Could God have created life on other planets? Whether supernaturally or by natural process is irrelevant to this discussion. What matters is the theology, does the biblical worldview have the ability to accomodate ET?
Does the Bible claim Earth is the only planet with life?
The more uncomfortable issue is sentient life. Species that can build machines, domesticate animals, and pray to God. This raises profound theological issues.
However, I must preface all my statements with this: I don't know if there are other physical sentient lifeforms in this universe. I can't know the answers to the theological questions this raises. But you know what? That's okay. We don't need to know everything. Trust that God is all powerful, all good, and all knowing and everything falls into place. We know that should there be intelligent extraterrestrials, God would treat them *exactly* as they should be treated. We need not fear this issue.
That being said, there is nothing in the science or the bible that rules out intelligent extraterrestrial life. There is no valid theological arguement I am aware of that says that humans are the center of God's attention, and God created no other sentient lifeform.
The bible is about humans, God, and everything in between. This is how knowledge of angels and demons is given to us. They are involved between us and God. But extraterrestrial life is irrelevant to our relationship with God and our ultimate destiny, the bible has no more reason to address them than it does to address quantum physics.
Does fine-tuning rule out extraterrestrial life?
However, I don't think it follows that this rules out life on other planets. In a way, I would argue it actually makes it more likely.
To explain, imagine you are some angelic being that roams around the universe and does not know about the earth and humanity. You would look at the universe and never imagine in a million years that humanity could exist. Because the fine-tuning is simply so extraordinary that you would never conceive of it as a possibility. You would never conceive that earth-like life *could* exist. In the real world, we only know this stuff in hindsight.
In the same vein, it is possible that God has fine-tuned other planets in the universe for life (earth-like or not) in ways that we couldn't possibly conceive of without the benefit of hindsight.
What about the sacrifice of Christ?
1. They aren't fallen. They never sinned and are in right relationship with God.
2. The sacrifice of Christ on Earth covers them as well as humans. Thus we would be allowed to be missionaries to them as well and allow them salvation. I dislike this option personally because if there really are a multitude are sentient species out there, what are the odds that we would be the one that was picked to kick off salvation?
3. God has other ways of ensuring salvation that he uses on other planets.
4. Christ incarnates and dies multiple times. Although many are unhappy with this option, I find it the most likely scenario. The traditional view (in this case, I agree with it) is that Christ has a dual nature, a divine nature and a human nature. He is at once fully God and fully man. In the same way he could possess an indefinite number of natures.
For example, I will take a species from the popular sci-fi show Star Trek, what if klingons were real? Well, Christ could take on a klingon nature and live and be sacrificed as a klingon. So that they could have salvation just as we do.
In the end, it is all speculation. We have no real reason to believe God created any other physical, sentient species in this universe besides humans. But even when/if we do, it is no reason to shake anybodies faith in the slightest, have trust in God and there is nothing to fear.
Thanks for reading!