For the over all structure of the Multi-verse, I will be going with the old TSR "Spelljammer" model of giant crystal spheres floating in an "Hyper-dimensional Sea." The interior of each sphere is known as “Wild-Space” and allows for standard or even aberrant physics if desired. This model also allows for the World of Áereth to be placed contiguously with other D&D realms.
For the world of Áereth the interior of the Sphere holds to "near Earth" normal for most physics, that is, Newtonian and Quantum calculations are the same here as on Earth with the following exceptions: the stars in the night sky (that are not one of Áereths' 3 sister planets or random comets) are "Portals" into the space beyond the crystal sphere, and that Áereth has a dangerously strong magic field.
By Spelljamming tradition, the system is named after its primary star and since this star is the embodiment of the Sun God Ormazd; the sphere Would be known as “Orm Space,” but I think that that is taken, so for now the Sphere is known as “Triad Space” or “Tri-Space”. The interior that would normally be thought of as vacuum actually has a thin but breathable atmosphere. Anything that is not Immune to cold damage or otherwise protected from the elements suffers 1d6 points of damage and 1 point of Dex damage per round of exposure. Anything that needs to breath that takes no precautions must make Fortitude saves versus fatigue/ exhaustion every 5 minutes: if creature is already “exhausted” and then fails, they pass out for 1d4 hours. The third time someone “passes out” they actually enter into a Coma and no longer make rolls until they are given healing and shelter... or starve to death, etc.
Numerous comets and asteroids of all sizes are trapped in a nearly infinite number of orbits... all unique and mostly uncharted: many are even inhabited. Áereth’s sister planets shall be known as Otergal, Pharisen, & Xherhu. A note for the sky-watchers of Áereth, due to the fact that there is a thin atmosphere within what would normally be “the Void,” all celestial objects that move, appear to have a faint comet tail. Additionally there is a dense ring of asteroids that orbits closer to the primary than Áereth, but nearly perpendicularly to the plane of the other orbits. It is known as the ring of Valdreth.
The orbital path of the Asteroids is such that from the surface and at the Equinoxes, the Asteroids appear to be a vertical line of sparkles and shadows nearly perpendicular to the horizon. Áereth passes closest to the belt as she passes her Vernal & Autumnal Equinoxes. Creating a spectacular show for the viewers. [ More about this later. ] To most of the Elder Humanoid Tribes of Áereth, this cycle is simply referred to as the “The Cycle of the Phoenix Lord, but then their view is the most spectacular of all. For most of the Tribes live where the Sun doesn’t set for half of the Year while this orbital dance occurs.
The second most noticeable of the non-planetary objects is the Shadow Star, a large purple comet: at least until it smacked into the planet to create the Burned Lands and the Devil’s Cauldron in the southern reaches of the Lostlands nearly 4399 years prior to the standard starting date for most games in this setting. It has been causing the newly dead to rise within an hour of death ever since.