Luna, the moon of Old Earth- the oldest Terragen colony world
Selenians are the archetypical "dead world", planets that have almost no heavy metals, and almost no geologic activity. Typically these worlds are formed in low metal systems, or through the collision of two bodies during the earliest formative periods of a solar system.
The two bodies typically meld together, along with most of the original heavy metals, while the remaining lighter material (if several other aspects to the collision have been satisfied) come together to form the Selenian world. Because of this, the majority of Selenian bodies are actually moons to larger planets, although a fraction of the worlds formed by this process are subsequently lost because of tidal effects, and become worlds in their own right. Many times there will be ample signs of past geologic activity, activity that was the by-product of the more cataclysmic form of creation.
Tartarus II, a Selenian type world in the HD 512 system
However, a mature Selenian world will experience the occasional out-gassing of deep volatiles that work their way towards the surface on the scale of millions of years, and nothing else. They are heavily cratered worlds, and typically devoid of any significant atmosphere.