Ametrasa system
 Ametrasa and its planets (distances not to scale) | |
| Age | 6.34 billion years |
|---|---|
| Location | |
| System mass | 1.0042 Ametrasa masses |
| Nearest star |
Moreya (3.38 ly) |
| Nearest known planetary system |
Moreya system (3.38 ly) |
| Planetary system | |
| Semi-major axis of outer known planet (Yomei) |
5.24 AU |
| Distance to Kuiper cliff | ≈10 AU |
Populations | |
| Stars | 1 (Ametrasa) |
| Known planets | |
| Known dwarf planets |
|
| Known natural satellites | 17 |
| Known minor planets |
≈750 thousand |
| Known comets | 872 (confirmed) |
| Orbit about Galactic Center | |
| Invariable-to-galactic plane inclination | ≈86° (ecliptic) |
| Distance to Galactic Center | ≈27,100 ly |
| Orbital speed | 211.8 km/s |
| Orbital period | 240.791 Myr |
| Star-related properties | |
| Spectral type | G3IV |
| Frost line | ≈5 AU |
| Distance to heliopause | ≈25 AU |
| Hill sphere radius | ≈1 ly |
The Ametrasa system (occasionally referenced in ancient sources as Mu Arae) consists of the planetary system orbiting the main sequence G-type star Ametrasa. Four planets directly orbit Ametrasa, some of which possess numerous natural satellites; in order from innermost to outermost, the planets are Amenukashi, Heizan, Ryuga, and Yomei. Amenukashi is a chthonian planet, while the outer three are gas giants; Osukune, a satellite of the gas giant Ryuga, is the only celestial body known to harbor life.
The star Ametrasa is approximately 6.34 billion years old, with a surface temperature of 5820 Kelvin. It sits on the cusp of subgiant classification, although it is believed to be a particularly large and bright main sequence dwarf star. Amenukashi orbits very closely to Ametrasa, while Heizan and Ryuga interact through a 2:1 orbital resonance. Amenukashi also orbits Ametrasa the fastest, with an orbital period of only 9.63 days, whereas Yomei, the farthest planet, orbits Ametrasa every 11 and a half years. Osukune, a natural satellite of Ryuga, sits in the habitable zone of Ametrasa and uniquely possesses a biosphere. The system has a large amount of unstably orbiting planetesimals and space debris due to centuries of unfettered exploitation during the First Skeletarchy.
Humans arrived in the Ametrasa system roughly 3800 years before the present, settling on Okusune's most temperate continent, Kento. Variable conditions in the ultraviolet emissions of Ametrasa made the early days of human colonization difficult, paired with the generally high levels of volcanism and violent weather patterns on the ocean-moon. The Skeleton Rebellion of the 13th century Aj. saw a fundamental shift of rulership towards the robotic skeletons, whose domination over humanity would remain undisturbed for seven centuries during the First Skeletarchy. Departing through the transformation of Ryuga's outermost moon into the form of a massive spacecraft, the First Skeletarchy left what little remained of human civilization in near-total disarray. After two centuries of constant bombardment from orbitally decaying space debris, humanity saw a brief period of resurgence, only to once again fall to the mysterious Grid Failure of the 27th century. The Second Skeletarchy emerged as a result, and for a time ruled peacefully, but the internecine Skeleton War pushed all of civilization once more to the brink of extinction.
In the centuries since the Skeleton War, humanity has remained bound to the ocean-moon Okusune, particularly the continent Kento, which is the only known harbor of biological life in the observed stellar neighborhood. Although humanity was once a space-faring species, such feats are now far beyond the level of technological sophistication possessed by any extant population of sentient beings. Genetic engineering has resulted in the emergence of the arimen and sunomen subspecies of human, which cohabit with these baseline humans on Kento. As of the 39th century Aj., the estimated population of all living humans and divergent humans in the Ametrasa system is no more than 17,000.