Planets' natural characteristics, such as age, mass and distance from their sun, place them in classes which have been assigned arbitrary alphabetic designation. A planet's distance from its sun, relative to that sun's luminosity, puts the planet in one of three thermal zones: hot, habitable or cold. A planet mass determines its internal heat generation and, in combination with its zone, its ability to hold an atmosphere. Some planets pass through distict stages as they form and age, which may put them in separate classes.
| Class A | Geo-Thermal |
|
Class A are young planets, typically 1 to 1/10 the mass
of Earth, and are in their sun's habitable or cold zones. Their surface
is patially molten and the atmosphere is primarily hydrogen compounds. These
planets cool to become Class C.
Example: Gothos |
| Class B | Geo-Morteus |
 |
Class B planets are small, typically 1 to 1/10 the mass of
Earth and are in their sun's hot zone. Due to a combination of weak gravity
and strong solar radiation, their atmospheres are very tenuous, with few
chemically active gases, and their surfaces are extremely hot.
Example: Mercury |
| Class C | Geo-Inactive |
 |
Class B planets are small, typically 1 to 1/10 the mass of
Earth and are in their sun's cold zone. Due to a combination of low solar
radiation and little heat, their atmospheres are permanently frozen.
Example: Pluto, Psi-2000 |
| Class D | Asteroid / Moon |
 |
Class D planets are small, typically 1 to 1/100 the mass
of Earth, or less. Due to low gravity, they have lost their atmospheres.
Their surfaces, directly exposed to radiation and meteor impact, are typically
lifeless and heavily cratered.
Example: Yonada, Regula |
| Class E | Geo-Plastic |
 |
Class E planets are typically of about the mass of Earth
and are in their sun's habitable zone. They are newly formed, and their
surfaces are still molten. Their atmospheres still retain many hydrogen
compounds, as well as reactive gases and rock vapors. These planets will
cool, becoming Class F.
Example: Excalbia |
| Class F | Geo-Metallic |
 |
Class F planets are typically about the mass of Earth and
are in their sun's habitable zone. They are younger than the Earth, and
their surfaces are covered with volcanic eruptions due to a large molten
core. Their atmospheres retain small amounts of toxic gases. As these planets
continue cooling they become Class G.
Example: Janus VI |
| Class G | Geo-Crystaline |
 |
Class G planets are typically about the mass of Earth and
are in their sun's habitable zone. They are younger than Earth, and their
surfaces are still crystalizing. As they continue to cool, these planet
may become Class K, L, M, N, O, or P.
Example: Delta Vega |
| Class H | Desert |
 |
Class H planets are typically the size of Earth and can be
found in all zones. They are extremely dry, possibly have oxygen-argon atmospheres
and possibly be bathed in lethal radiation, but can sometimes be habitable.
Example: Tau Cygna V, Rigel XII |
| Class J | Gas Giant |
 |
Class J planets are large, typically 10 to 100 times the
mass of Earth, and are in their sun's cold zone. Low solar radiation and
high gravity have allowed them to keep thick atmospheres of hydrogen and
hydrogen compounds. Wind speeds up to 10,000 kph are possible. Core pressure
may be high enough to generate heat.
Example: Jupiter, Saturn |
| Class K | Adaptable |
 |
Class K planets are typically 1 to 1/10 the size of Earth,
and are in their sun's habitable zone. They are unsuitable for humanoid
life but can be adapted through the use of pressure domes and life support
systems.
Example: Mars, Elba II Penal Colony |
| Class L | Marginal |
 |
Class L planets are typically the size of Earth and are in
their sun's habitable zone. They have oxygen-argon atmospheres and possibly
high levels of carbon-dioxide. Humanoids may need respirators or tri-ox
supplements to survive. They can possibly support life, but this is often
limited to plant life.
Example: Indrii VIII |
| Class M | Terrestrial (Minshara class) |
 |
Class M planets are typically about the mass of Earth and
are in their sun's habitable zone. Their atmospheres contain significant
oxygen, liquid water is a significant surface feature, and lifeforms are
generally abundant.
Example: Earth, Vulcan, Bajor |
| Class N | Reducing |
 |
Class N planets are typically of about the mass of Earth
and are in their sun's habitable zone. Due to the greenhouse effect of dense
atmospheres heavy in carbon dioxide, their surfaces are very hot and water
is found in vapor form, if present at all.
Example: Venus |
| Class O | Pelagic |
 |
Class O planets are typically about the mass of Earth and
are in their sun's habitable zone. Their atmospheres contain significant
oxygen, liquid water covers over 97 percent of the surface. Lifeforms are
generally abundant. With less water they would be Class M.
Example: Argo, Pacifica |
| Class P | Glaciated |
 |
Class P planets typically the mass of Earth and are in their
sun's outer habitable zone. Due to low solar radiation, their temperatures
are extremely low. The atmospheres can be permanently frozen.
Example: Breen Homeworld, Exo III |
| Class Q | Variable |
 |
Class Q planets are typically the size of Earth and can be found in all zones. Surfaces range from molten to water and/or carbon dioxide ice, due to eccentric orbit or variable output of star Example: Genesis |
| Class R | Rogue |
 |
Class R planets are typically the size of Earth and are found in interstellar space or cometary halos. Their surface may be temperate due to geothermal venting. Atmosphere's consist of volcanic outgassing. Example: Dakala |
| Class S | Gas Supergiant |
 |
Class S planets are very large, typically 3,000 times the
mass of the Earth, and are in their sun's cold zone. Low solar radiation
and high gravity have allowed them to keep thick atmospheres of hydrogen
and hydrogen compounds. High core temperatures cause them to radiate enough
heat that liquid water is present.
Example: Beta Pictoris B |
| Class U | Gas Ultragiant |
 |
Class U planets are very large, typically 10,000 times the
mass of Earth, and are in their sun's cold zone. Low solar radiation and
high gravity have allowed them to keep thick atmospheres of hydrogen and
hydrogen compounds. High core temperatures cause them to radiate visible
light. These are the largest possible planets, as more massive bodies generate
enough core heat to initiate fusion reactions and become stars.
Example: Beta Pictoris C |
| Class Y | Demon Class |
 |
Also known as Demon Planets, planets and planetoids of this
class can be found in any of a star's zones. They are typically 10,000 to
15 thousand kilometers in diameter. Atmospheric conditions are often turbulent
and saturated with poisonous chemicals and thermionic radiation. Surface
temperatures can reach in excess of 500 Kelvins. It can be dangerous for
a starship even to orbit such a planet.
Example: |