Class M planet

Class M is the classification used in Star Trek to refer to planets suitable for supporting life as we know it. They usually have Oxygen/Nitrogen atmospheres and are in a temperature range similar to Earth's.

Many people have complained about how many planets in Star Trek are Class M. The reason for this is simply one of budgets. Scenes taking place on a Class M planet can be easilly filmed on Earth. Other planets need detailed sets made in studios. It also explains why the aliens look like humans; under earthlike conditions life is likely to evolve in a similar way to life on Earth. This means that human actors can be used with little makeup. This is again cheaper.

A more scientific explanation is that, with a virtually infinite number of stars in the galaxy, if even a fraction of those have planets, a small fraction of that fraction is still a very large number.

Well known Class M planets in Star Trek are: Earth, Vulcan, Kronos and Bajor.

On the Star Trek series Enterprise, Sub-commander T'Pol[?] suggests that Class-M stands for Minshara-Class, a Vulcan term.