Class M planet

Class M is a fictional classification for planets and planetoids in the Star Trek science fiction media franchise. The term is explained in-universe as deriving from the Vulcan term "Minshara".

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; other Class M planets that appear throughout the franchise include Vulcan, Cardassia Prime, Bajor, Betazed, Romulus, Ferenginar, Qo'noS, and Nibiru.

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.