I doubt there are aliens out there to begin with, and I make this inference from both a scientific and theological perspective. There are lots of stars out there, yes, but the earth is one of the most rarest (if not the only) circumstance(s) that could support life. There are tons of factors in this: its placement in the solar system, unusually thin atmosophere, magnetic core (a very unique mix that protects us from radiation), crust activity, sun type, chemical compositions, pressure, gravity, size, planetary composition, ice age cycle, distance, water quantity, lunar counterweight (without it our winds would reach 400mph), galaxy type, placement within that galaxy, etc. etc.. It's amazing when you go through the details: even Jupiter, a rare planet formation that is almost always a bad planet to have in a solar system, protects the Earth by keeping back hordes of astroids and meteorites (we'd have 1,000-10,000x more collisions without it). Even if we found another planet like Earth, there would still have to be the genesis of life (which man himself has so far failed to prove it could be created materialistically).
I know it's a "what if" question, but really, the existance of aliens in the first place is hard to establish by both science and what we know of God's promises (i.e., the existance of the universe and its laws hinges on God's judgements of Earth, God creating man, etc.).
At least to me, asking that about aliens is like asking "what happens if dogs could suddenly talk" or "what happens if the world suddenly bursts into flames" ...it's just a moot point. What if? I don't know. I don't think it could ever happen. If it ever did happen, it would depend on the alien's own belief system and knowledge of science, so I really can't answer that.