Planet Classifications
Class A
Known as Geothermal planets, they are small, young with surfaces that are usually molten with the possibility of volcanic activity. If there was any atmospheres it would be very thin.
Class B
Geomorteus planets, are again young planets but are larger than Class A. They have partially molten surfaces and active volcanoes. Again if there was an atmospheres it would probably be very thin. Mercury would probably be a Class B planet, though Mercury's surface is solid and without known volcanoes.
Class C
Geoinactive planets, are young planets that are geologically inactive and may well be be covered in ice and/or frozen gases. Pluto could be considered a Class C.
Class D
Not really planets but such things as asteroids and moons. Some may also be so-called dwarf planets. They are small and geologically inactive. Again any atmosphere if any would be very thin. Earth's Moon could be considered Class D.
Class E
Geoplastic planets are small young and volcanically active worlds still in the process of forming. Their surfaces are largely molten.
Class F
Geometallic planets are very young worlds with volcanically active surfaces with heavy metallic cores rich in ores and minerals making them valuable to mining conglomerates. If any life was able to develop it may be silicon based.
Class G
Geocrystalline planets are young large worlds. Their 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 celled organisms.
Class H
These are desert worlds with barren surfaces that are either hot and arid, (if in close orbit of the parent star) or covered with icy tundra if in a more distant orbit. Any life attempting to form would have to be resilient to extreme temperatures and if the planet is close to the star, resistant to solar radiation.
Class I
These are Gas Supergiants, over 140,000 km in diameter. They are comprised almost entirely 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 and also water ice rings.
Class J
Again these are Gas Giants less than 140,000 km in diameter. They are mainly comprised of hydrogen and helium gases and could have a solid rocky or metallic core. They can have many moons as well as water ice rings. Jupiter and Saturn could be Class J planets.
Class K
Adaptable planets, are rocky and barren with little if any surface water. Their atmospheres are thin, and comprise mostly carbon dioxide. Life forms would be limited to single celled organisms and algae. They are "adaptable" because through terraforming they are suitable for human colonization. Mars could be considered a Class K planet.
Class L
Marginal planets, have rocky, barren surfaces with little water. Their atmospheres are mainly oxygen and argon with a high concentrations of carbon dioxide. Life forms would be limited to simple plant life. Class L planets are suitable for human colonization with some terraforming.
Class M
Terrestrial planets, are the most commonly visited planets. They are also called Minshara Class, a Vulcan term. They are always located in the ecological region of a star where they are provided enough warmth and energy to develop and sustain carbon-based life. Their surfaces comprises a thin tectonic 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 other trace gases. Life forms are nearly always present often flourishing as extensive plant and animal life. Earth is a prime example of a Class M world. Other would be Vulcan, Risa, Bajor, Cardassia Prime, and Qo'noS
Class N
Reducing planets, are barren and rocky with extremely high surface temperatures caused by an intense greenhouse effect. Their atmospheres are extremely dense comprising mainly carbon dioxide and corrosive sulfides. Sometimes water vapor is present. Life would be limited to single celled organisms living in the upper layers of atmosphere. Venus could be a Class N planet.
Class O
Pelagic planets, are similar to Class M worlds. Life forms would mainly be aquatic in nature due to the surface being 80% covered in water.
Class P
Glaciated planets, are similar to Class O water planets with an abundance of surface water, however almost all of it is locked in frozen ice. Liquid water may be present under the ice but at extreme depths. Any life developing on the surface would have to adapt to extremely frigid conditions. Moons Europa, Callisto, and Ganymede could be considered Class P planets.
Class Q
Variable planets have variable surface conditions because they either orbit a variable star (whose power output fluctuates), or their distance from their primary star varies greatly due to a sufficiently eccentric orbit. The result is a world where temperatures fluctuate within extreme levels. Any life existing on such a world will have adapted to rapid climate changes. A Class Q designation might also be applied to planets whose conditions vary due to other effects, either natural or artificial.
Class R
Rogue planets, do not orbit stars. Instead, they float in the cold darkness of space. They most likely normally formed within a solar system but have been flung deep into space usually caused by the collision with another planetary body. The surface of a rogue planet would usually be barren, but some might be temperate due to geothermal venting with atmospheres filled with volcanic gases. Life would be rare, but some may have unusual life that gather near volcanic regions. The surface life would exist in complete darkness with plant life that is non-photosynthetic in nature.
Class S T
Ultragiant planets are Super Gas Giants or brown dwarfs. Their diameters range in size from 10 to 50 million km (Class S) and 50 to 120 million km (Class T). Any larger and they would be classified as stars. They radiate considerable heat and generate enormous amounts of 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 planet - oceans or other large bodies of water would experience gigantic waves and powerful currents.
Class X Y Z
These are reserved for strange planets referred to 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 on these worlds they usually take on many bizarre forms, like living crystal or rock, liquid or gaseous physical states, non-corporeal or dimensional states or be energy-based.
