The Horizon System: Acidianus-Thermococcus, at the Edge of Existence
Tur
Acidianus (Temperate Marine Terra, Planet)
System - Horizon-Actinophrys
Mass - 0.903 Earths
Radius - 6,559 kilometers (1.030 Earths)
Global Average Temperature - 35.11°C
Day Length - 12.64 days (Mutual Lock)
Year Length - 1.625 years
Number of Satellites - 0
ESI - 0.892
Etymology - From the thermophilic, extremely acidophilic archaeon Acidianus infernus, for its ability to survive in extremely hostile chemical environments.
Thermococcus (Temperate Marine Terra, Planet)
System - Horizon-Actinophrys
Mass - 0.342 Earths
Radius - 4,446 kilometers (0.698 Earths)
Global Average Temperature - 18.77°C
Day Length - 12.64 days (Mutual Lock)
Year Length - 1.625 years
Number of Satellites - 0
ESI - 0.904
Etymology - From the thermophilic archaeon Thermococcus gammatolerans, the most radiation-resistant organism on Earth.
Overview
The inner edge of Actinophrys’s habitable zone is home to a wide pair of magnificent, blue-green worlds; a wide binary dancing to the dulcet tones of the musica universalis. But though their tones may resemble those of Earth, these two planets are nothing like our accommodating home world.
Acidianus and Thermococcus are acid worlds. Their oceans are filled with not with alkaline saltwater but a dilute solution of sulphuric acid not chemically dissimilar from the inside of a human stomach. Even on land, where freshwater rains fall, abundant volcanic activity quickly pollutes the water with acids. This harsh chemical environment is directly responsible for the worm-eaten appearance of rain-soaked Acidianus and the swirling, chromatic dunes of arid Thermococcus. Seemingly not content with just being acidic, both planets are also unusually radioactive, with thousands of natural nuclear reactors scattered across their volcanic plains.
Though we would be hard-pressed to find Earth lifeforms able to survive and thrive in these conditions, Acidianus and Thermococcus are nevertheless host to a thriving biota of strange life.
Lifeforms
Though Acidianus and Thermococcus are much less hostile than the likes of Riftia and Tonicella, the acidic conditions still require native life to undertake a suite of unusual adaptations in order to survive. Unlike most Earth acidophiles, their cells do not attempt to keep out the acid, instead hardening all of their internal structures against low pH. Like the inhabitants of Riftia, they store their genome as a DNA alternative, but as a pH-resistant peptide nucleic acid. Thus, Acidianian plant and animal life can thrive in solutions as harsh as battery acid, in which only the toughest of Earth archaea can barely survive.
Due to their proximity and similar surface conditions, the biological histories of Acidianus and Thermococcus are closely intertwined. Their floristic history in particular is riddled with panspermia events as lineages migrated from one world to the other and then back again, escaping extinction or seeding new floral dynasties. All descend from a violet algal ancestor native to Horizon’s oceans which crawled onto land in the absence competition from photosynthetic animals. Unlike Earth plants and the vast majority of Horizonian eukaryotes, these flora do not have alternation of generations, reproducing in a manner similar to the one used by Earth animals. Acidianus’s forests are dominated by pteridosperm-like forms that thrive in wet conditions, while the vegetation of drier Thermococcus is highly adapted to drought, oftentimes resembling desert plants on Earth.
The fauna is much more divergent between Acidianus and Thermococcus than the flora is. They are not closely related to the fauna of Horizon, instead being a ‘primitive’ eukaryote lineage closely related to Acidianian ‘plants’. As such, they do not have the alternation of generations that Horizonian fauna do, making them much more similar to Earth animals than any of their otherworldly brethren. With their relatively delicate physiology they have a much harder time world-hopping than their Horizonian counterparts, but a fair number of evolutionarily significant exchanges have occurred in the 120 million years of their existence.
Acidianus’s warm seas and humid, acid-eaten rainforests are rich with life. Most life on land belongs to a group of animals somewhere between arthropod, echinoderm, and reptile while swift-swimming eelworms dominate its seas. All are clad in scales and shells of insoluble sapphire, while their flesh is hardened with biological plastics that easily shrug off the harsh chemical environment. All manner of urchin-backed amphibians and sucker-mouthed jetleeches scurry through its primeval cloud forests and clamber through acid-eaten karst caverns, making full use of the abundant resources of land just forty million years after crawling out from the steaming oceans and into the nascent coal swamps.
On Thermococcus, things are quite different. The vast supercontinent that covers half of its surface is a practically uninhabitable desert with the exception of its coastlines and the monsoon forests of the eastern mountain ranges, while its seas are salty enough to mummify most Acidianian lifeforms. Fauna in these oceans are largely limited to small crustacean-like forms, but on land the low gravity allows them to get much larger. Elephantine tripods, pterosaur-size birdbugs, and spindly, three-meter-tall moon spiders are but some of the many alarmingly large arthropodal creatures that one may stumble across in the endless sands of Thermococcus. Of course, outside of rare geothermal springs and the eastern forests, life of any kind is few and far between.
“Are you afraid, Bacillaria? Does the sight of me strike hatred into your magmatic heart as you are consumed by visions of the destruction of your people?
Well, all the better! I can feel your talent rising now - so very close to the magnificence I desire.
So come forth! Show this horrid tyrant the power of the Idealist will! Proclaim to the Universe the might of your pure hearts beating as one! Show me your grand desire to overpower even the most daunting of obstacles! Oh, how beautiful, how GLORIOUS IT ALL IS!“
- The Seventh Archon, “Endless Rain Upon Viridian Skies”, “An Open Address to Bacillaria of the Idealist Coalition“ (2220)
Past & Future
Unlike Earth and the Moon, which formed out of fragments of a planetary collision, Acidianus and Thermococcus formed separately and captured each other, just like every other Horizonian binary. Though the only equivalent object in our Solar System is the Pluto-Charon system, the close encounters that produce binary planets and giant impacts are not uncommon during the planetary formation process. Unlike the chaotic, multi-moon systems of Riftia and Changxing, the simple Acidianus-Thermococcus binary has undergone very little evolution since its formation, only shrinking its orbit slightly as Actinophrys saps their orbital energy through its tides.
Though Acidianus and Thermococcus have been in the habitable zone for all their lives, they will be pushed past its inner boundary in about 1 billion years, much like Earth. Unlike Earth, they will take some time to succumb to the runaway greenhouse, since their slow rotation lets reflective cloud formations condense on their sun-facing sides, deflecting enough solar radiation to let both stay habitable for some 500 million years after passing the conventional inner limit. But even this cannot protect the from the warming sun forever - within 1.7 billion years, Thermococcus will have become a runaway greenhouse even harsher than modern-day Riftia, while Acidianus’s thicker atmosphere will let it stick it out to some 2 billion years in the future before it follows its smaller sibling into oblivion.
Civilization
Though Acidianus and Thermococcus are usually visible only as a single blue-green star, when they are their closest to Horizon it is sometimes possible to distinguish the pair. Under those conditions they appear as a green star and a blue star about 5 arcminutes apart. The natives of Horizon have been aware of this since prehistoric times. Mythological explanations for this phenomenon are diverse, from the recurring battles of warring gods to the work of pernicious trickster spirits. However, the cultural understanding of the pair was revamped along with that of all the other planets when telescopes were invented, revealing two seemingly-habitable worlds not visually different from their home world. Though modern Horizonians’ view of their solar system has returned to the spiritual realm, their beliefs regarding Acidianus and Thermococcus are surprisingly accurate to reality. They are universally regarded as a pair of intertwined spiritual planes, populated with strange beasts but devoid of thinking men - not all too different from the real Acidianus and Thermococcus.
In modern times, the wealth of acidophilic lifeforms native to Acidianus and Thermococcus have been thoroughly exploited for bioengineering and industrial processing. Their crystal-beaded flora and fauna are common exotic pets, kept in hermetic terrariums or greenhouses by wealthy Horizonians for their unusual appearances. Additionally, a few species of fauna from both planets have recently been lifted to sapience to exploit acidic habitats unsuitable for other Horizonian bioforms. The freshly-designed inhabitants of Acidianus and Thermococcus have wasted no time in establishing themselves, constructing hyper-dense ‘hive cities’ in a few suitable sites on both worlds.
Whispers from the Reach
Dramatization of encounter between Lord of Life ‘Voltzia’ and the Horizon-born Agent of Enigmata Cerrejõn during the Great Dissonance (2272).
