Planetary Classes

Earth is classified as a medium temperate terra, as its temperature (14.9°C) and size (6371 km radius) fall within the criteria for both ranged categories, and it is the archetype for the terran class. Its moon, Luna, is a dwarf frigid selena.
— Example planetary classification

Planets are the comparatively tiny motes of dust and gas that provide a platform for the vibrant verdancy of the living cosmos. There is an incredibly broad range of planetary "phenotypes," but all planets have a few critical similarities that define them as such. According to the International Astronomical Union's 2050 definition, a planet is a celestial body that:

(a) has sufficient mass for its own self-generated gravity to overcome rigid-body forces and assume a hydrostatic equilibrium in a spheroid shape.

(b) does not undergo self-sustaining fusion or naturally emit light at any point in its life cycle (exceptions being surface lava, lightning, or fires).

This definition is broad enough to include moons and brown dwarfs, though these two types of objects must be specified by name. Beyond this, planets are described by a system of classifications split into three categories. The planetary class designations of planets and moons are one-word labels representing particular defining aspects of themselves, intended to convey to the reader a very general but fairly accurate assessment of what the planet is like in each category. A planet's biosphere is classified according to a separate system.

Planetary Classification System

Size

All planets and moons can be classified as one of the following size classes based on their average radius:
  • Substellar (radius of 64,000 km to 96,000 km)
  • Giant (radius of 32,000 km to 64,000 km)
  • Large (radius of 12,000 to 32,000 km)
  • Medium (radius of 4,000 to 12,000 km)
  • Small (radius of 1,000 km to 4,000 km)
  • Dwarf (radius of 1,000 km or less)

Temperature

All planets and moons can be classified as one of the following temperature classes based on their average surface temperature:
  • Frigid (-50°C and below)
  • Cold (-50°C to 0°C)
  • Temperate (1°C to 50°C)
  • Hot (51°C to 100°C)
  • Scorched (101°C and above)

 

Type

All planets and moons can be classified as one of the following types based on their overall geophysical structure, composition, solvent presence, and atmosphere.

Selenae

Luna.jpg

by Gregory Rivera

Luna, the selenic planet archetype.
Selenic planets (designated by s) are worlds with extremely thin to nonexistent atmospheres; in essence they are spherical asteroids. Because of this, their surface features are remarkably well-preserved, though they are also consequently subject to extreme temperature fluctuations between sunlight and shadow.   Selenae have heavier cores, typically iron or nickel, surrounded by thin, solid crusts of lighter elements. This is not to say that selenae are always tectonically dead; larger selenae retain some pseudo-tectonic activity caused by the extreme heating cycles of their surface. A selenic planet with endemic life is unheard of, though technically not impossible. Chemosynthetic or photovoltaic autotrophs could form colonies on the airless surfaces of selenae, though this is as yet unproven.  

Desertae

Mars.jpg

by ESA (Rosetta)

Mars, the desert planet archetype.
Desert planets (designated by d) are worlds with an overall arid climate and scarce precipitation or surface solvent. This is not to say desertae are entirely devoid of solvent; it often subsists under the surface, inside permanent polar ice caps, or even on the surface in liquid state.   Desert worlds have compositions mirroring terrae, though tectonics are not universal among desertae. Life is not common on desert worlds, but exists in relative abundance; typically organic but occasionally exotic.   Venusiae (designated by v) are a subset of desert worlds with heavy atmospheres, unique among planetary types in being a subclass. Venusian worlds rarely have endemic life, but some have unicellular biospheres composed of exotic extremophiles that thrive in the heat and high pressures.  

Cthoniae

Nergal.jpg

by UNAC (DSASF) / Doug Marshall

Nergal, the cthonic planet archetype.
Cthonic planets (designated by ct) are worlds defined largely by their geologic structure, which in turn is shaped by their temperature: they are molten planets of high metallic and metalloid content, with temperatures in the thousands on the centigrade scale.   Very few cthoniae are known, but those that do exist (Nergal (Altair) and Icarus (Fomalhaut), among others) show similar characteristics. In the case of Icarus, the life cycle of cthonic planets is evident: cthoniae are the remaining cores of joviae whose atmospheres have been stripped away by solar winds, usually because of extremely close proximity to their star.   A cthonic world with endemic life is unheard of, though technically not impossible. Chemosynthetic or thermosynthetic extremophiles with exotic chemical compositions may eke out stubborn existences on these exceedingly rare hellish worlds, though this is as yet unproven.

Cryonae

Europa.jpg

by NASA (JPL) / Bjorn Jonsson

Europa, the cryonic planet archetype.
Cryonic planets, better known as icy planets (designated by i) are worlds whose surfaces are covered primarily by solidified volatiles[1]. This glacial shell may span all the way to the planet's solid iron-silicate core. More frequently, however, these icy crusts hide global liquid oceans warmed by either a core dynamo effect between the core and whatever thin mantle may exist, or tidal heating in the case of tidally locked worlds, planetary binaries, or moons of other, more massive planets.   Icy planets may have substantial atmospheres if they are large enough, but smaller cryonic worlds have hyper-rarefied atmospheres typically characteristic of selenae. Cryonae sometimes have native life, most often in whatever liquid ocean exists beneath the icy crust but occasionally playing host to surface-dwelling microbes.  

Titanae

Titan, the titanoid planet archetype.
Titanoid planets (designated by ti) are worlds that blur the line between cryonae and terrae, with the volatile composition of cryonae but surface features mirroring terrae. The internal structure of titanae is the strangest of any planetary class, fusing the tectonic crust of terrae with the global subglacial ocean of cryonae; the icy continents drift atop a mantle-like sea.   Titanae have solvent cycles, as terrae do, but the solvent is typically ammonia or liquid hydrocarbon rather than water due to the extremely low temperatures typical of titanae.   Life on titanoid worlds is typically organic in the sense that it is based on carbon chemistry, but this chemistry is far removed from the far more common water-based organic life. In rare cases, the mantle seas host entirely separate biospheres from the surface.  

Terrae

Earth

by NASA (Earth Observatory)

Earth, the terran planet archetype.
Terran planets (designated by tr) are terrestrial ("rocky") worlds that have a self-perpetuating tectonic cycle maintained by the planet's inner heat and the lubricating effect of the oceans.   Terran worlds are rigidly stratified into three or more layers: a solid inner core, a molten mantle, and a solid crust. They are composed mostly of stable transition metals and silica, though this is concentrated in the inner layers (especially the core). The crust and surface, by contrast, consists largely of lighter metalloids and nonmetals (such as silicon, carbon, and aluminum).   Life is common on terrae, typically organic but with the occasional exotic biosphere. The presence of life maintains the oceans and atmosphere, just as the oceans and atmosphere allow for the presence of life in the first place. In light of this elaborate circular dance of geology and biology, terrae are by far the most complex of the nine planetary archetypes.

Oceaniae

Pon'whe.jpg

by UNAC (Henry Stanley) / Doug Marshall

Pon/whe, the oceanic planet archetype.
Oceanic planets (designated by o) are worlds whose surfaces are completely covered by solvent, and often a substantial portion of their mass is said solvent. The global seas lay atop a dense nickel-iron and/or silicate core, which is typically surrounded by a mantle of high-pressure ice.   In cases where high-mass, high-pressure oceanic worlds have dense, hot atmospheres, the solvent may even enter supercritical state[2], blurring the distinction of sea and sky. The solvent mass of oceaniae is almost always water, though an ammonia oceanic world exists: Tiamat in the Altair system. Life on oceanic worlds is common and virtually always organic, even on oceaniae with supercritical atmospheres, as only carbon-based life interacts with water as a safe, non-acidic biological solvent.  

Neptuniae

Neptune.jpg

by NASA (JPL - Voyager 2)

Neptune, the neptunian planet archetype.
Neptunian planets (designated by n), also called ice giants, are large vaporous worlds composed primarily of elements heavier than hydrogen and helium, typically volatiles.   Like joviae, neptunian worlds usually have a solid core of silicates, iron, and nickel at their center, surrounded by the hot, supercritical mantle of volatiles which is in turn enveloped by an atmosphere of gaseous hydrogen, helium, and volatiles. In essence, ice giants occupy the planetary spectrum between oceaniae and joviae.   Life on neptuniae is actually rather common, but tends to be exotic more often than organic: based on nitrogen, phosphorus, and other alternative biochemistries. Most neptunian biospheres are aerial in nature, though in denser strata the inhabitants function more like marine life.  

Joviae

Jupiter.jpg

by NASA/ESA (Hubble Space Telescope)

Jupiter, the jovian planet archetype.
Jovian planets (designated by j), also called gas giants, are large worlds composed primarily of gaseous hydrogen and helium. Unlike the sharply defined boundary between atmosphere and surface characteristic of terrestrial planets, joviae do not have a surface in any conventional sense; rather, jovian atmospheres have a density gradient toward their cores.   The mantle of a gas giant is composed of supercritical metallic hydrogen and neutral helium, gradually fading to gaseous hydrogen. The outermost layers of joviae are the most complex and beautiful: trace elements form an ever-shifting labyrinth of multicolored cloud strata and cyclones.   Life on joviae is fairly rare and almost always exotic in biochemistry, metabolizing the abundant hydrogen in jovian atmospheres. The native lifeforms of Bellerophon (EQ Pegasi) are carbon-based, but this case is unique among studied worlds.

 

Footnotes

[1] Volatiles are chemical compounds with freezing points above about 100 K, such as water, methane, ammonia, or the carbon oxides.
[2] Supercritical fluids occur when a substance is subjected to a temperature and pressure high enough that distinct liquid and gas phases do not exist. It can effuse through solids like a gas, and dissolve materials like a liquid.  

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Comments

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3 Jun, 2019 15:09

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4 Jun, 2019 18:29

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4 Jun, 2019 06:47

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4 Jun, 2019 18:28

thank you so much! I'm glad you enjoyed it!