The cosmology of Conan's World is such that the various Inner and Outer Planes orbit each other, each plane affecting the others much as gravity affects the other planets. This is the system offered in WotC's Manual of the Planes, page 217, the Orrery Cosmology.

At any time, certain planes are closer to the Material Plane, and thus more powerful. These ascendant planes make certain spells stronger on the Material Plane.

Essentially, the transitive planes of Astral, Ethereal, and Shadow are unchanged from most other D&D campaigns, being coterminous with the Material and each other, but the Material Plane is off center in regards to the Inner Planes. The Plane of Fire is closest during the summer, Earth during Autumn, Water during Winter, and Air during Spring.

The Outer Planes are not some Great Wheel, but individual, separate planes that move in a complicated system of rotations and orbits, not colliding because they function at different harmonic levels of reality. They can, however, become briefly coterminous, and creatures can slip easily from one plane to another when these planes are in close proximity.

The number of planes are not fixed. Over the centuries some planes drift away and others come closer. Occasionally a plane drifts so far away it effectively becomes lost. New planes sometimes suddenly appear and sweep though the planes, knocking the others out of their orbits.

During the solstices and equinoxes (the three day period when the four main Outer Planes and Inner Planes are closest), an individual with the teleport spell or ability can move travel to those close planes without needing the transitive planes as if the two planes were one. Spellcasters in Conan's world have access to a new feat that allows them to take advantage of the movements of the planes.

Orrery Astrology [General]

By watching the stars and planets, and by understanding the movements of the Inner and Outer Planes, you have learned to enhance your magic.

Prerequisites: The ability to cast spells (divine or arcane)

Benefit: You can increase your caster level by making a successful Knowledge (Astrology) check versus a DC of 20. You are not permitted to take 10 or 20 on this check, and retries are not permitted. A successful check increases your caster level by +1 for 24 hours.

The Outside

The spaces outside the planes are called the Outside (or the Far Realm). Here reside the Old Ones and other unspeakable entities. These beings are too different and alien for a normal mind to accept without being damaged. From this realm where unspeakable, titanic creatures swim though nothingness comes the power of Magic. The knowledge of the Outside, for a mortal, is a triumph of mind over the boundaries of matter, space, and time... and in this triumph often hides madness.

This is the realm of pseudo-natural creatures (detailed in The Manual of the Planes and in Tome and Magic) and is where the sorcerers and wizards of Conan's World listen to the secrets of magic.

The Outside is detailed fully in The Manual of the Planes on page 211-212. There are no known links to the Outside.