A/N: Rephrased from the game, so that grammar is better. Classification borrowed from Star Trek franchise.
Class A – Small, young worlds with surfaces that are usually molten with possible volcanic activity. Atmosphere, if any, would be very thin.
Class B – Young planets larger than Class A. They have partially molten surfaces and active volcanoes. Atmospheres, if any, are very thin.
Class C – Young planets that are geologically inactive and may be covered in ice and/or frozen gases.
Class D –
Class E – Small, young and volcanically active worlds still in the process of forming. Their surfaces are largely molten.
Class F – Very young worlds with volcanically active surfaces with heavy metallic cores. They are mineral and ore-rich, making them valuable to mining conglomerates. If any life develops, it might be silicon-based.
Class G – Young but large worlds, with surfaces that are still solidifying. Their atmospheres may contain carbon dioxide and other toxic gases. If there is any life present it would be limited to primitive single-cell organisms.
Class H – Desert worlds with barren surfaces which are either hot and arid or covered with an icy tundra. Life forms developing on such a world would have to be resilient to either extreme hot or cold temperatures and to solar radiation, if the planet is close to the star.
Class I – Gas supergiants, over 140k km in diameter. They are almost entirely comprised of hydrogen and helium, but may contain water vapor as well. They may contain a solid metallic mass core and radiate heat. They can have hundreds of moons as well as water ice rings.
Class J – Gas giants with less than 140k km in diameter. They are mainly comprised of hydrogen and helium and could have a solid rocky or metallic core. They can have many moons as well as water or ice rings.
Class K – Rocky and barren with little, if any, surface water. Their atmospheres are thin and comprise mostly carbon dioxide. Life forms, if any, are limited to single-cell organisms and algae. They are “adaptable” because through terraforming they are suitable for human colonization.
Class L – Rocky, barren surfaces with little water. Their atmospheres are mainly oxygen and argon with a high concentration of oxygen dioxide. Life forms, if any, would be limited to simple plant life.
Class M – Always located in a star’s habitable zone. They are provided enough warmth and energy to develop and sustain carbon-based life. Their surfaces are a thin volcanic layer floating on a molten rock mantle and they usually have many active volcanoes. Most importantly, they have plenty of liquid water necessary for life to exist. Their atmospheres contain oxygen and nitrogen with traces of other gases. Life forms are almost always present, often flourishing as extensive plant and animal life.
Class N – Barren and rocky with extremely high surface temperatures caused by an intense greenhouse effect. Their atmospheres are impossibly dense, comprised mostly of carbon dioxide and corrosive sulfides. Sometimes, water vapor is present. Life would be limited to single-cell organisms in the upper layers of atmosphere. Some class N planets might fall into class Y, especially if a bizarre form of life exists on them.
Class O – Similar to class M worlds. However, over 80% of the surface is covered in water. Life forms would be mainly aquatic in nature.
Class P – Similar to class O planets. With an abundance of surface water, however, almost all of it is locked in frozen ice. Liquid water might be present under the ice, but at extreme depths. Any life developing on the surface would have to adapt to extremely frigid conditions.
Class Q – Planets with variable surface conditions due to either a highly eccentric orbit or orbiting around a variable output star. Variable conditions might also be caused by some other effect, either natural or artificial. The result is an extremely harsh world where temperatures rise and fall to extreme levels. Any life existing on such a world would have to be extremely adapted to rapid climate changes.
Class R – Rogue planets do not orbit stars. Instead, they float in the cold darkness of space. Most likely, they formed naturally within a star system, but have been flung deep into space. Their surface is usually barren, but some might be temperate due to geothermal venting, with atmospheres filled with volcanic gases.
Class S – Super gas giants with diameters from 50k to 100k km. They radiate considerable heat and generate enormous gravity, often harboring hundreds or even thousands of moons. In most cases, these moons would be subject to considerable tidal forces and gravitational effects caused by their parent planets.
Class T – “Near star” planets are super gas giants with diameters from 100k to 200k km. They radiate considerable heat and generate enormous gravity, often harboring hundreds or even thousands of moons. In most cases, these moons would be subject to considerable tidal forces and gravitational effects caused by their parent planets.
Class Y – Reserved for strange planets known as “demon” worlds where surface conditions do not fall into any other recognized category. Such worlds are usually hostile and lethal to humanoid life. If life develops, it usually takes on many bizarre forms, such as living crystal or rock, liquid or gaseous physical states, non-corporeal or dimensional states or energy-based.