Pope Francis would absolutely baptize an alien from Mars, if one showed up at the Vatican and asked for it. During his weekly homily on Monday, Franics said that aliens — which he imagines could be "Green, with that long nose and big ears, just like children paint them" — should be baptized just like anyone else who asks for it, because it's not up to any human to decide who should receive the Holy Spirit. In other words: if God prompts some Martians to come to Earth, find the Pope, and say "we want in on this Catholicism thing." The pope would probably say "OK. cool." But probably in Latin.
Francis laid out the hypothetical situation as part of a discussion of the early Church's baptizing of Gentiles. Basically, the earliest Church contained some tension over whether there was any difference between the very early Jewish followers of Christ, and Gentiles who converted from paganism to Christianity, who would be considered "unclean" by the standards of Jewish law. Peter eventually persuades his followers that everyone is open to becoming Christian by saying, "If then God gave them the same gift He gave to us when we came to believe in the Lord Jesus Christ, who was I to be able to hinder God?"
So here's how Francis tried to illustrate that point on Monday, with a much more modern example:
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?"
What would happen? They'd get baptized, that's what would happen. He goes on:
"When the Lord shows us the way, who are we to say, 'No, Lord, it is not prudent! No, lets do it this way'... 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."
The Vatican's astronomer — the same one who dismissed 'Intelligent Design' as 'bad theology' — said in 2010 that he'd baptize an alien because "any entity – no matter how many tentacles it has – has a soul." But, again, only if they request it. So glad that's settled. Now all we have to do is wait.