The letter-based system used by Starfleet and Federation science to sort planets by atmosphere, surface conditions and suitability for life, from habitable Class M worlds to gas giants and toxic Class Y bodies.
Planetary classification is the shorthand Starfleet crews use when a sensor sweep first describes a new world. Each planet is assigned a letter that sums up its composition, atmosphere and whether humanoids can live there, so that a single phrase from the science officer tells the captain what kind of landing party, if any, is possible.
The best-known grade is Class M, the Earth-like standard heard from The Original Series onwards. Other letters established on screen include Class D for barren planetoids and moons, Class J for gas giants, Class K for worlds habitable only under pressure domes, Class L for marginal worlds with thin or unusual air, and Class Y for the so-called demon planets met by Voyager.
Enterprise shows that the system has Vulcan roots: T'Pol speaks of Minshara-class planets in Strange New World, the term later shortened to Class M.