In reality, M class refers not to a planet, but the planets star. M class, as you can see on the Hertzsprung - Russell Diagram, typicly (but not always) refers to a Main sequence star with fairly low luminosity. About 78.6% of all stars fit this catagory.
However I think Mr Bot was refering to the Fictional Classification created by "Star trek" wich refers to a planet able to support life. Seems kind of vague to me.
Sorry to waste your time with such a trivial post.
What are some ways you think planets should be classified? Commical or not.
