Mu is a classification of planets with atmospheres are composed of nitrogen and oxygen and have an abundance of liquid water necessary for carbon-based life to exist. A planet with moderate (as necessary for liquid water) temperatures, more technically speaking: the thermal surface requirements necessary to support complex (multicellular) life.
Mu-class worlds come in four stages.
Life has evolved and can range from simple microbes to more advanced plant and animal life. These worlds however do not contain any native sentient life-forms. These worlds are typically found in the Outer Territories and are prime canidates for colonization by the corporations with no issues of previous ownership to contend with.
Extensive plant and animal life has been since established, and supports one or more sentient species in early development. Sentient development can range from stone age civilizations to post industrial civilizations. Earth in it’s prime was is a textbook example of a Mu Clas world.
Planets with Sentient life capable of inventing hyperspace engines and initiating space exploration.
A planet entering its final life cycle due to war, environmental destruction, or astronomical events such as a dying star or meteor impact. Earth entered stage 4 when humans were forced to leave their planet and migrate across space, eventually settling in the Nexis galaxy. Records were lost on exact cause of Earth entering stage 4 collapse.