Class A, B and C
Typically small, young planets whose class depends on their age and solidity of their cores.
Class D objects are planetoids like asteroids and some moons.
Class E, F and G
Typically, Proto-Earth-sized planets whose class depends on their age and solidity of their cores.
Class H planets appear in the series as harsh desert worlds.
Class of gas giant, larger than Class J, and smaller than Class S and T.
Class J and T
Class J and Class T planets are gas giants. Class J are smaller than Class T which are considered “super”, or “ultra”, gas giants.
Class K planets are barren worlds with no native life. They do not possess breathable atmospheres, but have reasonably tolerable gravity, thus they can be colonized with atmospheric domes. However, through terraforming, they can be made into Class M worlds.
Class L planets are barely habitable worlds with primitive ecosystems.
Class N planets have a reducing environment and are barren and rocky with extremely high surface temperatures caused by thick atmospheres containing carbon dioxide and corrosive sulfides.
From the Vulcan term “Minshara,” is a fictional classification for planets and planetoids in the Star Trek science fiction media franchise.
Their atmospheres are composed of nitrogen and oxygen and have an abundance of liquid water necessary for carbon-based life to exist. Extensive plant and animal life often flourishes; often, a sentient race is also present. Earth is a textbook example of a Class M world.
Class O and P
Planets covered almost completely with water (class O), or water–ice (class P).
Planets with continually changing environments caused by peculiar orbits, an orbit around a variable output star, or some other factor which causes conditions to drastically change over time.
A rogue planetary body, which is one that does not orbit a star but drifts freely in space. However, not all rogue planets are classified as Class R.
Class of gas giant smaller than Class T and the next larger size up from Class I.
The largest class of gas giant. Smaller gas giants are, in order of decreasing size, Class S, I, and finally J.
Class Y planets are referred to as “Demon” worlds, where surface conditions do not fall into any other recognized category. Such worlds are usually hostile and lethal to humanoid life. If life develops on these worlds they usually take on many bizarre forms, like living crystal or rock, liquid or gaseous physical states, or incorporeal, dimensional, or energy-based states.
Class X and Z
Reserved for other designations of “demon” planets.