These are Endothermic Planemo worlds, found in the dark reaches of space between star systems. They are too small to have retained their internal heat, but just large enough to have retained their original atmosphere.
They are rocky worlds, often high in silicates or ices. The surfaces are often ice covered, and they may even have kilometre deep frozen oceans.
The atmosphere is mostly Helium, with some Hydrogen, and the temperature is generally below 100K. The older the world, the lower the temperature.
They are geologically dead, and their surfaces are ancient. Out here in the space between stars, there are few asteroids or comets to leave a mark on the surfaces of such worlds, so they are generally uncratered. The lack of external heating means the atmosphere is a consistent temperature, so there is little weather, or even climate.