STAR CHART: PLANETS -
PLANETARY CLASSIFICATION - M

Class-M is the designation for small, rocky terrestrial worlds with oxygen-nitrogen atmospheres. The definition is first established in [TOS: The Cage], the first pilot of [TOS]. A Class-M planet generally has nucleogenic particles in its atmosphere. The lack of nucleogenic particles would result in an absence of precipitation and surface water. This is inferred from [#1 and #2 Caretaker] - see dialogue extract below. The Star Trek format calls for most stories to involve missions to Class-M planets, since showing non-Earthlike planets would be far more costly from the production budget point of view. While this is probably unrealistic scientifically, it is a key reason for making the production of Star Trek practical on a television budget.

An explanation for the preponderance of humanoid species in the galaxy is given in [TNG: The Chase], as follows. Intelligent bipedal life-forms, generally mammalian, commonly found on many Class-M planets. Humans, Vulcans, Klingons, Cardassians, and Romulans are among the many humanoid species known throughout the galaxy. Despite the vast distances separating these planets, many humanoid species have been found to share a remarkable commonality in form and genetic coding. These similarities were believed to be evidence of a common ancestry, a humanoid species that lived in our galaxy some four billion years ago. This species apparently seeded the oceans of many Class-M planets with genetic material, from which a number of humanoid forms eventually evolved. Archaeologist Richard Galen of Earth uncovered the similarities between certain DNA sequences in lifeforms from widely separated planets. He discovered that these DNA sequences were a puzzle deliberately left behind by these ancient progenitors. The DNA sequences, when assembled by protein-link compatibilities, formed an ingenious computer program, a message of peace and goodwill to their progeny. This message, assembled in 2369 in an unprecedented example of interstellar cooperation, was a confirmation that many humanoid species in this galaxy are indeed members of the same family, despite their significant differences. (Of course, another reason for there being so many humanoid lifeforms in Star Trek is the need to use human actors.)

Janeway, of the fifth planet which turns out to be the homeworld of the Ocampa: "It's virtually a desert - not one ocean, not one river. It has all the basic characteristics of an M-class planet except there are no nucleogenic particles in the atmosphere."
Tuvok: "That would mean the planet is incapable of producing rain."
Janeway: "I've studied thousands of M-class planets. I've never seen an atmosphere without nucleogenics. There must have been some kind of extraordinary environmental disaster."