Read this article here for some good Scriptural points as to why aliens are un-Biblical.
How Do Aliens And UFO's Fit With The Bible's Teaching?
Granted, aliens do exist, but they are not what they appear to be, though (For they are actually demons). I think the best way to understand this is if you realize that owls spoken of in the Bible sometimes refers to evil spirits (demons). Why is this important? Because certain owls have black almond shaped eyes like the popular grey aliens do. In fact, I remember seeing a movie trailer involving alien encounters and the appearance of owls.
Also, when people have encountered aliens or ghosts, they sometimes will smell brimestone which is something that is associated with Hell, too.
In additon, those who are involved in the occult at some point in their life will also sometimes have alien encounters, as well. In other words, you reap what you sow.
There is of course a lot more I could share on this, but I will let you research it for yourself.