In the Federation standard system of planetary classification, a class M or Minshara class 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 early 22nd century, Humanity (and later, the Federation) has been terraforming lifeless worlds such as Mars into class M worlds. Environmental conditions on Federation starships mimicked the class M environment. (TOS: "The Cage"; TNG: "Justice", "Home Soil", "Final Mission"; VOY: "Caretaker")
During the mid-22nd century, Vulcan science used the term Minshara-class for such planets. Vulcans were not able to confirm 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, that term was in general use in Starfleet, even though the term "Earth-type" was also occasionally used.
- Size: Class M worlds were typically terrestrial planets or moons, although planetoids and even large asteroids could qualify if they were massive enough to retain an atmosphere. (DS9: "The Maquis, Part I") The gravity on these worlds was generally Earth normal, but could be slightly less or slightly more. (TOS: "The Cage"; TAS: "The Eye of the Beholder")
- Orbit: Class M worlds were rare in binary star systems. (TAS: "Mudd's Passion") These planets were typically located in the habitable zones of their systems (TNG-R: "The Battle", "Chain of Command, Part II"), though terrestrial rogue planets could quantify as class M when sufficient heat from their interiors was vented outward by geological activities. (DS9: "The Search, Part I", "The Search, Part II", "The Die is Cast")
- Atmosphere: The atmosphere contained large percentages of nitrogen and oxygen, and smaller percentages of trace elements. Most planets had nucleogenic particles in the atmosphere; those without were incapable of producing rain. The atmosphere on these worlds was approximately Earth normal. (TAS: "The Eye of the Beholder"; TNG: "Clues"; VOY: "Caretaker") Temperatures might potentially be hot, but there was an upper limit. (TAS: "Mudd's Passion")
- Surface: The surface of these planets was abundant in water. Minshara-class planets were sometimes described as appearing blue and green from space, indicating water and plant life respectively, although this was not a universal quality. (ENT: "Twilight")
- Interior: Beneath the surface, there were additional sources of water. These worlds were geologically active and rich in minerals. (VOY: "Caretaker", "Investigations", "Once Upon a Time", "Dragon's Teeth") The planetary core of these planets was nickel-iron. (TNG: "Clues"; VOY: "Once Upon a Time") Some of them had a bemonite mantle. (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 plant and animal life; any non-carbon-based would have to be non-indigenous. A smaller number of these worlds had proto-humanoid, humanoid, and Proto-Vulcan humanoid lifeforms. (TNG: "Angel One", "The Chase"; VOY: "Parturition"; ENT: "Bound", "Observer Effect")
List of class M worlds
|D • H • J • K • L • M • N • R • T • Y • Other classes|
- The term "class M" was first used in "The Cage" to describe the planet Talos IV. The variant "M class" was also often used. The term has become Star Trek shorthand for Earth-like, and due to obvious production practicalities, this environment is the "default" for planetary surface scenes. It should be noted, however, that not all "Earth-like" planets are necessarily class M: class L worlds such as Kelis' homeworld (VOY: "Muse") and even the occasional class D (such as the one seen in VOY: "Gravity") can support life as well.
- 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 is supported by the reference book Star Trek: Star Charts. From an in-universe standpoint, the term M-class was first seen chronologically in a text within the Handbook of Exobiology in ENT: "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 "Home", in which Archer used it to describe Archer IV, a planet implied (but not explicitly confirmed) to be Minshara class in "Strange New World". The term continued to be used several more times in the final season, while "Minshara-class" made no more appearances. The last use of "Minshara class" was in season three's "Twilight", where it was still used in 2156 in an alternate timeline. The term also appeared on Enterprise's computer displays, indicating that Starfleet adapted it eventually.
- In TOS: "Return to Tomorrow", Spock called planet Arret class M despite the atmosphere having been ripped away, suggesting that the class M designation 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.
- Ceti Alpha V was described as "barely Minshara class" in "Twilight", but also as no more inhospitable and savage than the site of Australia's Botany Bay colony before colonization in "Space Seed", suggesting that even on a class M planet like Earth, there might be locations that stretch the definition. Alternatively, the planet might have become more hospitable in the century between the two episodes.
- According to Star Trek: Star Charts, class M planets have ages that range from 3 billion to 10 billion years and a diameter between 10,000 and 15,000 kilometers. They are located within the ecosphere of a star system.