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 described as a great rock in space with a few unremarkable ores. It was a Class D world. (Star Trek II: The Wrath of Khan)
Voyager encountered a gravitational sinkhole. Everything inside it was out of phase with normal space. A star system comprised of a type-3 sun and three planets was inside of it. One of these planets was class D. It was described as hot, arid, and rocky. (VOY: "Gravity")
Voyager found a ringed planet in the Delta Quadrant which was also Class D. Some of the asteroids comprising the rings were big enough to have class M environments inside them. (VOY: "Emanations")
he planet known as Weytahn to the Andorians but to the Vulcans was known as Paan Mokar was Class D before terraforming. It was described as being not much larger than the moon. (ENT: "Cease Fire")
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.