The Federation classifies planets it catalogs based on criteria such as atmospheric composition, surface temperature, and conditions, the size of the body, and the presence of animal and plant life. This system is used to determine the suitability of the planet for exploration, colonization, and scientific research.

Each class of planet is assigned a letter in the alphabet based on its suitability. For example, Class M (sharing meaning with the Vulcan designation Minshara class) is applied to planets that can support life without any special circumstances, such as Earth, Vulcan, or Cardassia.

The Klingons had a separate classification system, one of which is a Q’tahL class.

Asteroid Belt

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Asteroid belts are a region of space where millions of asteroids are found orbiting their star. Asteroid belts are made up of material that was never able to form into a planet, or of the remains of a planet which broke apart. The asteroids in an asteroid belt come in a variety of sizes. Some are very small (less than a mile across), while others are quite large.

Class A

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Class A planets are generally small, barren worlds rife with volcanic activity. This activity traps carbon dioxide in the atmosphere, causing a greenhouse effect that keeps temperatures very hot, regardless of the planet’s location in a star system. When the volcanic activity eventually ceases, the planet “dies” and becomes a Class C world.

Class B

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Class B planets are small, rocky worlds located within a star system’s hot zone. Unsuitable for humanoid life, Class B planets have highly unstable molten surfaces. The thin atmospheres composed primarily of helium and sodium. In the harsh daylight, temperatures can approach 450° Celsius; because there is little atmosphere to retain that heat, it can get as cold as -200° Celsius at night. No life forms have ever been observed on Class B planetoids.

Class C

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When all volcanic activity on a Class A world ceases, the planet is then considered Class C. Essentially dead, these small, rocky worlds have cold, barren surfaces and no atmosphere. While remaining highly unsuitable for humanoid life, Class C planets are often rich in minerals and suitable for mining.

Class D

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A Class D planetoid is a tiny world that generally does not meet the criteria for a planet; this includes moons, asteroids, and small planet-like objects. Dwarf planetoids can be composed of rock or ice; many are not even spherical, and have eccentric orbits cluttered with various even smaller objects. Most of these planetoids are not suitable for humanoid life, though many can be colonized via pressure domes.

Class E

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Class E worlds represent the earliest stage in the formation of a habitable planet. The core and crust are completely molten, making the planet susceptible to solar winds and radiation, and subject to extremely high surface temperatures. The atmosphere is very thin, and composed of hydrogen and helium. As the planet cools, the core and crust begin to harden and the planet becomes Class F.

Class F

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These barren worlds are witness to much geologic activity; steam expelled from volcanic eruptions condenses into water and forms the first shallow seas, in which bacteria may develop and ultimately thrive. As the core of a Class F planet cools, the volcanic activity lessens and the planet eventually transitions to Class G.

Class G

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After the core of a Class F planet is sufficiently cool, volcanic activity lessens and the planet becomes Class G. Oxygen and nitrogen are present in some abundance in the atmosphere, giving rise to increasingly complex organisms such as primitive vegetation like algae, and animal akin to sponges and jellyfish. As a Class G planet continues to cool, it can finally transition into the final stage of its evolution, a Class H, K, L, M, N, O, or P world.

Class H

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Dry, arid planets with less than 20% of the surface covered in water are considered Class H. Though many of these worlds are very hot and sandy, these are not requisite for desert classification. In fact, desert worlds may be both cold and rocky. Though precipitation is rare, drought-resistant plants and animals are common here, and many Class H worlds are inhabited by humanoid populations.

Class I

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Also known as Uranian planetoids, these frozen giants are vastly different in composition from their gaseous brethren. The core is mostly rock and ice, surrounded by tenuous layers of water, methane, and ammonia. Additionally, the magnetic field is sharply inclined to the axis of rotation. Class I planets form on the fringe of a star system.

Class J

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Class J planets, also known as Jovian planets, are massive spheres of liquid and gaseous hydrogen, with small cores of metallic hydrogen. Their atmospheres are extremely turbulent, with wind speeds in the most severe storms reaching 600 kph. Many Class J planets also possess impressive ring systems, composed primarily of rock, dust, and ice. They form in the cold zone of a star system, though typically much closer than Class I, S, or U planets.

Class K

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Though similar in appearance to Class H worlds, Class K planets lack the robust atmosphere of their desert counterparts. Though rare, primitive single-celled organisms have been known to exist, though more complex life never evolves. Humanoid colonization is, however, possible through the use of pressure domes and in some cases, terraforming.

Class L

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Also known as “Super Earths”, these terrestrial worlds are generally similar to Class M planets, but on a much larger scale. Class L planets tend to be rocky and heavily forested, and while they are suitable for humanoid and animal life, the scarce water supply makes natural evolution of these populations unlikely. As such, Class L planets are prime candidates for terraforming.

Class M

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Class M planets are robust and varied worlds composed primarily of silicate rocks, and are highly suited for humanoid life. To be considered Class M, between 20% and 80% of the surface must be covered in water; it must have a breathable oxygen-nitrogen atmosphere and temperate climate.

Class N

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Though frequently found in the ecosphere, Class N planets are not conducive to life. The terrain is barren, with surface temperatures in excess of 500° and an atmospheric pressure more than 90 times that of a Class-M world. Additionally, the atmosphere is very dense and composed of carbon dioxide; water exists only in the form of thick, vaporous clouds that shroud most of the planet.

Class O

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Any planet with more than 80% of the surface covered in liquid water is considered Class O. These worlds are usually very warm and possess vast cetacean populations in addition to tropical vegetation and animal life. Though rare, humanoid populations have also formed on Class O planets.

Class P

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Any planet whose surface is more than 80% frozen water is considered Class P. These glaciated worlds are typically very cold, with temperatures rarely exceeding the freezing point. Though not prime conditions for life, hearty plants and animals are not uncommon, and some species, such as the Aenar and the Andorians, have evolved on Class P worlds.

Class Q

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Exceedingly rare in nature, Class Q planets typically develop with a highly eccentric orbit or near stars with a variable output. They can also be generated artificially, such as Planet Genesis in 2285. Given their unstable nature, surface conditions on Class Q planets are widely varied; deserts and rain forests can exist within a few kilometers of each other, while glaciers simultaneously lie near the equator. Very basic bacteria and vegetation can form here, but given the extreme conditions, evolving more advanced life is virtually impossible.

Class R

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A Class R planet begins its life as a normal world in a star system, but at some point in its evolution, the planet is violently ejected, likely the result of a catastrophic asteroid impact or a wandering Class U planet. The transition radically changes the planet’s evolution; many simply die, but geologically active planets can sustain a habitable surface via volcanic outgassing and geothermal venting.

Class S

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Aside from their colossal size, there is little that differentiates a Class S world from its Class J counterparts. Located in a star system’s cold zone, they often boast impressive ring systems and harbor dozens of moons. Giant worlds like Class S and the other gaseous planetoids tend to act as “shields” for the terrestrial planets in the ecosphere, as their powerful gravitational fields tend to divert comets away from the interior of a solar system.

Class T

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Class T planets, share the same characteristics as other gas giants but are defined by massive ring systems.

Class U

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A Class U planet represents the upper limits of planetary masses. Structurally similar to their Class J and T counterparts, only on a far more grandiose scale, most Class U planets are content to loom on the edges of a star system. However, the great mass of these giant worlds occasionally causes them to assume eccentric orbits that cause them to spiral inward toward the heart of the star system and become a “Hot Jupiter”, a gas giant orbiting extremely close to its parent star. This destructive process disrupts the entire star system, ejecting smaller planets into interstellar space, and ultimately ends with the Class U planet’s demise as a desolate Class X world.

Class X

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When a Class U planet spirals into the hot zone of a star system, its proximity to the parent star typically results in gravitational forces stripping the atmosphere from the planet. The end result is a Class X planet, which is little more than the exposed core of the Class U world. These dense, metal-rich worlds are valuable some have surfaces composed of diamond—but are short lived. Doomed by their inward spiral from the cold zone, Class X planets are ultimately absorbed by their parent star and completely obliterated.

Class Y

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Perhaps the most environmentally unfriendly planets in the galaxy, Class Y planets are inhospitable to life in every way imaginable. The toxic atmosphere is plagued by exceptionally violent storms that discharge thermionic radiation; surface temperatures exceed 200° Celsius, and winds can exceed 500 kph. Incredibly, a type of biomimetic life form was discovered on a Class Y planet in the Delta Quadrant in 2374.

Ring System

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A ring system is a disc or ring orbiting an astronomical object that is composed of solid material such as dust and moonlets, and is a common component of satellite systems around giant planets. A ring system around a planet is also known as a planetary ring system.

The most prominent and most famous planetary rings in the Solar System are those around Saturn, but the other three giant planets (Jupiter, Uranus, and Neptune) also have ring systems. Recent evidence suggests that ring systems may also be found around other types of astronomical objects, including minor planets, moons, and brown dwarfs.

Source(s):
Memory Alpha
Star Treh the Final Frontier