The topic came up while Francis was speaking about acceptance, inclusion and converting non-believers into Christians, according to Vatican Radio reports.
Francis brought up a Bible story in which Peter sees a group of non-Christians receiving the Holy Spirit and is later criticized for associating with them.
Then aliens came up...
"That was unthinkable. If -- for example -- tomorrow an expedition of Martians came, and some of them came to us, here... Martians, right? Green, with that long nose and big ears, just like children paint them... And one says, 'But I want to be baptized!' What would happen?"
He referenced the story and the extra-terrestrials in order to remind people that the Catholic Church is supposed to have open doors.
"Who are we to close doors? In the early Church, even today, there is the ministry of the ostiary [usher]. And what did the ostiary do? He opened the door, received the people, allowed them to pass. But it was never the ministry of the closed door, never."