Class M Planet
Class M, from the Vulcan term "Minshara," is a fictional classification for planets and planetoids in the Star Trek science fiction media franchise.
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, and Qo'noS.
In fiction, Class M planets are similar to those suggested to be found in the real-world astronomical theory of life supporting planets found 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", where 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, more technically speaking: the thermal surface requirements necessary to support complex (multicellular) Earth-like life.
Read more about Class M Planet: Other Planet Classifications Based On The Star Trek Series
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