Trekipedia

Planet, Class-M

Terrestrial. Age ranged from 3-10 billion years. Diameter was 10,000 to 15,000 km. Typically located in the Ecosphere of a planetary system. Surface water was abundant; if water or ice covered more than 80% of the surface, the planet was considered Class O or Class P. Atmosphere was primarily primarily nitrogen, oxygen, and trace elements. Contained extensive vegitation and animal life.^[8] Capable of supporting carbon-based, humanoid life.^[1]

The designation apparently stemmed from the Vulcan word “Minshara,” which was used to designate planets capable of supporting life.^[7] Class-M planets could vary widely in color, cloud cover, and overall appearance. Most Class-M planets were characterized by a relatively thin, tectonically active crust floating on a molten rock mantle, which in turn surrounded a liquid metal outer core and a solid inner core composed of metal crystals.

In the Alpha/Beta Quadrants, one otherwise unnamed world among several Class-M planets in the Mar Oscura Nebula winked out after showing up on sensors, then blinked back, a result of the dark-matter spatial deformations. A sensor scan and three probes revealed no life, nor anything unusual.^[3] Another unidentified and rather hot planet with twin moons, possibly not even real, was where Q whisked away the bridge crew to stage his “games” during his second visit to the Enterprise.^[2]

In the Delta Quadrant, the Ocampa homeworld was notable for being Class-M, despite having no nucleogenic particles in its atmosphere.^[4] One barren world, despite its rating, is nicknamed “Planet Hell.” It turned out to be the nesting place of a member of a reptillian spacefaring race. Another was an unidentified system’s fourth planet, where the mutated Paris and Janeway were found to have mated and borne young.^[5] Another was small planet some three hours away from the point where Janeway left Voyager for the Tak Tak world, populated by Garan miners who contracted a mutating macrovirus; the Tak Tak “purified” it by killing all those infected.^[6]