its year changes slightly as it swaps orbits with its larger companion.
Horizon’s thick atmosphere and rapid rotation help it effectively distribute heat around its globe, preventing its oceans from freezing catastrophically despite the planet’s proximity to the edge of the habitable zone. Receiving only about half as much sunlight as Earth does, conditions on Horizon’s surface are nonetheless almost always muggy and tropical - though this is far from the norm for its history. With abundant precipitation and active volcanism, this young world’s world ocean is pierced through with continents of jagged aluminum lavas and cave-riddled carbonate platforms, providing the perfect environment for exotic forms of life unlike any seen on Earth. Its five continents, covering a combined surface area equal to the sum Africa and South America, have arranged themselves in a manner similar to Earth’s in the Campanian, encouraging the growth of near-global rainforests.
Though an unprotected human would be killed in seconds by Horizon’s lethal atmosphere, plenty of life has found a way to make this strange and beautiful world a home.
Lifeforms
Though Horizon is the birthplace of all life in its namesake system, its alien nature has cultivated a biological system completely foreign to our own. Two-thirds of Earth’s gravity and a soupy CO2 atmosphere are not insurmountable challenges, but necessitate some odd adaptations regardless. Though now a choking, humid hothouse, Horizon has historically been a frigid world; its history does well to reflect that.
Life on Horizon is not altogether foreign biochemically from the Earth organisms we are familiar with, with a few key differences. Though their genome is composed of DNA, it has six bases instead of four to accommodate an extended set of protein-building amino acids. Elements such as arsenic, lead, and vanadium are not toxic to Horizonian life and are in fact vital components of its biochemistry; their vanadium-based hemoviridin blood does not bind to CO2, allowing them to survive atmospheric concentrations exceeding 65%.
For most of Horizon’s history, it was a cold, dim world hidden beneath a shroud of CO2 clouds. To adapt to this dim, cold environment, two strategies were formed; the blue-green graptolite trees adapted to make the most of poor sunlight, while the reddish jellytrees and hive corals adapted their strange colors to stay warm and growing as long as possible. All of Horizon’s terrestrial flora are photosynthetic animals - polypous, sessile beings not unlike corals or bryozoans, with leaves of flesh and stems of gnarled cellulose. Competition between flora for light and living space is fierce; some host armies of defensive symbionts to rip apart competitors, others grow phalanxes of jaws or spines to pluck off fouling organisms, while yet others may project mesenteries covered in digestive fluids to eat their neighbors alive. Though they fix large amounts of carbon to fuel their fast metabolism, their dead tissues decay quickly, preventing their rapid rate of photosynthesis from freezing the planet over.
The division between fauna and flora on Horizon is murky to the point of nonexistence. Critically, all the major members of the Horizonian fauna engage in a haplodiplontic life cycle, with multicellular haploid and diploid stages, as opposed to Earth animals who only have a diploid stage. Furthermore, the dominant floral and faunal group Neozoa descend from a photosynthetic ancestor, giving all lineages of ‘animals’ on this world the ancestral ability to photosynthesize. Oftentimes, a single species can have both ‘plant’ and ‘animal’ stages; the insectoid armies that jellytrees use to defend themselves are in fact the haploid stages of their own species. Even the megafauna are involved in this madness; the saurian mollusks of Xenotheria range from leaf-winged fliers to elephantine giants overgrown by their vegetable progeny, the echinodermatous Triskeliozoa include all sorts of green anemones and walking crinoid-palms, and even the mammaloid annelids of the Squamatozoa have among them a smattering of leaf sheep and virescent sail-backed Dimetrodon analogues.
“From the depths of the abyss to the edge of the Galaxy, we hold the line.
For as long as the cosmos shines a blazing blue, we persist uninhibited.
In the face of utter destruction, we regenerate again and again.
For we are the Will of Horizon.
And we shall live forever.“
- The Nexus-Emperor, “Emerald Stars Upon Horizon“, Declaration of Intents (2268)
Past & Future
For the majority of its history, even Horizon’s thick atmosphere was insufficient to keep it much warmer than 0°C. Since its formation and the stripping of its primordial atmosphere, it has spent around 80% of its life with polar ice caps. This is in contrast to Earth, which has spent the vast majority of its life as a greenhouse world. Up until about 200 million years ago, it was cold enough for carbon dioxide to partially condense in the high atmosphere, forming an insulating layer of dry ice clouds which shielded the surface below from most light. Nevertheless, complex life appeared and thrived in this environment, diversifying from a Charnia-like ancestor starting some 300 million years ago.
Horizon is the only planet in its namesake system to have naturally evolved a sapient species. Around 1.5 million years ago, a species of lizard-like arboreal xenothere native Horizon’s equatorial montane rainforests became sapient, eventually giving rise to a spacefaring civilization that spread Horizonian life far and wide across the Gordian Reach some 1 million years ago before abruptly vanishing, leaving behind only derelicts and the petrified remains of their biomechanical cities about 200,000 years after the peak of their civilization’s extent. Unlike humanity, this civilization did not extensively modify the environment of its home planet, but rather had an outsize effect on the natural history of the galaxy as a whole by seeding many thousands of worlds with Horizonian organisms. But all that is left of these people themselves are a few pastoralist communities clinging to life on Horizon’s Southern Continent, having long since forgotten the majesty of their predecessors.
Civilization
To the relict populations of native Horizonians left on the planet, Horizon is home. Much like ancient humanity, they do not know of the true expansiveness of their world - beyond the shores of the Southern Continent, all is terra incognita to them. Most live simple lives, tending to flocks of arboreal grazers or harvesting the sporophores of graptolite-trees, unaware of the expansiveness of the universe around them or even the cosmic past of their ancestors. In the absence of their technological civilization, they have very much become a part of the ecosystem again - many of the Southern Continent’s predators are more than happy to eat them, and in the rigors of nature they rarely live for more than ten of their years. Though this is an existence filled with fear, it is perhaps preferable to the cosmic immensity of the Gordian Reach where you are but one man among trillions.
But that is not all that Horizon is. For the last four centuries, a new interstellar civilization has come to call Horizon and all their holdings from a million years ago home. Founded by a persistent relict from the golden age of Old Horizon and a bunch of foreigners from across the Six Great Clades, this new ‘Horizonian Imperium’ has worked more-or-less continuously to restore the technological and cultural titan that was its predecessor. Though both societies share a fascination with life and the creation of synthetic lifeforms, the new ‘Horizonian Imperium’ is not a society of ordinary men but one of alien minds spread across light-years of space and thousands of bodies, who are essentially divine immortals in comparison to their predecessors. Nonetheless, their presence has revitalized the previously crumbling cities of Old Horizon, giving its natives the chance to experience one of the wealthiest and most culturally rich societies in the Galaxy.
