Currently following D&D, Demi Planes exist. Demi Planes can take on many diffrent traits depending on what the creator made in it, Subjective gravity, Perhaps a cavern. Unlike most planes it is Finite, And therefor has a edge, It could be a number of types of edges. To create a Demi Plane you have to be an Archmage. Portals to it can exist, Or objects that take you there. Recommended to reserch Demi Planes before Creation.