Class K planet
In the Federation standard system of planetary classification, a class K (or K type) planet was characterized as being adaptable for Humans by use of pressure domes and life support systems. An example of a class K planet was Mudd. ()

The transjovian subtype of this class was used for harsh planets such as Theta VIII, with average surface temperatures well below 0 °C, wind speeds of up to 312 meters per second, and an atmosphere consisting of methane, nitrogen, and liquid neon. ()

Starfleet Transfer Regulations included provisions regarding special environmental support for class-K native personnel. ()

Starfleet Safety Protocols banned the use of the explosive ultritium 283 in class K planetary environments. ()

Appendices

Background information 
According to the non-canon publication Star Trek: Star Charts, class K planets are called "adaptable". These planets have an age that ranges from four to ten billion years and a diameter that ranges from 5,000 to 10,000 kilometers. A class K planet is found within the ecosphere of a star system. They are categorized by a barren surface with little or no surface water and a thin atmosphere comprised mainly of carbon dioxide. Lifeforms found on class K planets are limited to single-celled organisms, although class K planets can be adapted for humanoid life through the use of pressure domes. According to the criteria, Mars could be considered a class K planet (as could Tyrellia).

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Celestial objects