No clue for Mystara though.
Mystara exists in a completely separate multiverse where "Crystal Spheres" don't exist and an infinite number of Outer Planes do exist. Mystara exists in a solar system similar to our own, in a galaxy that may be the Milky Way because Mystara is very likely an Alternate Earth in a very Old School sense. The only way to travel between Mystara and what were once known as the "AD&D Worlds" is via a "reality shift" - a powerful magical effect that alters the very fabric of space and time and allows elves, dwarves and halflings to have character classes (unless the DM says otherwise, in which case it works however the DM says it does). Citations - The D&D Immortal Set and "Up, Away, and Beyond" in Dragon 160 by Bruce Heard.
Of course, later Mystara became an "AD&D World" itself and Mystaran characters showed up in Planescape and Ravenloft supplements, but that's how I continue to play it when I use Mystara. Also I do the same thing for Dark Sun and Eberron because the idea that these two worlds are somehow connected to the same Great Wheel cosmology of Planescape just breaks so much of my idea of what these worlds are about and adds nothing to the play experience in them that I refuse to accept it.