First of all: UFOs are not necessarily from the outer space. Many ufologists assume they are more likely to come from the future or some parallel universe. Or perhaps from past Earth, which they left at some point (the most popular time would be the Cretaceous).
Now, let's concentrate on actual aliens from another planet. There are countless estimations, more or less scientific, like the
Drake equation. I personally would like to point to the following necessary assumptions:
1) Life is not a one-time event.
Claiming that life is unique to Earth would be pretty heliocentric, since its chemical processes aren't particularly exotic. Even the development of DNA, or some equivalent, seems fairly probable to me, given enough time (and there is much time available).
2) Any cradle of life can create intelligent life.
However, highly organized life is a different story. Rising above bacteria level is extremely difficult for energy management reasons, and on Earth it worked only due to a miraculous and highly improbable symbiosis of cells with other cells, which evolved into mitochondria (otherwise it would be simple cellular life only). This is explained further in
this short article. And it is rather safe to say that complex life is necessary for intelligence as we know it.
Therefore, while I have little doubt that there is life out there, I am hesitant to say the same about aliens with plasma guns.
3) Alien intelligence is comparable to ours.
X-Com aliens are, well, alien, but are they really? Their physiology, society, culture and artefacts are parallel to ours - they understand transportation, engineering, warfare and so on. But is this true for a hypothetical alien intelligence out there, assuming it had the chance to appear at all?
The answer is, I don't know. However, I do know one thing: all living things must manage limited resources, and therefore understand such concepts as competition and hostility, but also economy, cooperation and trade. Therefore, no matter how alien they are, we can always communicate with them by exchange of resources (unless we have nothing to offer them, but I find it unlikely). I can't say anything more about other aspects of alien intelligence, because there is no data; refer to good sci-fi books for more elaboration (S. Lem, A. C. Clarke, W. J. Williams and more).
This perspective may not seem overly optimistic about meeting our space cousins, though it's not overly pessimistic either. Still, even there is none, there is still the parallel universes theory and other sources of weirdness that can also be taken into consideration as for the source of aliens, so do not despair! I would say that if our universe indeed keeps forking into other universes, then the existence of aliens is not just probable, it's inevitable. Of course many of these aliens - those from the nears regions of the multiverse - are completely like us, but that's another story...
