Galaxy: Unknown
Quadrant:
Sector: Unknown
Star System: Unknown
Stars: None
Known Planets: None
Class: Unknown

A system developed by the [[Federation]] to categorize [[planet]]s by many factors, such as atmospheric composition, age, surface temperature, size, and presence of life. The planetary classes are designated by the letters A through Z (however U, V, and W are not used). Classes near the letter M are generally more likely to support life; the farther from the letter M, the less likely the planet is to support life as we know it.

There are three temperature zones within a [[solar system]], and depending on the star's size, classification, and its energy output, these regions will vary in size:

Class Designation Age (109 std years) Diameter (103 km) Temperature Zone Surface Atmosphere Possible Lifeforms Notes Examples
A Geothermal < 2 1–10 Biozone/ coldzone Partially molten, may feature active volcanoes Primarily H2, if any Almost never Cool over time to evolve into Class C worlds [[Gothos]]
B Geomorteus < 10 1–10 Hotzone Partially molten, may feature active volcanoes with an overall high surface temperature If any, extremely tenuous, with few active gases Almost never Mercury (Sol I–though Mercury actually has no known geothermal activity, this class is the closest fit)
C Geoinactive 2–10 1–10 Biozone/ coldzone Geologically inactive, with usually cold temperatures Unpassable, usually frozen upon their surface Almost never Mars (Sol IV–can also be classed as Adaptable), Pluto (Sol IX), Sol's Kuiper belt object Quaoar, Psi 2000,
D Asteroid/ Moon 2–10 0.1–1 Any Barren, geologically inactive, covered with craters, normally freezing temperatures Tenuous Almost never Most asteroids and planetoids fall under Class D. They are commonly found orbiting planets as moons. Luna (Sol IIIa), Ceres, Regula, Rura Penthe
E Geoplastic < 10 10–15 Biozone Molten, high temperatures Primarily O2, with other reactive gases Carbon-based Cool over time to evolve into Class F worlds Excalbia, Janus IV
F Geometallic 1–3 10–15 Biozone Actively volcanic Primarily hydrogen compounds, sometimes O2 Silicon-based Cool over time to evolve into Class G worlds Janus VI
G Geocrystalline 3–4 10–15 Biozone Still crystalizing Contains CO2 and other toxic gases Primitive single-celled organisms Cool over time to evolve into Class N, O or P worlds. Ceti Alpha V, Delta Vega
H Desert 4–10 8–15 Any Barren, hot, and arid, except in the coldzone where they can be habitable since covered with empty tundra Varies, can contain O2, Ar, heavy gases and metal vapors Both drought and radiation-resistant flora and fauna Gothos, Ocampa, Nimbus III, Rigel XII, Tau Cygna V
I Gas Supergiant 2–10 140–10·103 Usually coldzone, but can be in any May be a solid metallic mass core Solid gas, tenuous, comprised mostly of H2 and hydrogen compounds, may contain water vapor. Temperatures vary in the cloud layers. The planets radiate heat. Uncertain; would have to exist in the biozone layers of the upper atmosphere. They may be single-celled organisms, or creatures and plants that would have to be constantly airborne. Can have hundreds of moons and also water ice rings. Excalbia, Q'tahL
J Gas Giant ("Jovian") 2–10 50–140 Usually coldzone, but can be in any May be a solid metallic mass core Solid gas, tenuous, comprised mostly of H2 and hydrogen compounds like CH4 and NH3, may contain water vapor. Temperatures vary in the cloud layers. The planets radiate some heat. Winds can speed up to 10,000 km/h in their turbulent atmospheres. Uncertain; would have to exist in the biozone layers of the upper atmosphere. They may be single-celled organisms, or creatures and plants that would have to be constantly airborne. Can have hundreds of moons and also water ice rings. Jupiter (Sol V), Saturn (Sol VI), Uranus (Sol VII), Neptune (Sol VIII)
K Adaptable 4–10 5–10 Biozone Rocky, barren with only trace amounts of water. Thin, mostly CO2 Limited to single-celled organisms and algae Gravity fields can fall within Class-M norms, suitable for human colonization through terraforming or the use of pressure domes and life-support systems. Mars (Sol IV), Mudd, Son'a Prime
L Marginal 4–10 10–15 Biozone Rocky, barren, with little water CO2, O2, Ar Limited to plant life Suitable for human colonization with some terraforming. Terrestrial planets conquered by the Borg are usually forcibly terraformed into Class L worlds as a result. Borg Prime (suspected), Indri VIII (before Klingon tampering), Psi 2000, Vaadwaur Prime
M Terrestrial (Vulcan: Minshara) 3–10 10–15 Biozone A relatively thin tectonic layer floating on a molten rock mantle, usually with active volcanoes present. Class M planets have an abundant amount of water necessary for life as we know it to exist. O2 / N2, (CO2, O3, Ar, He, Li) Almost always present, flourishing as extensive plant and animal life, usually at least one sentient species Small, rocky, terrestrial worlds. Highly supportive of organic life. Planets with more than 80% surface water fall under Class O or Class P. [[terra]] (Sol III), Vulcan (40 Eri A IV), Cardassia Prime, Risa, Bajor
N Reducing 3–10 10–15 Biozone Barren and rocky, with usually high temperatures due to an intense greenhouse effect. Water exists, but only as vapor. Extremely dense, containing CO2 and sulfides Would have to be adapted to such an extremely harsh environment; there may be single-celled organisms living in the upper layers of the atmosphere. Venus (Sol II–although some descriptions of Venus are also in keeping with that of demon-class worlds)
O Pelagic 3–10 10–15 Biozone Extreme abundance of water or water-like substances, covering more than 80% of the surface Class M-like: O2 / N2 and other trace elements Almost always, however almost all of it is aquatic plant and animal life, with little land masses to evolve surface creatures. Sentient races evolving on such worlds may be aquatic in nature. Argo, Monea
P Glaciated 3–10 10–15 Usually on the extreme edge of the biozone Extreme abundance of water or water-like substances, much of it is frozen in ice, covering more than 80% of the surface Class M-like: O2 / N2 and other trace elements Usually hardy plant and animal life surviving in the tundra Exo III, the Breen homeworld
Q Variable 2–10 4–15 Hotzone / biozone Ranges from molten rock to water and/or carbon dioxide ice due to the variable output of the star they orbit. Ranges from tenuous to extremely dense Would have to rapidly adapt to sudden changes in temperatures. Genesis Planet, Remus (Romulus B)
R Rogue 2–10 4–15 Do not orbit stars Usually barren, however some may be temperate due to geothermal venting Tend to be filled with volcanic outgassing Not believed to support life, however some may have animal and non-photosynthetic plant life Dakala, Founders' Homeworld
S
T		 Ultragiant 2–10

Class S: 10·103–50·103

Class T: 50·103–120·103

 Usually in the coldzone, but can be in any region Tenuous, composed of hydrogen, and hydrogen compounds; such planets radiate considerable heat and tremendous gravity varies in temperature, pressure and composition at different layers; there may also be water vapor present May only exist as single-celled organisms May have hundreds or thousands of moons, some of which may be terrestrial planets of Class M, O and P. Some extrasolar planets discovered by astronomers at the beginning of Terra's 21st century could classify as Ultragiants.
X
Y
Z		 Demon Undefined 10–50
Any Filled with toxic and corrosive gasses Turbulent, sometimes volcanic, with temperatures over 500 K Usually hostile to humanoid life; life forms, if any, will probably be silicon based or mimetic in nature, as experienced on Demon class worlds of the Delta Quadrant. Generate thermionic radiation Venus (Sol II, Class X), Tholian Homeworld (Class Y), Demon Planet (Class Y, Gamma Quadrant)

(Parts taken from Wikipedia, the free encyclopedia.)