In addition to such fictional references, the Catholic Church has theologians dedicated to thinking about the implications of life on other planets. José Gabriel Funes, head of the Vatican Observatory, has said that "Just as there is a multiplicity of creatures on earth, there can be other beings, even intelligent, created by God. This is not in contrast with our faith because we can't put limits on God's creative freedom."*
One of the first places I came upon the concept of exotheology was in the lyrics of a song written by Larry Norman and released on his In Another Land album in 1976. In the song "UFO" he writes,
And if there's life on other planets, then I'm sure that He must know. And He's been there once already, and has died to save their souls.
Norman's approach to exotheology is one valid perspective but is not the only perspective. Lewis, in the Cosmic Trilogy suggests that life on other planets may have taken a different path. He writes about other worlds where intelligent life had not yet fallen from grace with God; rather, they continued to walk in harmony with their Creator. Mary Doriah Russell is much more interested in the theme of how we reconcile a benevolent God with suffering in the world but her books allow exploration of other ways God might work with persons on other planets.
Of course, we do not know if there is such a thing as intelligent extraterrestrial life and we do not know if we will ever discover such if indeed it exists. Yet, the thought experiments related to such speculation are helpful as we think through our "endotheology" (my own word for the opposite of exotheology) and how we understand our own relationship with a Creator God. As is often the case, C.S. Lewis has some appropriate final words on the topic.
I look forward with horror to contact with the other inhabited planets, if there are such. We would only transport to them all of our sin and our acquisitiveness, and establish a new colonialism. I can’t bear to think of it. But if we on Earth were to get right with God, of course, all would be changed. Once we find ourselves spiritually awakened, we can go to outer space and take the good things with us. That is quite a different matter.#
^The trilogy of books sometimes called the Cosmic Trilogy consists of Out of the Silent Planet, Perelandra, and That Hideous Strength.
*"Vatican scientist says belief in God and aliens is OK," Reuters, May 14, 2008, and "Pope's astronomer insists alien life 'would be part of God's creation,'" The Independent, 15th May 2008.
#A 1963 interview with C.S. Lewis http://www.cbn.com/special/Narnia/articles/ans_LewisLastInterviewB.aspx.