In the Federation standard system of planetary classification, a Class D planet is a small and rock-based body, such as an asteroid, moon or other barren, cratered planetoid with little or no atmosphere.
Regula was a Class D world. (Star Trek II: The Wrath of Khan)
The satellites of a ringed planet found in the Delta Quadrant by the USS Voyager were also Class D. (VOY: "Emanations")
Background
The non-canon publication Star Trek: Star Charts specifies that a Class D world has an average age of 2 to 10 billion years, and includes Luna and Derna as additional bodies of this class.