The planet is awakened by unlocking any gray door locked by killing enemies in the room (not including bosses or minibosses).
Pit Room and Baby Kraid Room are two examples where this can be done.
Awakening the planet causes enemies to spawn in Parlor, Climb, Morph Ball Room, Construction Zone, and Blue Brinstar Energy Tank Room.
It also causes the item in the left side of Morph Ball Room and in The Final Missile to spawn.
The item and enemies in Pit Room do not spawn until entering with Morph and Missiles collected, regardless of whether the planet is awake.
See also: Tech Page