The standard system of classification used to indicate the habitability of a temperate planet. Temperate worlds are those which orbit their star at the correct distance for most of their water to be liquid, allowing a hydrological cycle to establish itself, and which produce geomagnetic fields strong enough to protect the planet from the stellar wind. On such worlds, life is practically inevitable, and because life follows the path of least resistance, life across the galaxy is broadly the same, and changes the environment to suit itself. Temperate worlds therefore also typically have a carbon cycle, photosynthesizing plants and plenty of free oxygen in the air.
Such planets can still vary wildly in terms of local conditions however, so the Dominion has a broad-strokes system for classifying worlds based on the ease with which a notional "average" being could survive there unassisted. The higher the number, the less universally hospitable a planet is to alien life.
The "galactic standard range" for habitability, temperate worlds in this range vary in hospitality to most life forms, though
- 1: Class 1 worlds are perfectly habitable, being completely suitable for any conceivable species to survive without effort. They are purely notional - none has ever been discovered, and their existence is considered impossible.
- 2: There is only a single Class 2 planet in the known galaxy, and it is the capitol planet of the Interspecies Dominion as a result. Any being placed on a Class 2 world would find it trivial to survive and would not face any life-threatening environmental conditions.
- 3: Class 3 worlds are easy to thrive upon, with just a little work required. There might exist a unique environmental factor which is uncomfortable for no more than one or two species, but nothing life-threatening. Most Class 3 worlds are garden worlds, resorts or heavily populated trade hubs. The Corti come from a class 3 planet.
- 4-6: These worlds require some effort for most species to thrive in the wild, and they may have environmental factors which are uncomfortable but not life-threatening for several or most species. Most species come from class 4-5 planets.
- 7-8: Survival becomes a skilled task, requiring hard work and specialist knowledge. The planet may have a few environmental factors which are genuinely dangerous to a handful of species.
- 9: The most dangerous classification of "habitable" temperate world. Survival on Class 9 worlds is a genuine challenge, though a skilled, careful and well-prepared being will do just fine. The world may have a single very dangerous local phenomenon. The Guvnuragnaguvendrugun originated on a Class 9 world: The planet Guvendruduvundraguvnegrugnuvenderelgureg-ugunduvug is notorious for the annual "Ugunduvug-vanrundrneg" (closest equivalent translation: "the planet releases a cathartic scream") also known as "The World-Storm", a week-long season of pounding rain, howling wind, lightning and flash-floods that gives way to clement and fair weather for the rest of the year.
- Garden (Class 2)
- Perfection (Class 3)
- Cimbrean (Class 4 before Jen Delaney 'corrupted' it, class 9 after human settlement)
- Guvendruduvundraguvnegrugnuvenderelgureg-ugunduvug (Class 9)
- Aru (Class 9)
Planets classified as 10 or higher are known collectively as "deathworlds." Survival becomes difficult in the extreme on such worlds throughout their year. Conventional wisdom has long held that it is impossible for a sapient race to evolve on a deathworld as the local conditions are so harsh that the budget of calories required to maintain a large brain capable of sapience would cause a potential sapient race's ancestors to die off long before their increased intellect became significantly beneficial to their survival.
- 10: Class Ten worlds have no more than three seasonal hazards or one year-round, permanent hazard which makes life there difficult if not impossible for most beings. This can just be high gravity or it can be something more akin to the World-Storm. While some moderate tectonic activity is a prerequisite for a world to be Temperate in the first place, Class 10 worlds are geologically highly active, and it will be a rare day on a Class 10 when there is not a volcanic eruption or minor earthquake somewhere on its surface.
- 11: Class Eleven planets are genuine hell-holes by the standards of all FTL-capable species, possessing multiple of the factors that individually make a Class 10 world dangerous. While pockets of the world's surface may be habitable and comfortable, most of it will be occupied by several persistent hazards that can easily kill - toxic organic compounds, dangerous predators, violent weather, extremes of temperature, and so on.
- 12: These planets possess ALL of the factors that make class 10 planets dangerous, usually meaning that there is nowhere on the planet that cannot kill even the most shrewd and experienced alien survivalist. Class 12 planets are high-gravity worlds, seething disease pits, menageries of death where even small and apparently benign creatures can do considerable harm, and worse. As such they are given a wide berth by all intelligent beings. Earth is a Class 12 temperate planet.
- 13: Only one Class thirteen planet is known to exist. The planet Nightmare would ordinarily be categorized as a Class 12, except for the eccentricity of its orbit, which results in brutal winters where the oceans freeze at the tropics, and oppressive summers where the oceans rise several meters as the polar ice melts. Conditions on Nightmare can change literally overnight, but all the possible conditions on its surface are deadly. Julian Etsicitty survived on Nightmare for several years after being abandoned by his captors.
- 14+: Classifications higher than 13, while theoretically possible, remain purely hypothetical for the time being - none have yet been officially discovered. To qualify, a planet would need to be even nastier than Nightmare - setting foot on such a world would almost certainly mean meeting an unpleasant end within hours, minutes or even seconds of landing.
- Gao (Class 10 [As of 13y11m AV])
- Although it is given the highest classification of any known planet, Nightmare is reckoned by Julian Etsicitty to be rather easier to survive on than Earth thanks to its predictable strong seasons and planet-wide homogeneous biome.
- The planet Gao was originally a 9.92 on the classification scale, but was upgraded to Class 10 13y11m AV. This was widely considered to be for political reasons.
- The planet Garden is the only known Class 2 planet in the galaxy.
- A Class 14 Deathworld is found by the crew on the BGEV Reclamation in MIA Chapter 22, but comes with a caveat - it is not a stable environment, instead quickly transitioning into a Violent World. The planet was cracked open in a long ago war, presumably by the Hierarchy, and the planet's core has been seeping out ever since. Only a few mountain ranges remain even slightly habitable, with no life below a certain elevation. It has not been reported and officially discovered, leaving Class 14 worlds hypothetical.