The Federation scientific community and Starfleet uses a planetary classification system to categorize planets and planetary bodies.
Asteroidal/Planetecimal[]
Terrestrial[]
Class A - Proto-planet[]
Protoplanets are planetary embryos within protoplanetary discs around young, forming stars. They are at least as large as 1022 kg (or around 1000 km in radius), or when they begin to accumulate an extended atmosphere, increasing the rate of capture of smaller planetesimals through atmospheric drag. They are formed through the accretion of dust particles and collisions between planetesimals.
Protoplanets are likely to still be in a process known as planetary differentiation, which occurs when the temperature of the planet is high enough to be at least partially molten. Heating, due to radioactive decay, impacts with other planetesimals and protoplanets, and gravitational pressures, melt parts of protoplanets, causing heavier elements to sink and form a planetary core, while lighter elements rise to the surface.
Protoplanetary atmospheres undergo dramatic turnovers, accreting materials from the surrounding protoplanetary disc impacts and losing materials through heating and impacts. Lost materials can also be replaced through out-gassing from the mantle.
Class B - Ferrinian, Ogunian[]
Massive stars and young stars exhibit intense stellar winds. These winds can erode and even immolate terrestrial planets that have formed near their star. In these cases, much of the lighter planet-building material is blown away, leaving behind only the heavier elements. There is almost no rocky material; instead the planet is composed almost entirely of materials such as iron, nickel and cobalt. These planets are typically 750 to 1500 km in radius with a mass of 1022 to 1023 kg. Their surfaces are typically heavily cratered, with signs of primordial geologic activity, and often have a substantial magnetic field.
The atmosphere on these worlds tends to be extremely rarified or completely absent, composed entirely of a thin, tenuous layer of hydrogen and helium which is constantly being removed and replenished by the stellar winds.
These worlds are quite rare. Most stars do not possess stellar winds strong enough to completely immolate planets in orbit around them. Far more common are Class C, or Hermean, planets.
Class C - Hermean[]
These worlds are formed in a manner similar to Class B (Ferrinian) planets. They have lost most of their lighter materials through erosion by stellar winds or from suffering a cataclysmic impact. What is left is a large metallic core and a thin, lighter rocky crust. The primary difference between these planets and class B planets is one of composition and density. These planets have significantly more lighter elements, such as silicates and carbonates, than Ferrinian worlds, and as such, have a lower average density. Their surfaces are typically heavily cratered, with signs of primordial geologic activity, and often have a substantial magnetic field.
The atmosphere on these worlds tends to be extremely rarified or completely absent, composed entirely of a thin, tenuous layer of hydrogen and helium which is constantly being removed and replenished by the stellar winds.
An example of this class of planets is Mercury, in the Sol system.
Class D - Selenian[]
Many worlds that form from planetary disks are neither very massive, nor composed of much heavy metals. These worlds are geologically dead. After creation, the only internal geological events are small quakes powered by internal gases slowly seeping to the surface. Besides being poor in heavy metals, Selenian worlds are virtual deserts of volitile materials, as well. Selenian worlds are often found as companions to larger, more metal rich bodies.
Selenian worlds are not massive enough to retain any atmosphere.
An example of this class of planet is Luna, Earth’s moon.
Class E - Lithic-Gelidian[]
Lithic-Gelidian worlds form past the snow line of the solar system. They are composed of rocky and ice materials and tend to be large, but have very low density. They may show evidence of primordial geologic activity, or may undergo geologic activity caused by tidal stresses.
Lithic-Gelidian worlds are typically not massive enough to retain atmospheres. Some may, and other may have atmospheres that are constantly replenished by core out-gassing. These atmospheres tend to be composed of nitrogen, oxygen, methane and other primordial gasses.
Examples of this class of planet include Ganymede, Calisto, Titania, and Pluto.
Class F - Europan[]
Europan worlds are like Lithic-Gelidian worlds, but are formed with much larger amount of water ice. Water is so plentiful that it covers the surface of the world in a massive ocean, ranging from several to hundreds of kilometers deep. This surface ocean is then sealed in a thick layer of ice. The planetary core may or may not be geological active, but the world tends to show some activity, caused by pressures of the ocean.
Europan worlds have no atmosphere.
Europan worlds can theoretically support life, at the bottom of its oceans around warm geological vents.
An example of a Europan world is Europa, a moon of Jupiter.
Class G - EoGaian[]
Class H - Setean[]
Class I - Salinean[]
Class J - Halogenic[]
Class K - Arean[]
Class L - MesoGaian[]
Class M - Gaian, Minshara[]
Class N - Cytherean[]
Class O - Paludal[]
Class P - Tundral[]
Class Q - Pelagic[]
Class R - Ionian[]
Class S - Titanian[]
Class T - Ymirian[]
Class U - Panthalassic[]
Class V - Variable[]
Class W - Hesperean[]
Class X - Unclassified[]
Class X is reserved for any planet without a known classification. Most planets are discovered from afar, using different telescopic techniques. While it is relatively simple to locate a planet from afar, it is often difficult to closely study and classify a planet until it can be visited in a starship. Until a newly discovered planet is studied and properly classified, it is classified as X.