Trekipedia

Planet, Class-K

Adaptable. Age ranged from 4-10 billion years. Diameter was 5000 to 10,000 km. Typically located in the Ecosphere of a planetary system. Surface was barren, with little or no surface water, atmosphere was thin and mostly carbon dioxide. Adaptable for humanoid colonization through the use of pressure domes and terraforming. May contain primitive single-celled organisms.^[1]

Class-K Planets:

• Mars (prior to terraforming in 22nd century)
• Mudd

References

• ^[1]STSC: Star Trek: Star Charts