Planets' natural characteristics, such as age, mass and distance from their sun, place them in classes which have been assigned arbitrary alphabetic designation (*see footnote*). 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 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
Life Forms: unknown
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
Life Forms: unknown
Class C 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
Life Forms: Tholians
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, asteroids
Life Forms: Fabrini, Vedala
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
Life Forms: Carbon-cycle
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 still crystallizing. Their atmospheres retain small amounts of toxic gases. As these planets continue cooling they may become classes C, L, M or N.
Example: Janus VI
Life Forms: Horta
Class G planets are typically about the mass of Earth and are in their sun's hot zone. Their gravity allows them to retain an atmosphere of heavy gases and metal vapors, but due to strong solar radiation, their surfaces are very hot.
Example: Medusa
Life Forms: Medusans
Class H planets are typically the size of Earth and are in their sun's habitable zone. 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
Life Forms: Humanoid
Class I planets typically the mass of Earth and are in their sun's outer habitable zone or cold zone. Due to low solar radiation, their temperatures are extremely low. The atmospheres can be permanently frozen.
Example: Breen Homeworld
Life Forms: Humanoid
Class J planets (Gas Giants) 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
Life Forms: unknown
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
Life Forms: Primitive invertebrates
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
Life Forms: Humanoid
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. With more water they would be Class N.
Example: Earth, Vulcan, Bajor
Life Forms: Humanoid
Class N 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
Life Forms: Humanoid
Class S planets (Gas Supergiants) 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
Life Forms: unknown
Class U planets (Gas Ultragiants) 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
Life Forms: unknown
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: Voyager's Demon Planet
Life Forms: mimetic
This Planetary Table is similar but not identical to most of the other charts that can be found on other Star Trek RPG web sites. An attempt was made to bring it closer to canon Trek planetary classifications as used on the various television series.