"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?"
This was probably the first time a Pontiff has ever mentioned Martians in a homily, to my knowledge. The comments are interesting and do catch the attention of anyone listening. This is exactly why Pope Francis used this analogy. It catches the attention of listeners present. The readings of the Mass had to do with opening the doors to the Gentiles for salvation via Baptism and that Christ is the door (see readings here).
The Pope was saying that anyone can be baptized who wishes it in good faith. Baptism should not be denied to anyone is basically what the Pope is telling all of us. Even if aliens from another world came to visit and wanted to be baptized, they should be baptized.
Is the Pope hinting that there is life elsewhere in the universe? No he is not. He is just using imagery to get the people's attention by using aliens as an example since they are not like us and despite this, should not be denied baptism if they ask for it just like Gentiles did during the Apostle's times. They were not Jews and were considered outsiders.
Perhaps Pope Francis has phoned ET home? :-)
Being the science student and nerd that I am, I would find it totally radical and awesome if aliens did come and wanted to be baptized.
The Catechism says this on who can be baptized:
IV. WHO CAN RECEIVE BAPTISM?
1246 "Every person not yet baptized and only such a person is able to be baptized."46 (http://www.scborromeo.org/ccc/p2s2c1a1.htm)
Notice that it does not say "humans only." Baptism unites us with Christ. If there are other intelligent and sentient lifeforms out there in the cosmos just like us, then I am sure they too are seeking the Creator who made them and would want to be baptized in order to grow in grace and encounter the Creator.
One may ask, "how can these aliens be baptized if they are not descendant of Adam and Eve and therefore don't have original sin?" My response to this would be that if Satan made Adam and Eve fall here on Earth, then rest assured that he will go bother any other species out there in the universe (if they are out there) and annoy them until they too fall into sin.