A friend of mine has listed as her twitter location that she lives on a Class M planet.

Not everyone tweets from Earth, but I assume she tweets from within the Solar System.

Which planets or moons in the real-life Solar system would match Star Trek's "Class M" designation?

Wikipedia and Memory Alpha have descriptions of what the class M criteria are, but doesn't try to analyze bodies in our Solar system (apart from the Earth) against those criteria.


According to Wikipedia, Earth is the only M Class-planet, because only it harbors and sustains natural life.

That said, Mars could be considered Class L - easily terraformable into a habitable planet, assuming that Star Trek terraforming projects mentioned in canon exist.

To wit, our solar system would be classified as follows:

  • Mercury: Class D (Planetoids that could support life underground)
  • Venus: Class N or Y (Potentially habitable, given atmospheric domes, but the toxic atmosphere would lend credence to it being a 'demon world.' In the case of Venus, there would be a high pressure to contend with)
  • Earth: Class M
  • Mars: Class L

  • Ceres: Class D (See Mercury)

  • Jupiter, Saturn, Uranus, Neptune (Class J or T - Gas Giants)

  • Europa, Titan, Io, and some of the other moons could range from Class D to K to L, depending on their suitability to sustain life.

  • Enceladus, being a water planet, would be Class O or P

  • Pluto, Eris, Xena, and the others (Class D)

And yes, I'm old enough to think that Pluto is a planet :)

  • That makes me wonder - has Pluto ever been referred to as a planet in Star Trek? – Andrew Grimm Oct 1 '13 at 11:35
  • @AndrewGrimm Looking at mentions of Pluto, I think I remember most of them saying the name "Pluto" without qualifying whether or not it was a planet. But either way, the map from TOS The Changeling would be incorrect, as there are now other dwarf planets that should be included with Pluto. – Izkata Oct 1 '13 at 12:41
  • Never heard of Encycladus. Do you mean Enceladus? Is that a water planet? What is a water planet? – user14111 Oct 1 '13 at 21:32
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    @Lèsemajesté Enceladus and Europa are both moons, not planets. Enceladua is a moon of saturn; europa, of jupiter. A lot of these are moons, actually. – ApproachingDarknessFish Oct 2 '13 at 5:47
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    Regarding Mars, it's not so easily terraformable (at least by our non-Trek technology standards). See: space.stackexchange.com/q/634/786 – MicE Oct 2 '13 at 9:00

Only Earth is a Class M planet capable of sustaining human life.

  • Possibly also Europa if you count moons. – bitmask Oct 1 '13 at 11:27
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    According the "Atmosphere" and "Potential for extraterrestrial life" sections of that article it sounds like it would still need significant terraforming to support human life. Once terraforming enters the picture, the question of what is or is not a Class M planet loses any meaning. – Meat Trademark Oct 1 '13 at 14:35
  • Fair enough. I added the "possibly", because as far as I understand it, Europa appears to be the best human-life-candidate in our solar system (except Earth). – bitmask Oct 1 '13 at 14:50
  • @bitmask: Next best capable of sustaining human life is practically meaningless in this context. Europa has a primarily oxygen atmosphere, but that atmosphere is only 0.000000001% the thickness of Earth's, and the mean temperature on the surface is -276° F (-171° C). So, yes, you might survive a few seconds longer on Europa without a spacesuit than you might on Venus or Mercury, but it still may as well be Venus or Mercury or Mars. It's a good target for a space mission or possibly a colony (after Mars), but that doesn't make it remotely class M. – Lèse majesté Oct 2 '13 at 0:07
  • Also, Europa is within Jupiter's radiation belt. So while it's probably shielded from solar winds, you'd need lead-lined space suits to visit it, or you'd be facing severe radiation poisoning and likely death within a day. – Lèse majesté Oct 2 '13 at 0:11

The original definition of a "Class M planet" referred to "Earth-Mars" conditions.

This web page (which is not necessarily definitive) quotes the original Star Trek pitch from 1964:

IV. Nature and duration of command:
Galaxy exploration and Class M investigation: 5 years
VII. Consistent with the equipment and limitations of your cruiser class vessel, you will confine your landings and contacts to planets approximating earth-Mars conditions, life, and social orders.

It's quite possible that the 'M' originally stood for "Mars". ("Class E" might have been too easily confused with the word "classy", but that's sheer speculation on my part.) The Wikipedia Class M planet article suggests that the "M" is from the Vulcan "Minshara", but that's a much later invention, from the "Star Trek: Enterprise" episode Strange New World.

Remember that this was written in early 1964, when much less was known about the actual conditions on the surface of Mars. Quoting the Wikipedia article:

Until the first successful Mars flyby in 1965 by Mariner 4, many speculated about the presence of liquid water on the planet's surface. This was based on observed periodic variations in light and dark patches, particularly in the polar latitudes, which appeared to be seas and continents; long, dark striations were interpreted by some as irrigation channels for liquid water.

A "class M Planet" is one whose surface conditions are suitable for human life without a spacesuit or equivalent. At the time the term was invented, it was thought that both Earth and Mars were class M planets. With closer examination of Mars starting about a year later in 1965, we now know that Earth is the only class M planet in our Solar System.

  • I mourn for the universe that could have been, in which we all live on a classy planet. – user867 Feb 26 '16 at 6:10

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