To the best of my knowledge, 5e doesn’t address this directly, though Groody the Hobgoblin’s fine answer does a decent job reading between the lines of what we do know.
Prior editions did address this directly—most notably in the 2e Spelljammer line, where spaceships would fly through the wild spaces of the Material Plane, from one campaign setting to the next. Where any of this stands in 5e is now very uncertain, since the recent Spelljammer: Adventures in Space ret-cons spelljammers entirely, moving them (inexplicably) to the Astral Plane. We just don’t know if the rest of the structure of the Material—which was mostly described for the sake of spelljammers traveling there—still applies.
But this is what we had, and maybe lost.
Crystal Spheres of the Material Plane
The various campaign settings were all found within the same Material Plane. (Mostly all; a few alternate Material Planes were known.) They were separated, however, in separate Crystal Spheres. The Crystal Spheres were spherical (natch), and made from some indestructible crystalline (doi) material. They were also enormous—think the size of our Solar System. They would include a star (or two, in a binary system), planets, moons, asteroids, comets, and so on. Within the Crystal Sphere, things worked more-or-less like they do in real life: the atmosphere thins as you get away from a planet, and in the vacuum between there is nothing.
The various Crystal Spheres floated in the phlogiston, an intensely flammable, intensely toxic, intensely magical substance. There were portals through the Crystal Spheres out into the space beyond, but the phlogiston could not get through them.
Relevant to this question, phlogiston absolutely blocked interplanar communication and travel—which under 2e rules included teleportation. Gods couldn’t even hear the prayers of their worshippers while those worshippers were within the phlogiston. You could plane shift away from the Material Plane inside one Crystal Sphere, and then plane shift back to the Material Plane in another Crystal Sphere, but you couldn’t teleport directly across the phlogiston from one Crystal Sphere to another. For that, you needed to travel physically through the phlogiston itself—in a spelljammer. Spelljammers were used to travel through the void within a Crystal Sphere, through a portal, and out into the phlogiston, flying to another Crystal Sphere and entering through one of its portals. This was, in a nutshell, the Spelljammer setting.
Realmspace
The name of the Crystal Sphere that contained the Forgotten Realms was known as Realmspace—which had its own dedicated sourcebook, cunningly titled Realmspace (1991). There are a number of details there, from the unique spells carved into the inner surface of the Crystal Sphere (found on no other Crystal Sphere), to the stable phlogiston currents connecting Realmspace with Greyspace (Greyhawk) and Krynnspace (Dragonlance) in a triangular neighborhood of significant import, to its proximity to the Arcane Inner Flow, a major spelljammer shipping lane.
But most relevantly to this question, we got information on precisely what the Realmspace Crystal Sphere contained:
- The Sun
- 8 planets
- The third of which is Toril, on which we find Faerûn and the rest of the Forgotten Realms
- Orbited by its single, large natural satellite, Selûne
- Which is tidally-locked with Toril, so one side always faces Toril; the far side is sometimes known as the “dark side” even though it is not always dark (just always facing away from Toril).
- Various comets, asteroids, and elder evils
Sounds familiar, doesn’t it?
They even include a map:
Anyway, Realmspace, at least, is confident that the moon is a moon, merely named for the goddess, and there are even people living on it—though they prefer to call themselves Leirans, and the moon Leira, favoring that godess over Selûne. The Leirans are primarily found on the “dark side” of Selûne/Leira, and thus their civilization is hidden from the inhabitants of Toril.
So in conclusion, Selûne (or Leira) was definitely within the same plane as Toril, even within the same Crystal Sphere, and teleportation between the two was possible, though before 5e, a teleport of 183,000 mi was rather difficult to achieve.
