Background

Xi’ialoa (/ʃiiælwʌ/), designated Exura-b, is a terrestrial planet with a mass of 2.06 ME and a radius of 1.27 RE. Formed 3.2 bya, it orbits Exura, an orange dwarf K-type main sequence star, HD 114386 (Centaurus), with 0.6 M☉. At a mere 0.56 AU, its 4.6 light-minute orbital radius lies in a habitable zone due to lower solar energy output. Its orbital period is 0.54 Earth years, with one day lasting 19 hours. An impact-originating axial tilt of 31.2 causes a seasonally variable climate regime with surface temperatures that range from -120°F to 155°F. Its two natural satellites – Lyridia, a modest-sized, life-bearing ice moon, and Phobeton, a small, captured, lifeless rock – share its orbital plane.

Iron-water reactions (3Fe + 4H2O → Fe3O4 + 4H2  as well as 3Fe2SiO4 + 2H2O → 2Fe3O4 + 3SiO2 + 2H2) during planetary formation contributed to Xi’aloa’s diatomic hydrogen dominant atmosphere, (70% H2, 13% CH4, 10% CO2, 3% N2, 2% O2, 1% H2O, 1% other) which extends far into space (about 60,000 km) due to its low density.  CO2 is sourced from oceanic outgassing, microbial organism decomposition, and atmospheric escape from the planet’s ice moon Lyridia. 

Though offset by higher gravity, Jeans Escape slowly pries H2 from the atmosphere’s upper reaches due to hydrogen’s lower escape velocity and high Maxwell-Boltzmann rating. Further dissipation from solar wind is abated by a far-reaching magnetosphere. For now, lost H2 is adequately replenished by undersea methane/water-ice vents (water and hydrocarbon reactions) and the native flora’s hydrogen-evolving photosynthesis. The steel blue sky is given color by atmospheric Rayleigh scattering, as Exura’s warm-colored yet lower wavelength light output is dispersed in the smaller particle-sized atmosphere. A low atmospheric pressure (580 mmHg at sea level) results in increased rates of water evaporation. Fire is an infrequent, suppressed phenomenon. 

With a Terran-analogous water cycle, Xi’ialoa’s hydrosphere covers almost 65% of its surface. Its primarily water-methane (15%, CH4 aq.) solution leads to frothier, milky-colored water bodies. With 17-24ppt salinity MgCl2, oceans reach depths of 11km and range from a frigid 23°F to a scalding 104°F. Life has adapted to these seasonally dynamic conditions, though more prevalent in temperate oceans. Hydrogen respiration occurs through gill-like structures that exchange dissolved H2 with respired CH4. Its tidally locked moons govern extreme tidal cycles which fluctuate with their aligned or opposing influences as they traverse their orbits. 

Due to low atmospheric O2, ferrous minerals in soil and rocks rust extremely slowly. Surface minerals are silica and hydrogen-rich, and with less iron cycling through ecosystems, organisms rely on TiO2-sequestering reactions to form mineralized exo- and internal skeletons based on titanium, iridium, and magnesium, alongside carbon, magnesium, phosphorus, and trace calcium.

Active plate tectonics govern Xi’ialoa’s landmasses. Familiar landforms like towering mountains, deep valleys, and flowing rivers populate the ever-changing landscape that defines the biosphere, with many endemic biomes defying Terran analogues. Seven main continents and frigid polar landmasses harbor life. 

Due to its axial tilt, seasons on Xi’ialoa are extreme yet short in duration. In scorching, wet summers, temperature and humidity soar. Frigid winters are cold and dry, with snowfall nearly reaching the equator, and many animals embark on migrations to escape the unforgiving climate. Floral and faunal life are well-equipped to handle these harsh, cyclical conditions. 

Xi’ialoa’s biosphere is supported by endemic plant-convergent flora. This incredibly diverse kingdom is thought to have evolved from cyanobacteria and reproduces via seeds, spores, and clonal fragmentation. Photosynthesis on Xi’ialoa uniquely uses the reaction

5CH4 + 5H2O + light → C5H10O5 + 10H2, where methane and water are combined into sugars (pentose, ribose) and hydrogen gas, used by animals and plants alike during digestion and respiration. In turn, methane is produced as a waste product from animals and incorporated into plants in a mirror role to CO2 on Earth. Plants combine these abiotic resources into hydrocarbons, units of energy storage and cell construction, which are consumed internally during cellular respiration, by predators, and by the rest of the food web.  Abundant, surface methanogenic bacteria react CO2 + 4H2 → CH4 + 2H2O, releasing atmospheric methane used by floral photosynthesis.