Astral Planes can only be opened with the abilities of certain cards, namely Astral Poets. All Astral Poets require you to have a number of force markers on your vanguard circle, after which they turn your back row center rear-guard circle into an Astral Plane.
As long as the specified conditions are met, your rear-guard circle becomes regarded as an "Astral Plane" in addition to its original type.
While an Astral Plane exists on your field, you will be able to call an Astral Deity to that zone. Astral Deities cannot be placed elsewhere, nor even be ridden. Only an Astral Deity can exist on that zone.
An Astral Plane cannot be chosen for the normal call of non-Astral Deity units. Should any non-Astral Deity unit or locked card be put as result of an ability, the card must be put into the drop zone.
However, a unit's "When placed" ability still activates. Resolve said ability, then proceed to retire that rear-guard.
At the end of your turn, if there is still an Astral Deity on an Astral Plane, you must put it on the bottom of your deck.
An Astral Deity can be normal called regardless of your vanguard's grade.
An Astral Deity can attack even from the back row, and perform drive checks, as well as activate its abilities. Any cards revealed for that unit's drive check must be put into the soul instead of going to the hand.
When an Astral Plane is open, if there was a non-Astral Deity rear-guard or locked card originally on it, that unit or card must be put into the drop zone.
When an Astral Plane is open, it retains all gift markers that are already on it. Likewise, you may further put gift markers on it.
An Astral Plane will close when your Opener is no longer your vanguard, or loses its ability to open the Astral Plane (such as by being deleted or Stridden).