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