Star Trek's Planet Classifications, Explained | CBR - CBR - Comic Book Resources

The bright, optimistic future of Star Trek entailed regular scientific exploration, which was part of Starfleet’s mantra. That included an entire lexicon of terms, to better sell the show’s setting and to provide the sheen of rigor to its various dramatic plots. Planets were grouped according to class – each one with different features and details – which became a part of the world-building and continued to be used in subsequent Star Trek series. The most enticing was Class M, which was a planet capable of sustaining humanoid life. That meant new alien beings, new cultures and civilizations, or even just a suitable spot to set up a colony.

But class M wasn’t the only type of planet in the Star Trek lexicon. Nine others were mentioned at one point or another during the series, each with a letter demarking their status. A list of all classifications follows, along with a brief description of their conditions.

Class D: Barren Rock

Class D referred to a planet or planetoid completely devoid of atmosphere. The most prominent example in canon was Regula, the planetoid from Star Trek II: The Wrath of Khan. Spock described it as “a great rock in space,” composed of unremarkable elements and of little note beyond that.

Class H: Uninhabitable

The class H designation in Star Trek canon was vague, though largely uninhabitable by most humanoid species. One exception was the Sheliak, an “R-3” lifeform consisting of what appeared to be sentient blobs. They laid claim over several Class H worlds, which the Federation ceded to them as part of a treaty. However, a class H planet named Tau Cygna V was inhabited by human colonists, provoking a diplomatic incident with the Sheliak that the Enterprise-D resolved in Star Trek: the Next Generation Season 3, Episode 2, “The Ensigns of Command.”

Class J: Gas Giant

Class J was designated a manner of gas giant, akin to Jupiter. They are uninhabitable by humanoid life forms though their varying layers of atmosphere could conceivably carry life of a non-humanoid sort. The most prominent onscreen appearance came in Star Trek: Deep Space Nine Season 4, Episode 7, “Starship Down.” The Defiant engaged in a game of cat-and-mouse with the Jem’hadar in the atmosphere of a remote planet fitting the classification.

Class K: Habitable with Modifications

A class K planet was deemed habitable in many ways, but with surface conditions too harsh to support humanoid life. That could include anything from extreme temperatures to lack of a breathable atmosphere. People could live on such worlds with help from technological devices such as pressurized air domes or underground structures, but exposure to the surface without protective gear would be lethal. The Original Series Season 2, Episode 12, “I, Mudd” was set on a class K planet, beneath a dome controlled by androids where Harry Mudd set himself up as king.

Class L: Marginally Habitable

Class L planets held all of the components suitable for human life, and could often support such life for extended periods of time. They are usually quite barren, though some contain arable land and others are able to support colonies of hundreds of thousands of people. Class L planets are mentioned regularly throughout the franchise, notably in multiple episodes of Star Trek: Voyager.

Class M: Habitable

The vast majority of Star Trek’s alien worlds are class M, featuring a sustainable oxygen atmosphere, viable ecology and other Earth-like qualities. That allows all manner of life to develop on them and provides a convenient location for franchise's stories. Most civilized worlds in the Star Trek universe are class M, including Earth, Vulcan, Qo’noS and Andoria -- though Andoria is technically a moon. The “M” stands for “Minshara,” a Vulcan term used in Star Trek: Enterprise, though none of the other planetary classes had in-canon designations beyond the letters.

Classes N and R: Habitable With Unknown Modifiers

Both class N and class R planets are deemed habitable, though they differ from Class M planets in a manner that the Star Trek canon has yet to lay out. In both cases, their respective atmospheres are sensitive to specific types of explosives. Beyond that, their properties are unknown. Both were mentioned for the first time in The Next Generation Season 4, Episode 17, “Night Terrors.”

Class T: Gas Giant

A class T is a gas giant, similar to a class J. As with classes N and R, there is no current in-canon explanation for the differences between the two. Class T planets have thus far only been mentioned once in the franchise: Voyager Season 6, Episode 20, “Good Shepherd.” The Delta flyer encounters one bearing rings, though it’s unknown if those are the distinguishing factor for the class or not.

Class Y: Demon Planet

Class Y planets were designated “demon planets” due to their overtly toxic and dangerous atmospheres, and for their often hellish surface appearance. That could include surface temperatures higher than 500 degrees Kelvin and radiation discharges that were actively harmful to humanoid life. Its most notable appearance came in Voyager Season 4, Episode 24, “Demon,” which used a class Y planet as its primary focus.

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