Class K - Adaptable
Age: 4-10 billion years
Diameter: 5,000 - 10,000 km
Surface: Barren, little or no surface water
Atmosphere: Thin, mostly carbon dioxide
Life-Forms: Primitive single-celled organisms; adaptable for humanoid colonization through the use of pressure domes
Examples: Mars, Mudd
Class K planets generally reside in a star's habitable zone. Smaller than Earth, they range in size from 5,000 to 10,000 kilometers in diameter. Class K planets have a thin atmosphere and a barren surface, with only small amounts of water. Heavy atmospheric pressure, active volcano networks, carbon dioxide and sulphur dioxide clouds contribute further to an inhospitable habitat. Never the less, colonization is possible through the use of pressure domes and other life support devices.
Mars in the Earth star system is a Class K planet, as is the planet referred to as Mudd. Mudd was colonized by androids from the Andromeda Galaxy and was named after Harry Mudd in "I, Mudd" (TOS).