Aliens would not be descendants of Adam and Eve and all of God's communication has been with humans.
So I do not see how the existence or nonexistence of alien creatures would have any effect on God's plan for the salvation of Man.
Remember that Scripture tells us there are other intelligent beings created by God, who are not descendants of Adam and Eve, and are not saved by Christ. Namely, angels.
Although angels are spiritual creatures, can we truly say with any certainty that there could be no similar corporeal creatures?
By the way, have you read any of C.S. Lewis' space trilogy -
Out of the Silent Planet,
Perelandra, and
That Hideous Strength?