Class-M planets, various
Star Trek: The Original Series
Episode: TOS 001 - Cage, The
Enterprise leaves Earth
Planets that are highly supportive of organic life. Both Earth and Vulcan are Class-M planets.
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.
Another unidentified and rather hot planet with twin moons, possibly not even real, is where Q whisked away the bridge crew to stage his "games" during his second visit to the Enterprise.
In the Delta Quadrant, one barren world, despite its rating, is nicknamed "Planet Hell." It turned out to be the home 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.
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.
