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 such as 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||An uninhabitable planetoid, moon, or small planet with little to no atmosphere. Some were viable candidates for terraforming.||Regula, Weytahn|
|Class H||Generally uninhabitable for Humans, though viable for Sheliak.||Tau Cygna V|
|Class J||A type of gas giant.|
|Class K||Adaptable for Humans by use of artificial biospheres.||Mudd, Theta VIII (transjovian)|
|Class L||Marginally habitable, with vegetation but usually no animal life.||Indri VIII, Kelis' homeworld|
|Class M (Minshara)||Earth-like, with atmospheres containing oxygen and, typically, nucleogenic particles. Largely habitable for humanoid life forms.||Earth, Vulcan, Organia|
|Class T||A type of gas giant.|
|Class Y ("Demon")||A world with a toxic atmosphere and surface temperatures exceeding 500 Kelvin. Prone to thermionic radiation discharges.||The Silver Blood homeworld, Ha'Dara|
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.
The Worlds of the Federation also features an expanded classification system, though it differs from the Star Charts version. The Memory Beta article (linked below) lists both systems in their entirety.
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 and 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: