Historical Rules · 1 February 2010
Rule 309.6
Official source text from the 1 February 2010 Rules release
A 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 face up." This is called the "planeswalking ability." A face-up plane card that's turned face down becomes a new object.