In the Federation standard system of planetary classification a Class M planet, moon, or planetoid was considered to be suitable for humanoid life. By the mid-24th century, thousands of Class M planets had been charted by the Federation. These worlds were the first choice for colonization. Since the late 23rd century, the Federation has been terraforming lifeless worlds into Class M worlds. Environmental conditions on Federation starships mimic the Class M environment. (TOS: "The Cage"; TNG: "Justice", "Home Soil", "Final Mission"; VOY: "Caretaker")
- Atmosphere: The atmosphere contained large percentages of nitrogen and oxygen, and smaller percentages of trace elements. Most planets had nucleogenic particles in the atmosphere. (Atmospheres that lack nucleogenic particles were incapable of producing rain.) The atmosphere on these worlds was Earth normal. The atmosphere, as measured in millibars, might be slightly less or slightly more.(TAS: "The Eye of the Beholder"; TNG: "Clues"; VOY: "Caretaker")
- Surface: The surface of these planets was abundant in water. Under the surface, there were additional sources of water. These worlds were geologically active. (VOY: "Caretaker", "Once Upon a Time", "Dragon's Teeth")
- Mantle: Some of these worlds had a bemonite mantle. (VOY: "Once Upon a Time")
- Planetary Core: The planetary core of these planets was nickel-iron. (TNG: "Clues"; VOY: "Once Upon a Time")
- Life: These worlds had ecosystems and contained amino acids and protein readings. High percentages of both indicated healthy plant life. Many of these worlds supported carbon-based vegetation and animal life. A smaller number of these worlds had proto-humanoid, humanoid, and vulcanoid life-forms. (TNG: "Angel One", "The Chase"; VOY: "Parturition"; ENT: "Bound", "Observer Effect")
- Other: Class M worlds emitted electromagnetic radiation. (TNG: "In Theory") These worlds were rich in minerals. (VOY: "Investigations") The gravity on these worlds was Earth normal. The gravity might be slightly less or slightly more. (TOS: "The Cage"; TAS: "The Eye of the Beholder")
During the mid-22nd century, Vulcan science used the term Minshara class for such planets. Vulcans were not able to determine if a planet was Minshara class through orbital scans, instead sending down probes to collect the necessary data. (ENT: "Strange New World"). In 2151, Starfleet officers were not familiar with the term and Enterprise chose to adopt it. However, by 2154, Starfleet had adopted the term Class M. A century later the term was in general use in Starfleet, even though the term "Earth-type" was also occasionally used.
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- The term "class M" was first used in "The Cage" to describe the planet Talos IV. The variant "M class" is also often used. The term has become Star Trek shorthand for Earthlike, and due to obvious production practicalities is the "default" for planetary surface scenes.
- The Vulcan term Minshara class (first used in ENT: "Strange New World") was used in Enterprise to denote planets that in other series would have been called class M by the writers, the implied consequence being that the two terms meant the same, and possibly even that M stood for Minshara. This assumption has been contested by some fans, but is at least supported by the Star Charts. From an in-universe standpoint The term M-class was first seen chronologically in a text within the Handbook of Exobiology in "Strange New World", the same episode that introduced Minshara class. However, this mention was barely legible on-screen and may have been included by an art department not yet aware of the intention to use "Minshara class". The first spoken use of the word was in ENT: "Home", where Archer used it to describe Archer IV, a planet implied (but not confirmed) to be Minshara class in ENT: "Strange New World".
- Spock called the planet from TOS: "Return to Tomorrow" class M despite the atmosphere being ripped away, suggesting that class M goes beyond being able to support life. Alternately, he could have been saying that the planet prior to losing its atmosphere would have been categorized as class M.
- According to the Star Trek: Star Charts, class M planets have ages that range from three to ten billion years and a diameter between 10,000 and 15,000 kilometers. They are located within the ecosphere of a star system.
- The novel Strangers from the Sky indicates that the term "Class M" was used by Humans prior to First Contact with Vulcans, or indeed before humanity reached any extra-solar planets.