There isn't one, as such. Whether life exists on other planets is a scientific, not a theological, question. If life on other planets is ever discovered, there are theological questions which can be considered.
In his novel Perelandra C. S. Lewis speculated about the possibility of a fallen race (such as ours) influencing unfallen extra-terrestrials. We were affected by the fallen angel Satan. There's nothing in the Bible to say humanity couldn't have a similar effect on another race.
On the other hand, we might also play a part in the redemption of another race--a role in their salvation history. The good angels played an important part in ours (Mt 28:2-5; Acts 7:38, 53).
It's possible God has set up multiple worlds, some fallen, some not, but there's not the slightest scientific evidence he has. A few scientists, pandering to the tabloids, claim extra-terrestrials must exist, based on the mathematical likelihood of other stars having planets, but their theories are scoffed at by nearly the entire scientific community.
The argument runs like this: If some of those planets have atmospheres like Earth's, and if some of those Earth-like planets spontaneously generate amino acids, and if some of those amino acids result in higher life-forms, then intelligent life exists on other worlds. You'll note a lot of "ifs" there.
To insist there must be intelligent life elsewhere in the universe is to overstate the case. It's also theologically irrelevant because the central tenets of Christianity remain intact with or without little green men.