Planetary classification systems were used by many races to categorize planets. Various factors were taken into consideration, including atmospheric composition, surface temperature, vegetation and size. The classification system used by the Federation used single-letter designations, for example class M to describe a planet able to support humanoid life for long periods, while the Vulcans used the term "Minshara class" to describe a similar planet.
|Class D||Planetoid or moon; uninhabitable||Paan Mokar, Regula|
|Class H||Generally uninhabitable||Tau Cygna V|
|Class J||Gas giant||Jupiter, Saturn|
|Class K||Adaptable with pressure domes||Mudd, Theta 116 VIII (class K transjovian)|
|Class L||Marginally habitable||Indri VIII|
|Class M||Terrestrial||Earth, Risa, Bajor|
|Class T||Gas giant|
|Class Y||"Demon"||Silver Blood planet|
The Star Trek: Star Charts book, which was authored and advised by Trek staffers, listed many other planetary classes which may one day be recognized on-screen, but as of now they remain conjectural.
According to the Star Charts, a Class P planet is a "glaciated" planet. They have an age that ranges from three to ten billion years and a diameter between 10,000 to 15,000 kilometers. Class P planets are located in the ecosphere of a star system. A glaciated planet is categorized by a surface comprised of 80%+ water ice and an atmosphere consisting of nitrogen, oxygen and trace elements. Lifeforms found on class P planets include hardy vegetation, animal life, and occasionally native humanoids. It is widely believed that Breen is also a class P planet. However, according to Weyoun, it was in fact a lush world with a temperate climate, in which case it would have to be categorized as class M. (DS9: "When It Rains...")
Examples of class P planets: