Habitability rating

Habitability rating is a system for classifying planetary bodies based on their suitability for habitation by humans and other Earth biota. 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. Importantly, the system focuses only on a planet's habitability for humans, not in general; a superhabitable planet may host ideal conditions for life to evolve from a non-anthropocentric standpoint and still be classified as lower than Earthlike worlds.

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 in most regions of the planet. 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 that fall short of the criteria of full terrestrial habitability for one reason or another, but are still partially habitable without terraforming. Category-II worlds include those with clearly earth-divergent day-night cycles or limited habitable zones, usually due to extremes of temperature or other hazards in certain parts of the planet, e.g. lowlands or highlands, equatorial or polar regions. Other hazards, such as radiation or dangerous native biota, may also be present. Terraforming is sometimes used to alleviate such issues, though in many cases it has been deemed too costly or risky to the local biosphere and only the already-habitable parts of the world are colonized. Most colonized worlds with native biospheres are either cat-II or cat-IV.
 * 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, but also not too far from human habitability for the terraforming process to take inordinate amounts of time and energy. Such worlds may take up to a century to terraform and may undergo ongoing or active atmospheric processing and climate control while colonized. Cat-III worlds can usually be explored with light protective equipment, sometimes as little as a breathing mask.
 * Examples: Actium, Bolder, 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 category-IV worlds are still colonized for various reasons (as they may have other unique advantages), and can usually be made habitable either partially or fully through additional effort. E.g. ocean worlds without dry land, harsh terrain or flora/fauna, exceedingly low/high gravity, Earth-incompatible native ecosystems, inclement weather, widespread active volcanism/geological instability, or tidally-locked worlds with only a small habitable belt along the terminator line.
 * Examples: Baikonur, Circumstance, Ruthersburg, Sunburn, Venezia, Verne.
 * V: Worlds that can be explored in a normal spacesuit or equivalent protective gear. This includes airless worlds and worlds with low to moderate level ambient hazards, e.g. "snowballs", worlds with toxic atmospheres and/or biota, or scorching temperatures. Most planets, moons, asteroids, and comets fit this category. Usually only colonized for scientific, industrial or military purposes. Outside historic exceptions such as Mars, such worlds are 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: Astronomical bodies that cannot be landed on, even with the best protective equipment available.
 * Examples: Gas giants, ice giants, stars.