Typically 1,000 - 10,000 km in diameter, with a partially molten surface and an atmosphere primarily composed of hydrogen compounds. These planets cool to become class C. They are not habitable. Examples include Gothos.1
Up to 10 billion years old, these planets are 1,000 - 10,000 km in diameter. They are partially molten, with a high surface temperature. They have virtually no atmosphere. Examples include Mercury (Sol I).1
Aged 2 to 10 billion years, these worlds are 1,000 - 10,000 km in diameter. They have a low surface temperature, with any atmosphere frozen on the surface. They do not harbour life. Examples include Pluto (Sol IX) ans Psi 2000.1
A small rocky planetoid with an airless surface - essentially a great rock in space. The class D planetoid Regula was chosen as the site for the second phase of the Genesis project, and the Starfleet corps of engineers spent ten months in space suits tunnelling out a complex under the surface to facilitate the project. The second phase was a success, creating a cavern several miles across in the planetoid.
Class D objects are not habitable by Human life forms without extensive life support facilities.2
Aged 0 to 2 billion years, these worlds are 10,000 - 15,000 km in diameter. They have a high surface temperature, with a molten surface and an atmosphere composed of hydrogen compounds and reactive gases. They are rarely inhabited, but at least one such world - Excalbia - has been found to harbour carbon-cycle life forms.3
Aged 1 to 3 billion years, these worlds are 10,000 - 15,000 km in diameter. They are volcanically active and have an atmosphere composed of hydrogen compounds. They cool to become class G planets. Some class F worlds have evolved silicon based life, such as the Horta which inhabit Janus IV.3
Age 3 to 4 billion years, Class G planets are 10,000 - 15,000 km in diameter. Their surfaces are largely composed of crystalline material, and they have a carbon dioxide atmosphere with some toxic gases mixed in. They cool to become Class K, L, M, N, O, or P worlds. They are generally inhabited only by simple life forms such as single celled organisms. Examples include Delta Vega.4
Generally extremely dry, class H planets are marginally habitable. Tau Cygna V is a good example of a habitable class H planet. With an atmosphere capable of supporting Human life, the planet was bathed in levels of hyperonic radiation thought lethal to Human life. Nevertheless, when a colony vessel crash-landed on the planet in the 2270s the colonists were eventually able to adapt to the radiation and survive. Class H planets are also suitable for colonization by the Sheliak.5
Gas supergiants, these worlds are 140,000 - 10,000,000 kilometres in diameter. They have no solid surface as such; atmospheric zones vary in composition, temperature and pressure throughout. No known Class I planet is inhabited. Examples include Q'tahL.6
Class J planets are gas giants. Largely composed of Hydrogen and Helium, they have turbulent atmospheres with wind speeds in excess of 10,000 kilometres per hour. Jupiter and Saturn in the Sol system are examples of class J planets. Both planets have extensive collections of moons, while Saturn also has a major ring system comprising many hundreds of individual rings. There are several Starfleet facilities located near the planets, including the Jupiter station and a Starfleet Academy training range.7
Class K planets have gravity fields which are suitable for Humanoid life, but are otherwise uninhabitable. They are only suitable for habitation with the use of pressurized domes or underground habitats using life support systems. Planet Mudd was class K and at one time sustained a sophisticated civilization. This died out some time ago, leaving advanced robot technology behind which was utilized by Harcourt Fenton Mudd in order to briefly capture the Starship Enterprise.8
These are small rocky worlds with an oxygen-argon atmosphere. They are sometimes capable of supporting life, but this tends to be limited to plant life only. Class L planets tend to have a high concentration of atmospheric carbon dioxide. In 2373 a Starfleet runabout containing two personnel from the Deep Space Nine station crash-landed on a class L planet.9 In 2340 the class L planet Indri VIII was discovered by the Federation. It became the subject of a scientific research project by several governments in 2369 when it became apparent that an ancient species had seeded many planets, including this one, with genetic codes designed to direct evolution on the planet towards a Humanoid form. The codes apparently failed to perform this function on Indri VIII. In order to prevent anybody from recovering samples from the planet, a Klingon vessel destroyed the entire biosphere with a plasma reaction.10
Voyager encountered a class L planet in the Delta Quadrant which had a flourishing Human civilization upon it, descended from Humans abducted from Earth in the 20th century.11 Other class L planets are not so habitable, however - in 2374 the crew of the USS Defiant raced to rescue a Starfleet officer from a class L planet whose atmosphere was so inhospitable that she required regular doses of tri-ox compound. The attempt was unsuccessful.12
Class M is the Federation designation for planets which have similar environmental conditions to those on Earth. The term comes from the Vulcan designation, Minshara class. In our galaxy only one in every 42,000 planets is M class.13 They are approximately 12,000 kilometres in diameter and have temperatures which allow liquid water to exist on large parts of the surface. The atmospheres tend to be an oxygen-nitrogen mix, highly supportive of organic life. Starfleet tends to focus much of its effort on class M planets, since these are the ones most commonly inhabited by Humanoid life.
Aged 3 to 10 billion years and 10,000 to 15,000 km in diameter, these worlds have a high surface temperature due to a runaway greenhouse effect in the atmosphere. Water exists only as a vapour on a Class N world. The atmosphere is extremely dense with very high levels of carbon dioxide and sulfides. No known Class N world has evolved life. Examples include Venus (Sol II).14
Age 3 to 10 billion years, Class O planets are 10,000 - 15,000 km in diameter. Their surfaces are largely covered in liquid water, with only 20% or less land area. The atmospheres are composed of nitrogen and oxygen with trace elements. Life forms generally include aquatic vegetation and animal life. Examples include Argo.4
Age 3 to 10 billion years, Class P planets are 4,000 - 15,000 km in diameter. Their surfaces are heavily glaciated, with water ice covering 80% or more of the total surface area. The atmosphere is composed of nitrogen and oxygen, with trace elements. Surface life includes hardy plant and animal life and occasionally humanoids. Examples include Exo III.14
Age 2 to 10 billion years, Class Q planets are 4,000 - 15,000 km in diameter. Their surfaces are highly variable as a result of their eccentric orbits or a variable output from their star; surface temperatures range from molten rock to liquid water to solid carbon dioxide in a single year. Atmosphere can range from virtually none to highly dense. Examples include the Genesis planet.4
Rouge worlds, Class R planets have been thrown from their solar systems and travel through deep space. They are usually 4,000 - 15,000 km in diameter. Their atmospheres are generally composed of volcanic outgassings; volcanic activity can create large zones which are warm enough to be habitable, and non-photosynthetic plants can thrive in these areas along with animal life. Examples include Dakala.4
An ultragiant planet, S class worlds are 10 - 50 million kilometres in diameter. They are largely composed of gaseous hydrogen, though the atmosphere varies considerably in composition, pressure and temperature at different levels. They radiate a great deal of heat, though they are not large enough to be classed as stars.4
A class of gas giants. T class planets can have orbital ring systems, sometimes including radiogenic particles.15
Also known as a Demon class planet, these are the most inhospitable worlds known to the Federation. The surface temperatures are at least 500 Klevin, and the atmosphere is highly toxic and corrosive. Worst of all, Demon class planets have very high levels of thermionic radiation which make it dangerous for a vessel even to enter orbit.
Voyager encountered a Demon class planet in 2374 which had extensive deposits of Deuterium on the surface contained within some form of liquid metal. The ship, which was very low on fuel at the time, landed on the planet n order to mine supplies. The crew were surprised to discover that the liquid metal was in fact a life form.16