These are worlds which have frozen over time. Their orbit may have shifted outwards, or their star could have cooled. They are mostly rocky worlds, which may have had water and atmosphere at some point in the past. However, they have frozen over into dormant, dead worlds of frozen ices.
Due to their tendency to have carbon dioxide atmospheres earlier in their life cycle, such worlds are often vacuum worlds with a thick layer of frozen carbon dioxide covering the surface. Other ices may form as well, depending on initial atmospheric and hydrographic compositions.