Planet Classification
Class A  -  1,000 to 3,000 km
Class C  -  2,000 to 10,000 km
Class D  -  100 to 2,000 km
Class B  -  3,000 to 5,000 km
Class E  -  3,000 to 5,000 km
Class H  -  10,000 to 15,000 km
Class J  -  50,000 to 140,000 km
Class I  -  140,000 to 10,000,000 km
Class K  -  5,000 to 15,000 km
Class G  -  10,000 to 15,000 km
Class F  -  5,000 to 10,000 km
Class L  -  8,000 to 15,000 km
Class M  -  10,000 to 15,000 km
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Geothermal planets which are small, young worlds with surfaces that are usually molten with possible volcanic activity. Atmospheres, if any, would be very thin. Would be similar to Jupiter's moon Io.
Geomorteus planets are also small young planets larger than Class A with partially molten surfaces and possible active volcanoes. Atmosphere, if any, would be thin. In Sol, Mercury would be a Class B planet, although it has no volcanoes.
Geoinactive planets are young planets that are geologically inactive, hence their name, and may be covered in ice and/or frozen gases. In Sol, Pluto could be considered a Class C planetoid. Psi 2000 is also Class C.
Class D objects are asteroids and moons. Some may also be so-called dwarf planets. They are small and almost always geologically inactive. Atmosphere, if any, would be very thin. Earth's moon could be considered Class D.
Class E, or "Geoplastic" planets are very young and volcanically active worlds still in the process of forming. Their surfaces are largely molten. An example of a Class E planet would be in the case of Excaliba.
Class F planets are also known as Geometallic planets. These planets are still young and volcanically active, however they have heavy metallic cores rich in ores and minerals. This makes them ideal for mining operations. If any life were to be developing on a Class F planet, it would most likely be in the form of a silicon-based lifeform as in the case of Janus VI, which is Class F.
Geocrystalline planets are also still in the process of forming, larger than a Class F and with less volcanic activity. Since the surfaces of these planets are still solidifying, their atmospheres would be thicker than Class F, containing mostly carbon dioxide and other toxic gases. Any life present would be confined to cingle celled organisms. Delta Vega is a Class G planet.
Class H planets are desert planets that lie just before or just after the Habitable Zone, where L, M, and O Class planets usually form. Their surfaces are hot and arid (in a closer orbit to the parent star) or covered in icy tundra (in a more distant orbit). Life forms developing on such worlds would have to be resilient to either extreme hot or cold temperatures, and solar radiation if the planet is too close to the star. Rigel XII is an example of a distant Class H, where the Ocampan homeworld would fall into a hotter Class H.
Gas Supergiants, larger than regular gas giants (such as Jupiter or Saturn), usually have a diameter of between 140,000 kilometers to 10 million kilometers. They are comprised almost entirely of hydrogen and helium gases, however may contain water vapors as well. Some may contain small solid metallic mass cores and radiate heat. They can have hundreds of moons and also water ice rings.
Similar to Class I Supergiants, Class J Gas Giants are different only by way of being smaller. Any Class J Giant that grows larger than 140,000 kilometers in diameter is reclassified as a Class I. These gas giants are mainly comprised of hydrogen and helium, with possible traces of water vapor and either a small rock or metallic core. They can have many moons as well as water ice rings. Jupiter and Saturn are prime examples of a Class J Gas Giant.
Class K planets, also known as "Adaptable" planets, are rocky and barren with little, if any, surface water. Their atmospheres are thin and usually made up of carbon dioxide. Life forms, if any, are limited to cingle celled organisms, bacteria, and algae. The reason they are adaptable is because through the process of terraforming they are suitable for human colonization. Mars would be classified a K Class planet.
Better known as "Marginal" or "Jungle" planets, Class L's have lush, temperate climates with massive forests covering the surface. Their atmospheres are mainly oxygen and carbon cioxide with varying amounts or argon. Life forms native to Class L planets can vary from small animals to primitive civilization. An example of a Class L world would be the Klingon homeworld of Qo'NoS (Kronos).
Class M planets are "Terrestrial" planets, meaning they are the most suitable for human colonization. They are named after the Vulcan term "Minshara". They are always located in the ecological region of a star also known as the "Habitable Zone" where they are provided enough warmth and energy to develop and sustain carbon-based life. Their surfaces comprise 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 almost always present, often flourishing as extensive plant and animal life. Sometimes a sentient race is present, as is the case with Earth and Vulcan.