In Planetary Classification a Class K planet is an 'Adaptable' planet. Adaptable planets have an age that ranges from 4-10 billion years and have a diameter that ranges from 5,000-10,000 km. A Class K planet is foudn within the Ecosphere of a Solar System. They are catagorized by a barren surface with little or no surface water and a thin atmosphere comprised mainly of carbon dioxide. Life-forms found on Class K planets are limited to single-celled organisms; although they can be adapted for humanoid life through the use of pressure domes.
Examples of Class K planets include: Mars and Mudd.
The Transjovian subtype of this class is used for harsh planets such as Theta 116 VIII, with average surface temperatures well below 0°C and an atmosphere consisting of methane, nitrogene and liquid neon.
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The first part is partly information from the Star Trek Star Charts. The second part is conjecture based on on-screen information from (TNG: "The Royale").