Habitability rating

Habitability rating is a system for classifying planetary bodies based on their suitability for human habitation. A planet's rating is aggregated from numerous factors, including gravity, liquid water, atmosphere, biosphere characteristics, geological features such as terrain and volcanism, local stellar activity, and more. It is a broad system used largely as shorthand; others such as the Gresham Habitability Index are used for more thorough and scientific analysis.

Established by the Colonial Administration Authority in the mid-24th century to broadly categorize surveyed worlds and prioritize them for colonization, it remains in widespread use within the UNSC and civilian organizations such as the Phoenix Initiative. Note that all examples below are given based on how those worlds were categorized in initial surveys; since then, some of these worlds have undergone terraforming processes and now occupy a higher rating.


 * I: Earthlike garden worlds, with a breathable atmosphere, comfortable gravity, Earth-compatible ecosystem and a "shirtsleeve" climate. Compatible or near-compatible with human settlement from the start, requiring no terraforming; minor acclimation (e.g. injections against allergic reactions) may be necessary. The most rare category.
 * Examples: Arcadia, Gao, Harvest, Reach.
 * II: Near-Earthlike worlds with limited habitable zones, usually due to extremes of temperature in certain parts of the planet, e.g. lowlands or highlands, equatorial or polar regions. Terraforming is sometimes used to alleviate this, though in some cases it may be deemed too costly on a large scale and only the habitable regions are colonized.
 * Examples: Cascade, Gilgamesh, Luyten, Parvati, Skopje.
 * III: Pseudo-Earthlike but terraformable worlds, with temperatures, atmospheric composition, climate and/or native microecology that make them uninhabitable or only barely habitable without modification. May take up to a century to terraform and may undergo ongoing or active atmospheric processing and climate control while colonized.
 * Examples: Actium, Bolder, Draco III, Eridanus II, Harmony, Miridem, Tribute.
 * IV: Borderline habitable worlds. These usually have one or more pervasive disadvantages that make them suboptimal for large-scale settlement, and common terraforming measures are ineffective or inapplicable; many are still colonized for various reasons (as they may have other unique advantages), and can usually be made habitable through additional effort. E.g. ocean worlds without dry land, harsh terrain or flora/fauna, exceedingly low/high gravity, inclement weather, or widespread active volcanism/geological instability, or tidally-locked worlds.
 * Examples: Baikonur, Circumstance, Ruthersburg, Sunburn, Venezia, Verne.
 * V: Airless worlds and worlds with low to moderate level ambient hazards, e.g. "snowballs", worlds with toxic atmospheres, or scorching temperatures. As a general rule, can be explored in a standard spacesuit. Most planets and moons fit this category. Usually only colonized for scientific, industrial or military purposes. Outside historic exceptions such as Mars, rarely terraformed due to the effort required.
 * Examples: Beta Gabriel, Callisto, Europa, Ganymede, Josaphat's Folly, Mars, Mercury, Luna, Neos Atlantis, Pharos, Siberia Prime.
 * VI: Worlds that are not merely uninhabitable, but actively inimical to human life, e.g. extremes of temperature (cryoworlds and hothouses), pressure or radiation, or biohazards. Require specific protective equipment to traverse. Only colonized for scientific, industrial or military purposes. Sometimes called "Hellworlds". The line between the least hospitable category-V worlds and the most hospitable cat-VI ones can be tenuous, though in debatable cases, a world is typically placed in the latter class.
 * Examples: Csodaszarvas, Io, Titan, Venus.
 * VII: Miscellaneous/inapplicable.
 * Examples: Gas giants, ice giants, asteroids.