It is not unchristian to believe in aliens unless is gets in the way of our belief in God because, as far as I know, there is no verse or passage that says there isn't life out there somewhere.
Aliens may have come here long ago. I also believe that if they came here in spacecrafts, they would have to know how to bend space to get here, otherwise they'd never be able to make it here in a reasonable amount of time. But here's a thought...if they can bend space, and space and time are woven together as Einstein described, wouldn't it be safe to assume they can bend time too?
Have you noticed how the cave paintings and early artwork of spaceships look exactly like the spaceships we see today? Wouldn't you think after thousands of years of development, an alien race would have improved upon their design? I mean, look at us and our cars. We come up with new models every year, and if you compare our cars to what we had even a hundred years ago, they look completely different. But with spaceships and a span of thousands of years, they look the exact same.
Is it possible that they are able to bend time, and therefore take thier spaceships they have now into the past? It would explain why they have always looked the same. Maybe they haven't exactly "always been there" in our past, in the way of linear time. But maybe they are here now, and they are traveling to the past. If that is the case, then the question would be "why?"
This is just a thought and nothing to do with spirituality or the Bible. Obviously, there are no Bible verses to back this up. The only Bible verse I can think of that might even remotely have anything to do with aliens is in Genesys when the "sons of God" are described as inpregnating women and creating the race of giants. It is widely believed that this is refering to demons, but I just can't see how a spirit can inpregnate a woman, and if it was possible, why is it not still happening? We don't have giants anymore, not like those described in Genesis.
I'm not saying the "sons of God" were aliens for sure because I don't know. Maybe at that time spirits could inpregnate women and God took that ability away from them. There's a lot of possibilities. It's just something to think about.