Planetary classification

In planetology, **planetary classification** is the classification of planets and moons based on their environments. Inspired by stellar classification, it also uses letters for each planetary type. There are 5 main classes of planets: *T*, *D*, *O*, *S*, and *G*. Additionally, there are subclasses for some of the classes as well as additional classes for rarer planets.

## Standard classification

### *T*-class planets

The most commonly colonised planet class is the *T* class. *T*-class planets (for ***T**erra*), also known as Earthlike planets or Earth analogues, are characterised by an atmospheric density greater than 1 nanobar and the presence of water on their surface. This can be in the form of oceans, lakes or simply river systems.

#### Notable examples

* Earth
* Avalon
* Proxima

#### Subclasses

*T*-class planets with one predominant biome are often categorised with their dominant biome. For example, Tempest is primarily arid and is thus classified as a Barren *T*-class planet. Seraph's landmasses are almost entirely covered in ice year-round, leading to its classification as a Cold *T*-class planet. Planets primarily covered in lush forests may be classified as Lush *T*-class planets.

Along with *D*- and *O*-class planets, *T*-class planets with a radius greater than that of Earth (6,371 km) are referred to as Super-Earths while planets with a radius smaller than that of earth are referred to as Sub-Earths.

### *D*-class planets

*D*-class planets (for ***D**esert*) are planets characterised by an atmospheric density greater than 1 nanobar but no surface water. Subterranean oceans do not make a planet *T*-class.

#### Notable examples

* Mars
* Venus
* Trafalgar

#### Subclasses

There are few generally agreed-upon subclasses for *D*-class planets. The three most common ones are dependent on the planet's surface temperature. If the planet's temperature is too cold for humans, such as on Mars, the subclass Cold *D*-class is used. Habitable *D*-class planets such ass Trafalgar are dubbed Temperate *D*-class. Finally, planets with temperatures too hot for humans like Venus are classified as Hot *D*-class planets.

### *O*-class planets

Formerly considered a subclass of the *T* class, *O*-class planets (for ***O**ceania*) are characterised by an atmospheric density greater than 1 nanobar and a surface covered entirely with water. Note that the water does not have to be liquid, as there are *O*-class planets with polar ice caps.

### *S*-class planets

The final official class of rocky planets, *S*-class planets (for ***S**elena*) are characterised by an atmospheric density smaller than 1 nanobar. These are usually barren, uninhabitable planets with settlements in the form of domed cities or pressurised habitat modules.

#### Notable examples

* Luna
* Mercury
* Pluto

#### Subclasses

A notable distinction often made with *S*-class planets is based on their composition: planets composed of silicate rock are classified as Rocky *S*-class planets whereas planets composed of water ice are classified as Icy *S*-class planets.

### *G*-class planets

*G*-class planets (for ***G**aseous*) are gas giants with no solid surface.

#### Notable examples

* Saturn
* Uranus
* Baal

#### Subclasses

Similar to *S*-class planets, *G*-class planets are often split into two subclasses based on composition. *Gα*-class planets are gas giants primarily composed of light gases such as hydrogen and helium, whereas *Gβ*-class planets (sometimes known as *I*-class) are ice giants that lie beyond their star's frost line and are composed of heavier elements such as carbon, sulfur, oxygen and nitrogen.

## Further classes

### *C*-class planets

*C*-class planets (for ***C**arbon*) are theoretical planets that contain more carbon than oxygen. Such a planet would probably have an iron- or steel-rich core like the known terrestrial planets. Surrounding that would be molten silicon carbide and titanium carbide. Above that, a layer of carbon in the form of graphite, possibly with a kilometers-thick substratum of diamond if there is sufficient pressure.

### *F*-class planets

*F*-class planets (for ***F**errum*) are planets that consist primarily of an iron-rich core with little or no mantle. Iron-rich planets are smaller and denser than other types of planets of comparable mass. Such planets would have no plate tectonics or strong magnetic field as they cool rapidly after formation. Wet iron planets in the habitable zone of their host stars may be covered by lakes of iron carbonyl and other exotic volatiles rather than water.