In “The Man Trap”, the 1st episode of Star Trek aired on television, the starship Enterprise went to an archaeological excavation site on the planet M-113. The planet was enclosed by a desert with little vegetation and the remnants of a former civilization. The atmosphere is seemingly breathable by humanoids, a plot device used in a number of later episodes.
In the 3rd episode “Where No Man Has Gone Before”, the crew of the Enterprise is stuck on the planet Delta Vega, a planet like Earth except it had a somewhat smaller size, with a lithium cracking station functioning there.
In many episodes of the original Star Trek series, the Enterprise searches the geology of numerous planets, sometimes occupied by humanoids or alien lifeforms. The class of planets in the Star Trek universe is grounded on size, composition, geological activity, atmosphere, and includes over 13 planet types.
For instance, planets fit for humans, small, rocky worlds with geological activity and oxygen-atmosphere, are categorized as M after Minshara, the native name of Vulcan, home of Commander Spock. Aboard the Enterprise, Spock can simply scan and categorize a planet.
In the real world, the first exoplanet was found in January 1992 and today over 3,600 planets in other star systems are known. However, we can only guess about the environment discovered on an exoplanet. The shape of the orbit can be utilized to estimate the mass of an exoplanet.
Therefore, even if the category system in Star Trek is fictional, true science might adopt a similar one. M-class planets are well-known in the Star Trek universe. In the real world, it is only a couple of rocky planets that may host liquid water, a critical ingredient of life. Desert planets are the most visited by the crew, a plot scheme to limit the costs of the set.