If any REAL space alien of any kind shows up on earth, my faith will be absolutely rocked. If aliens exist, then there is no God. The two are not compatible.
Any system of worlds that a supreme being would create must be on different time schedules. Planet A and planet B are not experiencing things at the same time in the same way. Planet Z is way, way different. Never the twain shall meet.
What about the thousands of "sightings?" Military experiments, atmospheric disturbances, hoaxes, and just plain good-old imagination.
Which brings up the subject of ghosts. It is also probably true that most ghostly phenomena are attributable to physical disturbances, hoaxes, and just plain good-old imagination. There is one difference, however. I CAN see a just God allowing a departed soul a brief time to contact loved ones to set right some wrong. And, there might be cases where the departed are so hung-up on staying earth-bound, they just can't seem to tear themselves away "toward the light." A few devilish spirits might also be around, just to throw a little confusion in the mix. By and large, however, most ghost sightings are as mistaken as are alien sightings.
To be fair, if someone were to reverse my entire argument, and prove space aliens exist - and thus do away with the traditional concept of God by substituting Erich van Danikens ancient visitors - well, that would do away with ghosts with the same brush stroke. No real God - no immortality. No immortality - no spirits.