I assume by "aliens" you refer to extraterrestrial intelligence.
I strongly suspect there is more "life" in the Universe than just here on Earth, whether that life is carbon-based and manifests itself like we do is going to be the main difference.
Equally, the fact that tiny differences could have resulted in nothing on Earth could easily result in no other lifeforms, although with a Universe infinitely large, it seems likely there is something.
As for flying saucers etc, I'm sceptical as I'm never seen any 100% conclusive evidence, I'm not doubting there may have been something but without some solid irrefutable proof, I remain unconvinced at this point.