Rule 901.14 · Planechase
In Grand Melee Planechase games, multiple plane cards or phenomenon cards may be face up at the same time.
Subrules
Before the first turn of the game of the game, each player who will start the game with a turn marker sets a starting plane (see rule 901.5). Each of them is a planar controller.
If a player would leave the game and that player leaving the game would reduce the number of turn markers in the game, that player first ceases to be a planar controller (but no other player becomes a planar controller), then that player leaves the game. Each face-up plane card or phenomenon card that player controlled is put on the bottom of its owner’s planar deck. No player is considered to have planeswalked.