In the Federation standard system of planetary classification, a class D planet was characterized as a small, rock-based body, such as an asteroid, moon or other barren, cratered planetoid with little or no atmosphere.
The planet known as Weytahn to the Andorians and Paan Mokar to Vulcans was an example of a class D world. Weytahn was class D before terraforming. It was described as being not much larger than Earth's moon. (ENT: "Cease Fire") Regula, which was described as a "great rock in space" with a few "unremarkable ores," was also a class D planet. (Star Trek II: The Wrath of Khan)
In 2371, the USS 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")
A subspace sinkhole encountered by the Voyager in 2375 led to a subspace pocket, where a star system composed of a G-type star and three planets was located. One of these planets was class D. It was described as hot, arid, and rocky, but otherwise had an atmosphere breathable by humanoids, e.g., Humans and Vulcans. It also supported limited indigenous plants and a species of spiders. (VOY: "Gravity")
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