The following is based on the planetary classification system used in Gregory Mandell's
Star Trek Star Charts and Chris Adamek's variant found at
The Final Frontier, themselves based on the planetary classes so far named in televised
Star Trek (classes D, H, J, K, L, M, N, T and Y). I've tweaked it considerably, though, to hopefully make it more closely match both what we've seen on screen and the types of planets found in reality. It has also incorporated David Sudarsky's gas giant classification scheme.
The classification scheme used on
Trek is based around class-M being an Earth-type planet. In the original series we saw numerous class-M planets that ranged from being virtually identical to the Earth to all manner of oddly hued worlds, but all with a breathable atmosphere (except for Arret, which was described as class-M in spite of having lost its atmosphere). Other than the one class-K planet (Mudd), we hear very little about other classes, but the simple rule remained M = habitable.
From the movies and
TNG onwards, more classes were introduced, such as the barely habitable class-H, the gas giant class-J and the barren class-D. The latter has been used very inconsistently, applying to a ringed gas planet in
Voyager "Emanations" and the arid but habitable planet in
Voyager "Gravity."
TNG introduced class-L as a planet with a breathable atmosphere but otherwise unsuited to animal life (at least long-term), but
Voyager gave us several class-L planets with humanoid civilisations (in the episodes "Muse" and "The 37s," notably). Over the years, the idea that only class-M planets are habitable has been lost, with Mandell's scheme including various classes that would have been included under M in the original
Trek. I've tried to centre the scheme back on class-M here.
Enterprise revealed that M stands for "Minshara," a Vulcan term.
TNG "The Royale" featured an obscure "Transjovian" class-K with a thick cold atmosphere, that I've tried to incorporate into the below scheme. The hellish Class Y was created for
Voyager "Demon" and appeared a couple of times since, and the Class T ultragiant was featured in
Voyager "Good Shepherd." Other classes mentioned over the years, such as Theta-class planetoids, class-9 gas giant and the Klingon
Q'tahl class don't fit into this scheme.
* Updated with a new class to contain Essof IV from
Star Trek: Discovery, which I've also presumed is the same class as Elba II from the original series. Had to squeeze it in as Class N3 but that'll have to do unless I start importing more letters.
(Images taken from various sources. Classes F, G, H, L, T, V, X and Y rendered by Chris Adamek at
The Final Frontier. Classes A and O nicked from Wookiepedia. Classes B, D, E, I, J, K, M, N1, N2, P and Q are all photographs of real planetary bodies. Kudos if you can identify them all.)
Class A
Molten
Hot zone/lunar orbit
E.g. Gothos
Class A planets are young, rocky planetoids, the surface of which is kept at least 50% molten due to the proximity of the parent star or planet, via direct heating or gravitational effects. The atmosphere is thin, boiled away by the intense heat but replaced by volcanic outgassing. Due to the tenuous nature of the atmosphere, the heat released by the volcanic activity quickly dissipates into space.
Life forms: none
Class B
Ferrous/iron planet
Hot zone/ecosphere
E.g. Mercury, Kepler-10b, Proxima d
Small, mostly metallic rocky planetoids. Class B worlds exhibit a highly iron-rich crust, with a magnetic core and no mantle. Atmosphere thin to negligible, with little to no heat retention. The surface varies from extremely hot to cold dependent on position near star, and can exhibit molten surface areas. The night side of the planetoid will fail to retain the heat exhibited on the day side, left a frigid wasteland. These planetoids are inimical to life.
Life forms: none
Class C
Carbon planet
Hot zone/ecosphere
E.g Janssen (55 Cancri e)
Predominantly carbon-based planet, appearing blackened from orbit due to large deposits of graphite. The pressure within the mantle and outer core produces diamond deposits. The atmosphere is composed primarily of carbon dioxide, rich in hydrocarbons and monoxide smogs. Little to no surface water is to be expected on the surface of a carbon planet.
Life forms: anaerobic carbon-based life may be possible
Class D
Asteroidal/dwarf
Hot zone/ecosphere/cold zone/lunar orbit
E.g. Luna, Ceres, Regula, Paan Mokar
Rocky bodies varying in size from the tiniest planetessimal to planet-sized moons. Common around larger planetary bodies and in asteroid belts. Atmosphere tenuous, although water ice can manifest at the poles. Although naturally lifeless, Class D worlds may be adapted through use of pressure domes or oxygen caverns.
Life forms: none.
Class E
Ice dwarf
Cold zone/outer cloud
E.g. Pluto, Eris, Psi 2000
Small, sub-planetary bodies common in the outer star system, in the orbit of Class I planets, through the scattered disc and out into the Oort Belt. With a rocky crust covered in nitrogen ice, and an atmosphere tenuous in the extreme, Class E worlds are incapable of retaining the limited heat they receive from their distant parent star. There may, however, be subsurface water, heated by mantle activity, which can provide the basis for colonisation through pressure domes.
Life forms: rare, microbial.
Class F
Primordial
Hot zone/ecosphere
E.g. Excalbia
Young planets that are still developing, Class F planets represent the earliest stage of the formation of a habitable world. With partially molten surfaces, atmospheres rich in reactive gases and heavy vulcanism, Class F planets are inimical to life like ours, but have, on rare occasions, developed inorganic life, when present in the hot zone and continued in their plastic state for long enough. Those further out will cool over billions of years to become Class G, the next step in their evolution.
Life forms: metal-carbon complex (e.g Excalbian)
Class G
Developing
Hot zone/ecosphere
E.g. Janus VI
With a primarily silicate-based crust, these planets have cooled and solidified from Class F to form a more stable surface, although vulcanism is still rife. Water has begun to condense to form oceans, amid centuries of constant rainfall. The atmosphere and the life that may develop on the surface are intertwined; as the rich carbon dioxide atmosphere allows early photosynthetic life to flourish, these organisms flood the atmosphere with oxygen, pushing towards the next stage in its evolution. Over many millions of years further, these Cambrian-stage planets cool further to become classes H, K, L, M, N, O and P, dependent on various factors.
Life forms: primitive organic or silicon-based life, more rarely advanced silicon-based life (e.g Horta)
Class H
Extreme desert
Hot zone/ecosphere
E.g. Tau Cygna III, Shelia, Nimbus III
Rocky planets with primarily silicate crusts, Class H planets are true desert worlds. With very limited surface and atmospheric water, and high levels of surface radiation, Class H planets are not conducive to complex ecosystems, although hardy life may develop and flourish. Milder Class H environments may be colonised by humanoids with some adaptation. Class M planets can be reduced to Class H through environmental damage.
Life forms: radiation-resistant carbon-based organisms (e.g Sheliak). Not naturally conducive to humanoid life.
Class I
Ice giant/neptunian
Cold zone
E.g. Uranus, Neptune, Marijne VII
Cold worlds with thick atmospheres of hydrogen, water, methane and ammonia, commonly found in the outer reaches of a solar system. The hydrogen envelope is considerably thinner than on a Class J world, but this is still the dominant element of the planet. Such planets commonly attract a number of moons and impressive ring systems. In spite of the name, ice giants have little solid material and are mostly fluid.
Life forms: unknown
Class J
Gas giant/jovian
Ecosphere/cold zone
E.g. Jupiter, Saturn, Cherela
Huge planets with thick hydrogen and helium-based atmospheres, rich in hydrocarbons. Beneath the gaseous layers lies liquid hydrogen above a metallic hydrogen core. Class J planets commonly support many moons and ring systems, and these moons may themselves be habitable worlds in their own right. Class-J planets dominate a star system in the inner region of the cold zone. With sufficient engineering prowess, habitable Class M environments can be constructed between the cloud layers of a gas giant.
Class J planets correspond to classes I to III on the Sudarsky scale. The coolest are Class I jovians, Jupiter-type planets with ammonia clouds, often with complex and powerful weather systems. Warmer are the Class II jovians, which feature water vapour clouds. Class III jovians have no chemical components that form clouds and appear as featureless blue-white orbs.Those straying closer to the star are captured and are heated to Class-S.
Life forms: Jovian-type, hydrocarbon-based (e.g Lothra)
Class K
Adaptable
Ecosphere/cold zone
E.g. Mars, Mudd
Class K planets are essentially dead terrestrial planets, with a primarily silicate crust, rich mineral deposits and no magnetic field. The atmosphere is thin, predominantly carbon dioxide, and retains little heat, leading to a frigid desert landscape. Nonetheless, there can be some weather systems in a Class K atmosphere, and vulcanism can occur. Water and/or carbon dioxide ice may be found at the poles. Class-K environments can develop from the evolution of Class G, or through the long deterioration of classes G, L or M. Rich in mineral deposits. Although fundamentally lifeless except for the most basic of organisms, Class K planets are readily adaptable through use of pressure domes or oxygen caverns, and are prime targets for terraforming.
Life forms: microbial carbon or silicon-based life.
Class K/T
Transjovian
Cold zone
E.g. Theta-116-VIII
This subclass represents frozen class-K planets that have drifted or been expelled into the outer system, commonly by gravitational perturbation by a larger body. A thick atmosphere of nitrogen, neon and methane accretes and can develop turbulent weather systems. Transjovian-class planets are highly inhospitable and experience phenomenally low surface temperatures.
Life forms: none
Class L
Marginal
Ecosphere
E.g. Phylos, Kaijur 12, Kokytos
Similar to Class M planets, Class L are on the borderline of life-bearing environements. Typically rocky, silicate-crust planets, Class L worlds are commonly arid, but in some cases display oceans or tundra. Surface temperature varies considerably, and the atmosphere is thinner than on a Class M world, with high levels of argon, carbon dioxide, and often other toxic gases. Radiation levels are potentially dangerous. Class L environments may feature basic ecosystems, normally only with plant life. They may, however, be colonised by humanoid life, and are excellent targets for terraforming. (Planets assimilated by the Borg, where the atmosphere has been altered by pollution with carbon monoxide, methane and fluorine, may be considered a variant of Class L).
Life forms: Most have no native animal life. Plant life often abundant on more temperate examples.
Class M
Terrestrial
Ecosphere/lunar orbit
Also referred to as "Earth-type," S3 or Minshara-class, Class M planets are the cradles of life. With silicate crusts, those with rotating iron cores can display strong magnetic fields. Rich nitrogen-oxygen atmospheres with some carbon dioxide, water vapour and trace gases are ideal for the development of varied, complex biospheres. Class M planets feature high surface and atmospheric water content, essential for organic life. Surface conditions can vary considerably across the globe, from tundra, to temperate, to desert environments. Class M worlds are found in orbit of stars or larger Class-I, J and U planets, and can vary widely in visual appearance. Class M is divided into subtypes dependent on surface water levels and other features, and these can vary over the course of a planet's lifespan (for instance, Earth was a Type-4 ice-world during one period of its early history, and Exo-III was once a more hospitable Type-2).
Life forms: abundant carbon-based life, including humanoids
M Type-1 Arid. E.g. Vulcan, Cardassia Prime, Deneb IV, Lambda Paz
Surface water 25-50%
M Type-2 Temperate/varied. E.g. Earth, Bajor, Altamid, Kaminar
Surface water 50-80%
M Type-3 Pelagic. E.g. Argo, Azati Prime, Antede III
Surface water 80-95%
M Type-4 Glacial. E.g. Andoria, Exo-III, Rigel X, Delta Vega
Surface ice 50-95%
M Irregular E.g. Barzan II, Ba'ku planet, Planet Hell, Gaia
Class M but with unusual features, such as, atmospheric variances, radiation belts and ring systems.
Class N1
Reducing
Hot zone
E.g. Venus
Although similar to Class M planets in size and geological make-up, Class N planets are rendered as hugely different environments due to their atmospheric conditions. A thick carbon dioxide atmosphere causes a runaway greenhouse effect leading to extremely high surface temperature and pressure, utterly inimical to humanoid life. Some nitrogen, water and sulphur dioxide exist in the atmosphere, which is dominated by clouds of sulphuric acid, leading to corrosive rainfall. A Class N world may potentially be adapted to class-M by long-term terraforming, but this is a significant undertaking and such planets are usally overlooked in favor of more hospitable worlds.
Life forms: rare; microbial organisms may exist in cloud layer.
Class N2
Sulphuric
Hot zone/lunar orbit
E.g Tholia, Io
A variation of the Class N planet in which a considerably thinner atmosphere, composed mainly of sulphur dioxide and monoxide, sodium chloride vapours and molecular oxygen. Large deposits of sulphur exist on the surface giving a yellow-green colour from orbit. Temperature is lower than N1 conditions, but still high in comparison to Class M, with significant vulcanism caused by gravitational effects from the host planet or star, or by an unstable core. Unlike on N1 worlds, N2 enviroments may develop complex organic life, although such organisms will rely of sulphur respiration and use hydrogen sulphide as a biological solvent in place of water. This life form type is far rarer than the more common oxygen/water type organisms.
Life forms: sulphurphilic organisms (e.g Tholian)
Class N3
Corrosive
Hot zone/ecosphere
E.g. Elba II, Essof IV
Planets with less severe atmospheric effects than classes N1 and N2, Class N3 worlds are still highly dangerous for organic life forms. Displaying variable surface temperatures and pressures, N3 atmospheres are predominantly carbon monoxide, laced with corrosive and reactive chemicals such as sodium perchlorate, rapidly toxic to oxygen-breathing life. However, they are also rich in useful chemicals such as deuterium, and can be used for small habitats using pressure domes.
Life forms: none
Class O
Oceanic
Ecosphere
E.g. The Waters, Megara, Kepler-22 b
True ocean worlds with no surface land area. Oceans on Class O planets are typically thousands of kilometres deep, with phenomenal pressures at the depths. Turbulant atmospheres of nitrogen, oxygen, water vapour and carbon dioxide envelop the planet. On hotter variants of the Class O, the ocean surface may vapourise, giving a continuous fluid surface, rather than a delineated ocean and atmosphere, on the edge of becoming a Class U world.. Cooler Class O worlds can potentially be colonised with artificial habitats, and have considerable scope for food cultivation in the form of plankton and algae.
Life forms: abundant, marine carbon-based organisms.
Class P
Cryoterrestrial
Cold zone/lunar orbit
E.g. Titan, Breen
Similar in size and structure to Class M planets, but in far colder regions, Class P planetoids display enivronments that are like frigid shadows of terrestrial worlds. With a dense nitrogen-methane atmospheres, and surface rich in hydrocarbons, the seas and oceans on Class P worlds are comprised from short-chain hydrocarbons such as methane and ethane. In place of rock, mountains and landmasses form from water ice; cryovolcanism is apparent. These planetoids display a subzero ecosystem. In the later stages of a star's evolution, Class P worlds may be heated to another evolutionary stage, dooming existing ecosystems and pushing the planetoid towards classes K, L or M.
Life forms: hydrocarbon and ammonia-based
Class Q
Cryo-ocean
Cold zone/lunar orbit
E.g. Europa, Ganymede, Enceladus
Ocean worlds in colder regions, these are smaller planetoids enclosed in thick water ice crusts. Atmosphere is tenuous, beneath the ice layer exists an extremely deep ocean. Undersea heating from the planetary core, or gravitational effects from a host planet, can lead to non-photsynthetic ecosystems. Commonly form as moons around planets of classes I, J and U. Can potentially be colonised with artificial habitats, although care must be taken not to damage the existing, submarine environment.
Life forms: marine carbon-based organisms
Class R
Rogue/orphan planet
Interstellar
E.g. Dakala, Omarion
A varied class, containing those bodies that are planet-sized but not tied to a star's gravity. Such bodies, sometimes called planemos, can range from terrestrial to Jovian size; the largest are on the borderline with the brown dwarf class. Rogue planets form in the interstellar void from accreted material, while orphan planets are ejected from star systems by gravitational effects. Thick, carbon-rich atmospheres can lead to retained surface heat and non-photosynthetic ecosystems, sometimes displaying very unusual adaptations to their harsh environment.
Life forms: varies, from none to complex; carbon or silicon-based
Class S
Hot jovian/pegasid
Hot zone
E.g. Galileo (55 Cancri b), Osiris, 51 Pegasi b
Gas giants, similar to classes I and J but in short, close stellar orbit, maintaining an extremely high temperature. Carbon monoxide is the dominant carbon-carrying molecule. Class S planets correspond to classes IV and V on the Sudarsky scale, with Class IV being the cooler of the two, displaying alkali metal vapour clouds. The hottest planets are Class V, with silicates and even iron forming clouds. These planets glow red due to the high thermal output.
Life forms: none known
Class T
Gas supergiant/ultragiant
Cold zone
E.g. Kappa Andromedae b
Gigantic gaseous planets with thick hydrogen atmospheres and enormous gravitational pull, these planets are on the verge of becoming stars. Supergiants accrue complex systems of moons ranging from planetesimal to planetary size, effectively becoming miniature star systems in themselves. Any such bodies that exceed 13.6 Jupiter masses would begin deuterium fusion and become a brown dwarf or "substar."
Life forms: unknown
Class U
Transitional
Hot zone/ecosphere/cold zone
E.g, Dulcinea (Mu Arae c), Kepler-10c
Existing in size between the Class I ice giants and the Class V superterrestrials, Class U planets are large enough and with strong enough gravity to retain a thick atmosphere of hydrogen, helium and hydrocarbons. The atmosphere transitions to oceans of semisolid compressed water above a rocky core. Sometimes known as gas dwarfs - something of a misnomer for such large planets.
Life forms: Jovian-type, hydrocarbon-based.
Class V
Superterrestrial
Ecosphere/cold zone
E.g. COROT-7 b, Gliese 163 c, Persephone
The so-called "super-Earths," large rocky/metallic planets intermediate in size between terrestrial and ice giants. Their higher gravity allows them to retain dense, hydrogen-rich atmospheres. Surface temperature and pressure high and unsuitable for humanoid habitation, but complex high-temperature life can evolve, and they are potentially viable for colonisation using pressure domes.
Life forms: silicon or carbon-based, adapted for higher pressures
Class W
Divided/locked
Hot zone/ecosphere/lunar orbit
E.g. Daled IV, Klavdia III, Remus
Rocky planets kept tidally locked to the parent star or sister planet by the intense gravitational interaction of other bodies in their system. One side is overlit and heated, displaying molten areas and a burnt, desert-like surface. The far side is kept in perpetual darkness and cold, sometimes with a more temperate dividing line if the atmosphere is dense enough to mediate the heat. Such planets may be colonised, and some display native life that has adapted to the extreme environment, often in unusual ways.
Life forms: microbes and plants, some display higher organisms.
Class X
Chthonian
Hot zone
E.g. COROT-7b
The dead core of a Class-S or T planet, stripped of its atmosphere by millennia of stellar activity. Dense and metal-rich, these planetoids are rare and valuable. Uninhabitable and ultimately doomed to absorption by their parent star.
Life forms: none
Class Y
Demon-class
Hot zone
E.g. Ha'dara, Theta Zeta
Exceedingly unfriendly, these planets display thick atmospheres rich in toxic gases, high radiation levels, extreme surface pressure and corrosive conditions, even harsher than Class-N planets.
Life forms: rare, but mimetic life has been discovered.
Class Z
Pulsar planet
E.g. Draugr, Poltergeist, Phobetor
Planets found in orbit of pulsars (rapidly rotating neutron stars), bathed in intense magnetic radiation and inimical to all known life. Subdivided by origin, pulsar planets may form from the remains or cores of destroyed companion stars, or may be more ordinary planetoids captured by the pulsar's gravity.
Dandy also catches a giant cuttlefish alien which goes for 5000 woolongs, and a bunch of aliens at the ski resort that look like a snowman, an oni and even Santa Claus.
