Planet classifications

In the fictional Star Trek universe the Planet Classification System is a system developed by the Federation to categorize planets 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 (except that U, V, and W are not used).

The system was initially developed by Gene Roddenberry. Only a few classes have been mentioned in the Star Trek canon, the most common visited worlds being Class M or Earth-type planets for the obvious reason that advanced humanoid life will most likely exist there.

Temperature Zones
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:


 * The hotzone is nearest to the star. Planets within this region absorb a tremendous amount of direct energy from the star, and are usually too hot to contain life.
 * The biozone (or ecozone) is at just the right distance from the star for planets to have temperatures that support most forms of life. (Compare the concept of a habitable zone that is actually used by astronomers in the real world.)
 * The coldzone is the region past the biozone, where planets are too far away from the star to support an ecosystem. These planets are usually frozen rocks or gas giant worlds.

Class A Geothermal
Very young planets, Class A worlds are less than 2 billion years old. Their diameters range in size from 1,000 to 10,000 km. They are located in the biozone or coldzone regions of a stellar system. Their surfaces are partially molten and may feature active volcanoes. Their atmospheres, if any, are primarily hydrogen. Class A planets cool over time to evolve into Class C worlds. They almost never have life forms.

Example: Gothos

Class B Geomorteus
Young planets, Class B worlds are less than 10 billion years old. Their diameters range in size from 1,000 to 10,000 km. They are located in the hotzone region of a star's solar system. Their surfaces are partially molten and may feature active volcanoes with an overall high surface temperature. Their atmospheres, if any, are extremely tenuous, with few active gases. They almost never have life forms.

Example: Mercury (Sol I)

Class C Geoinactive
Class C worlds range in age from about 2 to 10 billion years old. Their diameters range in size from 1,000 to 10,000 km. They are located in the ecozone or coldzone regions of a star's solar system. Their surfaces are geologically inactive, with usually cold temperatures. Their atmospheres, if any, are usually frozen upon their surface. They almost never have life forms.

Examples: Pluto (Sol IX), Psi 2000

Class D Asteroid/Moon
Most asteroids and planetoids fall under Class D. They are commonly found orbiting planets as moons. They range in age from about 2 to 10 billion years old. Their diameters range in size from 100 to 1,000 km. They can be located in any temperature region of a star's solar system. Their barren surfaces are geologically inactive, covered with craters, and have normally freezing temperatures. Their atmospheres, if any, are tenuous. They almost never have life forms.

Examples: Luna (Sol IIIa), Regula, Rura Penthe, Ta'Deb

Class E Geoplastic
Class E worlds are younger than 1 billion years old. Their diameters range in size from 10,000 to 15,000 km. They are located in the biozone region of a star's solar system. Their surfaces are molten, and have high temperatures. Their atmospheres are primarily hydrogen with other reactive gases. Class E planets cool over time to evolve into Class F worlds. Life forms, if any, are Carbon-based.

Example: Excalbia

Class F Geometallic
Young planets, Class F worlds range in age from 1 to 3 billion years old. Their diameters range in size from 10,000 to 15,000 km. They are located in the biozone region of a star's solar system. Their surfaces are actively volcanic. Their atmospheres primarily contain hydrogen compounds. Class F planets cool over time to evolve into Class G worlds. Life forms, if any, are silicon based.

Example: Janus VI

Class G Geocrystaline
Young planets, Class G worlds range in age from 3 to 4 billion years old. Their diameters range in size from 10,000 to 15,000 km. They are located in the biozone region of a star's solar system. Their surfaces are still crystalizing. Their atmospheres contain carbon dioxide and other toxic gases. Class G planets cool over time to evolve into Class N, O or P worlds. Life forms, if any, are primitive single-celled organisms.

Example: Delta Vega

Class H Desert
Class H worlds range in age from 4 to 10 billion years old. Their diameters range in size from 8,000 to 15,000 km. They are located in the hotzone, biozone or coldzone regions of a star's solar system. Their surfaces are barren, hot, and arid, except in the coldzone where they can be covered with empty tundra. Their atmospheres may contain heavy gases and metal vapors. Life forms, if any, would have to be both drought and radiation-resistant flora and fauna.

Examples: Rigel XII, Tau Cygna V, Ocampa, Nimbus III

Class I Gas Supergiant
Class I planets range in age from 2 to 10 billion years old. Their diameters range in size from 140,000 to 10 million km. They are usually located in the coldzone region of a star's solar system, but can exist in any region. They are solid gas, tenuous, comprised mostly of hydrogen and helium compounds, and may have water vapor as well. Temperatures vary in the cloud layers. They may contain a solid metallic mass core. They also radiate heat. A Supergiant can have hundreds of moons and also water ice rings. Life existing on a Class I is uncertain. Life forms, if any, 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.

Example: Q'tahL

Class J Gas Giant
Class J planets range in age from 2 to 10 billion years old. Their diameters range in size from 50,000 to 140,000 km. They are usually located in the coldzone region of a star's solar system, but can exist in any region. They are solid gas, tenuous, comprised mostly of hydrogen and helium compounds, and may have water vapor as well. Temperatures vary in the cloud layers. They may contain a solid metallic mass core. They also radiate some heat. A Gas Giant can have dozens of moons and also water ice rings. Life existing on a Class J is uncertain. Life forms, if any, 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.

Examples: Jupiter (Sol V), Saturn (Sol VI)

Class K Adaptable
Class K planets range in age from 4 to 10 billion years old. Their diameters range in size from 5,000 to 10,000 km. They are located in the ecozone region of a star's solar system. They have rocky, barren surfaces with only trace amounts of water. Their atmospheres are thin, mostly carbon dioxide. Life forms, if any, are limited to single-celled organisms and algae. Class K planets are suitable for human colonization through terraforming.

Examples: Mars (Sol IV), "Mudd's planet", Son'a Prime

Class L Marginal
Class L planets range in age from 4 to 10 billion years old. Their diameters range in size from 10,000 to 15,000 km. They are located in the ecozone region of a star's solar system. They have rocky, barren surfaces with little water. Their atmospheres are oxygen/argon with a high concentration of carbon dioxide. Life forms, if any, are limited to plant life. Class L planets are suitable for human colonization with some terraforming.

Examples: Indri VIII, Vaadwaur Prime, and possibly Borg assimilated worlds.

Note: Earth may have become a Class L planet, as was seen briefly in the time conduit in Star Trek: First Contact, after the Borg altered the timeline and assimilated it.

Class M Terrestrial
Class M planets range in age from 3 to 10 billion years old. Their diameters range in size from 10,000 to 15,000 km. They are always located in the ecozone region of a star's solar system. Their surfaces are comprised with 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 to exist. Their atmospheres contain oxygen/nitrogen with other trace gases. Life forms are almost always present, flourishing as extensive plant and animal life. Usually a sentient race is also present.

Class M planets are also called Minshara Class, a Vulcan term.

Planets with more than 80% surface water fall under Class O or Class P.

Examples: Earth (Sol III), Vulcan (40 Eridani A), Cardassia Prime, Risa, Bajor, Delta Vega, Rigel VII, Qo'noS

Class N Reducing
Class N planets range in age from 3 to 10 billion years old. Their diameters range in size from 10,000 to 15,000 km. They are usually located in the ecozone region of a star's solar system. Class N worlds are barren and rocky. Their surfaces temperatures are usually high due to an intense greenhouse effect. Water exists, but only as vapor. They have extremely dense atmospheres containing carbon dioxide and sulfides. Life forms, if any, would have to be adapted to such an extremely harsh environment. There may be single-celled organisms living in the upper layers of atmosphere.

Example: Venus (Sol II)

Class O Pelagic
Class O planets range in age from 3 to 10 billion years old. Their diameters range in size from 10,000 to 15,000 km. They are always located in the ecozone region of a star's solar system. Class O worlds have an extreme abundance of water covering more than 80% of their surface. They have Class M-like atmospheres, with oxygen/nitrogen and other trace elements. There is almost always life, but almost all of it is aquatic plant and animal life, with little land mass to evolve surface creatures. Sentient races evolving on such worlds are likely to be aquatic in nature.

Example: Argo

Class P Glaciated
Class P planets range in age from 3 to 10 billion years old. Their diameters range in size from 10,000 to 15,000 km. They are usually located on the extreme edge of the ecozone region of a star's solar system. Class P worlds have an extreme abundance of water, but much of it is frozen in ice, covering more than 80% of the surface. They have Class M-like atmospheres, with oxygen, nitrogen, and other trace elements. There is usually hardy plant and animal life surviving in the tundra.

Examples: Exo III, The Breen homeworld

Class Q Variable
The environment of a Class Q fluctuates because they orbit a variable output star. These planets range in age from 2 to 10 billion years old. Their diameters range in size from 4,000 to 15,000 km. They are usually located in the hotzone or ecozone regions of a star's solar system. Surfaces range from molten rock to water and/or carbon dioxide ice due to the variable output of the star they orbit. Their atmospheres range from tenuous to extremely dense. Life, if any, would have to rapidly adapt to sudden changes in temperatures.

Examples: Genesis Planet

Class R Rogue
Class R rogue planets do not orbit stars. They range in age from 2 to 10 billion years old. Their diameters range in size from 4,000 to 15,000 km. Surfaces are usually barren, but some might be temperate due to geothermal venting. Atmospheres tend to be filled with volcanic outgassing. They are not believed to support life, but some might have animal and non-photosynthetic plant life.

Examples: Dakala, Founders' Homeworld, Gothos (as it was under the control of the Trelane entity).

Class S and T Ultragiants
Class S and Class T planets are super gas giants. They range in age from 2 to 10 billion years old. Their diameters range in size from 10 to 50 million km (Class S) and 50 to 120 million km (Class T). They are usually located in a solar system's coldzone, but they can be in any region. Their surface is tenuous, composed of hydrogen and helium compounds. They radiate considerable heat and tremendous gravity. The atmosphere varies in temperature, pressure and composition at different layers. There may also be water vapor present. Life forms may only exist as single-celled organisms if any exist at all. They may have hundreds or thousands of moons, some of which may be terrestrial planets of Class M, O and P.

Examples: N/A Note: In real life, some recently discovered extrasolar planets could classify as Ultragiants.

Class X, Y and Z Demon
Classes X, Y, and Z are reserved for planets referred to as "Demon" worlds. They are usually hostile to humanoid life because they contain turbulent, sometimes volcanic environments with atmospheres filled with toxic and corrosive gasses. They range from 10,000 to 50,000 km in diameter, and can reside in any region of a star's solar system. They generate thermionic radiation, and exhibit very high surface temperatures. Life forms, if any, will probably be silicon-based or mimetic in nature, as experienced on Demon class worlds of the Delta Quadrant.

Example: Tholian Homeworld (Class Y)

Planets in our real solar system
Our solar system does not have planets in all of the classes listed. The planets in our solar system are, by this classification:

Mercury: Class B

Venus: Class N or Class Y

Earth: Class M

Mars: Class C or Class H or Class K

Asteroid Belt: Class D objects

Jupiter: Class I or Class J

Saturn: Class J

Uranus: Class J

Neptune: Class J

Pluto: Class C

Kuiper Belt: Class C objects

Oort Cloud: Class C objects