According to a Vatican official, belief in aliens is not necessarily against the Catholic Faith ^[1]:

Father Jose Gabriel Funes, director of the Vatican Observatory, said this week there is no opposition between belief in the existence of aliens and at the same time belief in God. This position, he reminded, was held by Father Angelo Secchi, the 18th century Jesuit astronomer and director of the Observatory of the Roman College—today the Pontifical Gregorian University.

In an interview with L’Osservatore Romano, Father Funes explained that Father Secchi was the first scientist to classify the “stars according to their spectrum” and that the existence of aliens “could not be excluded a priori.”

Boy is this going to open up some evangelization opportunities, along with some unique challenges. But if you thought St. Francis Xavier had a hard time as a missionary to Asia, imagine how difficult it will be to convert Klingons (although they probably would appreciate our veneration of the martyrs). At least this site ^[2] will make the job a bit easier.