Historical Rules · 4 February 2011
Rule 901.7a
Official source text from the 4 February 2011 Rules release
Each plane card is treated as if its text box included “When you roll {P}, put this card on the bottom of its owner’s planar deck face down, then move the top card of your planar deck off your planar deck and turn it face up.” This is called the “planeswalking ability.” A face-up plane card that’s turned face down becomes a new object.