In fiction, Class M planets are similar to those suggested to be found within the real-world astronomical theory of life-supporting planets within the habitable zone (HZ), sometimes also referred to as the 'Goldilocks' zone. The Earth Similarity Index, a scale used in planetary science, includes within its classification of habitable planets (hClass) a "Class M". Here, the "M" stands for mesoplanet (not to be confused with Asimov's mesoplanet proposal), i.e. a planet with moderate (as necessary for liquid water) temperatures, or more technically speaking, the thermal surface requirements necessary to support complex (multicellular) Earth-like life.
Class J and Class T planets are gas giants. Class J are smaller than Class T which are considered "super", or "ultra", gas giants. In the Star Trek: Deep Space Nine episode "Starship Down", the USS Defiant enters the atmosphere of a Class J gas giant to rescue the crew of a Karemman ship attacked by the Jem'Hadar. The USS Voyager encounters a Class T Super-Giant in the Delta Quadrant with "radiogenic" rings in the Voyager episode "Good Shepherd".:26,28
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. In the original Star Trek series episode "I, Mudd", the planet Mudd was designated in dialogue as Class K.:26
Class L planets are barely habitable worlds with primitive ecosystems. In "The Chase", the planet Indri VIII is indicated in dialogue as Class L. In "The 37s", the planet on which Amelia Earhart and others are stranded is a Class L planet with an oxygen–argon atmosphere. In "Muse", the planet on which B'Elanna Torres' shuttle crash lands is described as a Class L planet, which also supports Bronze Age humanoid life. In "Timeless", the USS Voyager' crashes into a Class L planet with an arctic climate. In "The Ascent", Quark and Odo crash-land on a desolate Class L planet.:26
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; for instance, in Star Trek: Deep Space 9 episode "The Search", the Founders homeworld in the Omarion Nebula is referred to as a "rogue" Class M planet. But this is probably a rare situation as most planets that don't belong to a star system would not be able to support life.
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, J and I.
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. In the series, examples of Class Y "Demon" planets include Tholia, the "Silver Blood" planet discovered by the USS Voyager in the Delta Quadrant in the episode "Demon" and later mentioned in "Course: Oblivion", and the home world of the incorporeal Medusans.:28
Class X and Z
Reserved for other designations of "demon" planets.
^Erdmann, Terry J.; Paula M. Block (2008). Star Trek 101. Simon & Schuster. p. 21. ISBN0-7434-9723-6. What is Class M? It is a designation used to describe a planet capable of supporting life “as we know it”. With a breathable oxygen–nitrogen atmosphere and a temperature range that won’t boil the crew’s blood. Most of the planets the Enterprise stops at fit the classification and are very Earth-like. Which is very convenient for filming on location.