The following is the standard planetary classification system that is used in "Star Trek: Athena" and all the other series of "Star Trek"-based stories that the site creator has created over the years. This classification was first created in 1981 and has been slightly modified since then, and for this reason, it might not be fully canonical. However, when a particular planet class is needed for the purposes of an episode, this classification system is used.
Class-A
This is a gas giant planet, larger than Jupiter, consisting of primarily hydrogen and large enough that it emits more radiation than it absorbs from the star it orbits. Originally, it was thought that Class-A planets could not form close to their stars because the solar radiation would drive away the hydrogen, but evidence for such planets close to stars have been found. Although Class-A planets cannot support humanoid life, if they are close enough to their suns, satellites in orbit may be class-M.
Class-B
This is also a gas giant planet, but contains heavier gases like methane and ammonia. Such planets may also emit more radiation than they absorb, and they can display vivid cloud patterns and bands, and tend to be colourized according to the dominant consituents of the atmosphere. These planets are generally found a distance from their stars. Saturn and Uranus are examples of Class-B planets.
Class-C
This is a smaller version of a gas giant, and may have a large rocky core to hold a higher-density atmosphere consisting of carbon dioxide and nitrogen among others. Water may be present as water vapour in the atmosphere or as ice in the atmosphere or on the surface. The higher pressure may account for warm surface temperatures. An example would be a world like Venus, only much larger.
Class-D
A Class-D planet is a young, primodal planet that may be of Earth's diameter or larger, with a rocky crust and mantle and a metallic core, although the planet components might not be completely separated yet. The atmosphere is primitive, with carbon dioxide, hydrogen, helium and sulfur compounds in abundance, and volcanic activity may be extensive. Life is not yet possible on such a world, but may arise. An example of a Class-D planet would be Earth in the first five hundred million years of its existence.
Class-E
A Class-E world is the opposite of a Class-D one. This is an old planet, perhaps in orbit around an expanding red dwarf star. Life has expired on such a world, and its atmosphere is mostly carbon dioxide and nitrogen, perhaps with some water vapour and lighter gases if it is cool enough. Class-E worlds are no longer inhabitable, but archaeological remains might be found on them.
Class-F
This is a large, silicate or rocky world, of diameters equal to Earth or greater, without a significant atmosphere. Planets like this can be close to the sun and quite warm (almost like a super-sized Mercury) or more distant and covered with ices from a former atmosphere. Class-F planets are heavily cratered, and are not considered suitable for habitation, although mining operations might be conducted.
Class-G
A Class-G world is similar to a Class-F one, except that it has a dense atmosphere. A Class-G world may support life, but it would be life adapted to much higher gravity than is present on Earth, and so is not suitable for humanoid life. Because of higher pressure, the range of liquid water would be shifted, so the life that appears on such a planet would be quite different than life on Class-M worlds. This type of planet is rare in the galaxy, as most Class-G planets would have an atmosphere so dense that the temperature would exceed the boiling point of water.
Class-H
This type of world is so water-logged that it is oceanic planet-wide, and would have a very humid atmosphere, global circulation patterns in the ocean and atmosphere and would have very predictable weather. The oceans of a Class-H world would be teeming with life, but none of course would ever evolve into land creatures. Class-H worlds have a diameter similar to that of Earth, or larger.
Class-I
A Class-I planet is a silicate planet ranging in size from that of Mars to larger than that of Earth. It has a thick carbon dioxide atmosphere, but is remote enough from the star that the greenhouse effect is not a runaway one. The surface is mostly desert, with low water content, and heavily eroded. Some Class-M planets that are on a fine equilibrium could tend towards a Class-I status.
Class-J
This is a variation of a Class-I planet where the greenhouse effect has produced very high temperatures. The atmosphere is carbon dioxide and other gases, including sulfur compounds, and the temperature is well above the boiling point of water. The surface is normally enshourded in clouds. Venus is a Class-J planet.
Class-K
A Class-K planet is an Earth-sized world with an atmosphere that is not breathable, consisting either of nitrogen or carbon dioxide, with the possibility of other constituents in various amounts. It has gravity, atmospheric pressure and temperature ranges comparable to Earth, and so planets such as these are inhabitable with domed structures to contain an atmosphere. It is popular to place prisons on such planets, since escape would only be possible through controlled transporters or shuttlecraft.
Class-L
A Class-L planet is a silicate planet that is too small or too cold to support a significant atmosphere or liquid water on the surface. The atmosphere is thin and consists of nitrogen, nitrogen compounds or carbon dioxide. Frozen water may be present in the ground. Class-L planets are geologically active. Such worlds may be terraformed if large enough or otherwise inhabited by environmentally-sealed structures. Mars is a Class-L world.
Class-M
The Class-M planet is one which is capable of or is supporting carbon-based life. It has an atmosphere that contains oxygen and carbon dioxide, along with abundant water vapour. Liquid water exists freely on the surface, and there is geological activity or tidal activity to speed along evolutionary effects. A Class-M world is usually inhabitable planetwide. Earth, of course, is a Class-M planet. In addition, Class-M worlds can be further subdivided:
Class-M0: This is a world that is capable of supporting sentient life but does not currently do so, nor is it expected to do so any time soon, "soon" meaning within ten to fifty million years. Worlds such as this are freely colonizable.
Class-M1: This is a world in which there is sentient life, but that life is not yet technologically advanced for contact with interstellar-capable races. The Prime Directive applies to such worlds, and they cannot be, under most conditions, be approached.
Class-M2: This is a world in which there is sentient life currently technologically advanced that it has interstellar-flight capability.
Class-M3: This is a world on which sentient life once lived, but at the current time there is no sentient life on such a planet. Such a planet may be colonizable, but it likely has reduced biodiversity and could be environmentally degraded.
Class-N
A Class-N planet is similar to a Class-M world except that the average temperature is high enough that liquid water exists only in restricted areas of the planet. Lifeforms on such a planet tend to be limited in range, and the planet is not always suitable for colonization, although some races will attempt to colonize such a world.
Class-O
Going in the opposite direction of a Class-N world is Class-O, which is similar to a Class-M world in terms of size and atmospheric composition, but its average temperature is low enough that life can only be supported towards the equator, while much of the planet is covered in ice caps. The range of life on such a planet is restricted to forms adapted to cooler conditions.
Class-P
A Class-P world is similar to a Class-M world, except that the land area is restricted to five percent or the surface area or less. Life is widespread on such a world, but the land forms are limited. Such worlds tend to be colonized by small settlements, but Class-P worlds cannot support large populations because of limited resources and agricultural capability.
Class-Q
A Class-Q world is a small, rocky world, possibily irregular shaped, with a diameter of one thousand kilometres or less. They usually lack atmospheres. Such worlds, of which asteroids would be grouped, could be mined if they have useful resources.
Class-R
A Class-R world is similar to a Class-Q world, except that it is small enough to contain a light atmosphere.
Class-S
Class-S worlds are found far from the system star because these worlds are composed of a mixture of ice, of water and various gases, and rock, and they may not be fully differentiated. Pluto is an example of this, as are the larger moons of the outer planets. If a Class-S world is in orbit around another planet, such as a gas giant, the gravitational effects might be significant enough to induce volcanic activity on them (like Io) or could lead to liquid oceans under ice (like Europa). Such worlds could be heavily cratered, or with rift valleys and escarpments from cracking and shrinking. They contain a wide variety of potentially useful materials.
Class-T
This is a generic term referring to any geologically young and unstable planet that is undergoing rapid change and evolution (geologically speaking), and so is shifting categories as they change and their atmospheric composition changes. Class-T worlds could be fragile and unstable.
Class-U
This is a world, usually without an atmosphere, that is geologically dense and dead.
Class-V
A Class-V planet is one that is usually small with a silicate crust and an iron core, heavily cratered and usually located close to a primary so that it is rotationally locked (keeps one side permanently facing the star). It has no atmosphere, and is not inhabitable but may be mined because it is a rich source of heavy elements. Mercury is a Class-V world.
Class-W
This is another generic term, and indicates a planet whose category has been changed due to some rather cataclysmic event, such as collision with another planet. The Class-W stage is when the planet is attempting to reorganize and form under gravity again.
Class-X
A Class-X planet is a brown dwarf, a star-like object that may be orbiting another, and with its own collection of planets.
Class-Y
A Class-Y planet is approximately Earth-sized to larger, with a rocky surface and a large core that makes the planet magnetically active. Active volcanoes and other causes influence the atmosphere, which is usually carbon dioxide, methane, sulfur compounds and nitrogen, with significant trace atmosphere. The atmosphere, interacting with the planet's magnetic field and energized by an active primary, is also magnetically active, making communications, sensors and even transporters through it risky. Severe thermal differences also cause weather on a Class-Y world to be extreme.
Class-Z
A Class-Z world is a smaller world that is inhabitable, and contains an atmosphere that is unusually dense for the size of the planet. The atmosphere may be active and partly or fully contained within a planet-wide plasma shield. The planet is dense, and is rich in heavy and active elements. Class-Z planets are quite rare.
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