How is space different on other planes? If I headed “up” from the fire elemental plane a few hundred thousand miles and looked down, what would I see? Depends on the plane – for elemental planes, I believe you’d slip into the prime material. Not canonical, but I always thought of “up” from there are pointing to the center of the inner planes. Otherwise, at boundaries you’d likely enter the ethereal or astral.

— Mike Mearls, but Spooky (@mikemearls) October 23, 2018