Historical Rules · 1 April 2011
Rule 901.11b
Official source text from the 1 April 2011 Rules release
The planar controller is normally the primary player of the active team. However, if the current planar controller's team would leave the game, instead the primary player of the next team in turn order that wouldn't leave the game becomes the planar controller, then the old planar controller's team leaves the game. The new planar controller retains that designation until he or she leaves the game or a different team becomes the active team, whichever comes first.