From the Statutes:
Part II: THE ACADEMICIANS
Candidates for a seat in the Academy are chosen by the Academy on the basis of their eminent original scientific studies and of their acknowledged moral personality, without any ethnical or religious discrimination, and are appointed for life by sovereign act of the Holy Father.
In addition, by reason of their office, the Director of the Vatican Observatory; the Director of the Astrophysical Laboratory of the Vatican Observatory; the Prefect of the Vatican Library; the Prefect of the Secret Archives of the Vatican, are appointed « Academicians pro tempore ». The Academicians « pro tempore » enjoy the same rights and perform the same functions as the Pontifical Academicians.
By way of exception, and in a purely honorary capacity, on proposal by the Council of the Academy, persons who have deserved particularly well of the Academy, by honouring it and assisting it and its scientific undertakings, may be nominated by the Holy Father « Honorary Pontifical Academicians ».
The full complement of the Academy consists of 70 life members, chosen in such a way that as far as possible all the principal branches of science and all the major geographical regions are represented.1
1 On 8 January 1986 John Paul II augmented to 80 the number of Academy life members.