For the fifth time (at least) in the history of the Church, the pope is throwing in the towel.
Indeed, for a pope to leave office, death is much more common than renuntiatio. In the history of the Church, so far, there have only been four verified cases: Benedict IX (May 1, 1045), Gregory VI (December 20, 1046), Celestine V (December 13, 1294), and Gregory XII (July 4, 1415).
As decreed by Canon 188 of the Code of Canon Law, the essential element of each act of legally valid abdication -- including the current pope's -- is his freely expressed will.
"The abdication done out of grave fear, unjustly inflicted, by fraud or by substantial mistake, or with simony, is invalid by the law itself."
The second requirement for validity of the abdication is the form of the resignation. The legislation in force does not specify one. Thus, given that the pope is the supreme legislator of canon law, whatever form he adopts to resign his post is the proper form, ipso facto. No acceptance is needed from anyone.
In accordance with Canon 189, "the abdication, in order to be valid, whether or not it requires acceptance, must be made to the authority to which belongs the provision of the office in question."
Thus, one can assume that the abdication of the Papal Throne has been presented to the authority charged with filling the vacant office -- in other words, the College of Cardinals.
For example, the following are the words of Pope Celestine V:
I, Celestine V, moved by legitimate reasons -- for humility, improved life, unharmed conscience, weakness of body, defect of science, malignancy of the people, illness of the Person, and to find the peace of past life, expressly renounce the place, the Dignity, the honor, giving full and free right now to the Sacred College of Cardinals to elect and to provide for the Shepherd of the Universal Church.