The pope has said that he would baptise a Martian – but would they want our religions?

Do aliens believe in God? Did aliens create God? Did God create aliens? Is God, in fact, an alien? Religion and extraterrestrial beliefs don't tend to go together. So it was something of a surprise when Pope Francis himself waded in on the debate this Tuesday. "If … an expedition of Martians arrives and some of them come to us … and if one of them says: 'Me, I want to be baptised!', what would happen?" the pontiff jokingly speculated during Tuesday's morning mass. The church turns no one away, he confirmed, not even extraterrestrials.

Life on other planets is something most major religions have pondered – especially if you choose to interpret their texts as if you're researching a new Dan Brown novel. Ancient religions such as those of the Aztecs, Mayans and Egyptians were obsessed with the cosmos but short on supporting documentation, thus opening a gap for the likes of Erich von Däniken, whose fanciful 1968 book Chariots of the Gods? speculated that ancient civilisations had worshipped visitors from outer space, spawning a whole industry of speculative "exotheology". Faiths such as Hinduism and Buddhism are already fairly relaxed about notions of parallel worlds and alternate realities.

As for Judaism and Christianity, conspiracy theorists are fond of the Jacob's Ladder episode in the Book of Genesis, in which Jacob dreams of climbing a ladder into "heaven" and meeting "God", who tells him to spread his seed about the Earth – which chimes with modern-day descriptions of alien abduction. For some, even the Qu'ran hints at the existence of ET. Verse 12, Chapter 65 states: "Allah it is who hath created seven heavens, and of the earth the like thereof", a line irreverent analysts have interpreted as suggesting seven heavens and seven Earths. Who lives on the other six?

Obviously there's Scientology, whose interplanetary backstory devotees seem happy to accept, despite the fact it came from a science-fiction writer. And in Mormonism's Book of Abraham there's mention of an observed star or planet called "Kolob", which the Lord describes as "the great one … because it is near unto me". In other words, Kolob is next door to where God lives, somewhere in our universe.

Three years ago, a paper for the Royal Society found that roughly 90% of believers did not think they would suffer a crisis of faith should intelligent extraterrestrial life be discovered, though two-thirds of them believed other religions would have a crisis.

The Vatican has already covered the angles, it seems. In 2008, its chief astronomer, José Gabriel Funes, publicly accepted that there could be life on other planets. "Why can't we speak of a 'brother extraterrestrial'?" he said. "It would still be part of creation." Aliens might even be closer to God than us, Funes suggested, and humans could be the "lost sheep" of the universe. In other words, alien visitors might well have a more advanced religion than ours. Rather than baptising them, we might want to convert to their faith.