Why the "Class M" label?

Question

I've never understood the significance behind labeling habitable planets in the Star Trek universe as "Class M".

Does the "M" stand for something?

Or are there classes A-Z and the habitable class just happens to fall at "M"? It seems to make more sense to use habitable conditions as an endpoint rather than a midpoint, though, categorizing everything from least hospitable to hospitable or vice versa rather than falling smack dab in the middle of the alphabet.

What is the significance of the "M" and why not use another system that places a level of importance on the habitable range?

Answer 1

Because of the Vulcans

In Enterprise, they use a different name: Minshara-class. ¹

I would assume that over time, this was shortened to just M.

So that would explain why it's an M, which is smack-dab in the middle of the alphabet, and not at the beginning or end.

Continuing to assume, we might assume that the other letters were decided after Minshara was shortened to M, seeing as they appear to lead up to M being inhabitable.²

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¹

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 -Memory Alpha

²

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 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 "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", in which Archer used it to describe Archer IV, a planet implied (but not confirmed) to be Minshara class in ENT: "Strange New World". The term also appeared on Enterprise's computer displays, indicating that Starfleet adapted it eventually. -Same article as above

Answer 2

In-universe there appear to be a wide selection of planetary types (see below for a general list). They range from those that are deeply hostile to human life at one end of the spectrum to those that are more hospitable. No special reason is given in earlier sources regarding why M was chosen, other than that it's toward the end of the world types.

_{^{TNG: Worlds of the Federation}}

As you can see from the Star Trek: Star Charts, although Class-M planets are noted to be "Minshara-class" there's no special indication that the names are directly related, indeed, the classes seem to stem from planetary orbital position, going on order of likely distance from their local star.

Answer 3

Out of universe, the designation of "Class M" was probably based on the real-life system of Stellar Classification, which describes stars. This system consists of a seemingly rather random set of classes - Classes O, B, A, F, G, K, and M. Our sun is a Class G2, meaning it's a hot main-sequence star between 5,300 and 6,000 kelvin. The system we currently use is a development from an earlier system which classified stars in types from I - V; the system was then rearranged several times as more information was gathered.

It seems likely that the Federation's planetary classification scheme developed in a similar way. The M-Class designation for a terrestrial planet, as others have pointed out, probably descended from the Vulcan designation of "Minshara-class"; it's also worth noting that 'marginally habitable' planets, such as one with an oxygen/argon atmosphere, were classified as "Class-L". Whether Ls were so designated because they were close to Ms, or if M just happened to fall into the middle of a range of criteria, we do not currently know.

Answer 4

My recollection (and it's hard to find proof of this on the internet, I think we're going to have to dig up a pre-1970s published book on astronomy) is that at the time of TOS, "Class M" was the accepted designation for planets that are like Earth and Mars.

HERE'S THE IMPORTANT POINT: When the term was created, and when the TOS episodes were filmed, our best knowledge of Mars was based on Earth-bound telescopic observation. At that time, the scientific consensus was that there was a good chance that Mars could support life. In fact, the telescopic observations at the time noted shifting patterns of light/dark on on the Martian surface, and many scientists attributed this to seasonal patterns of plant growth.

The Marinar 9 mission in 1971 proved that assumption to be wrong. It was after that mission that we started to learn what we know to be true today: Mars is dry, and there are no obvious signs of life.

As a result of this knowledge, "Class M" was no longer an appropriate name for planets that might support life. The astronomical classifications were changed, and now we use the more accurate (and easier to understand) terms "Earthlike" or "Terrestrial."

Unfortunately, everything I've just said poses a problem for ST canon: we have episodes where characters are referring to planets using terms that are no longer in use. How to explain that?

Simple: create the new classification, and write in that the terms were developed in the 22nd Century.

The only real-world problem with this is (as I've experienced over the past 24 hours) that because of the popularity of sites like this, people who look for explanations of the term "Class M" do web searches. Search return rankings based on site popularity will drive sites like this to the top, and thus people will assume the information is true. (After all, why wouldn't we?)