As in intellectual extraterrestrials flying around in space crafts with the ability to reach another solar system well within a single life time?
Why would I say that when it's impossible to travel faster than light? Especially since it's been proven time and time again with particle accelerators.
Albert Einstein said faster than light travel is impossible, HOWEVER space can be warped in which the 2 distances between the star systems is zero. And this is the same Albert Einstein who postulated the theory of gravity waves 100 years ago, and they were proven recently.
Even those researchers hope the discovery of gravity waves can allow us to travel the stars like on Star Trek while upholding the laws of physics.
Plus discoveries into the Alcubierre warp drive, zero point effect, and so forth, it's safe to assume that it's possible to bypass the light barrier without breaking it.
That's all nice, but what about alien visitations? Since the question also added a detail on UFOs.
Anything like that always comes down to physical evidence and the old adage of Sir Arthur Conan Doyle -- "Once you eliminate the impossible, whatever remains, however improbable, must be the truth."
Even though sceptics will foolishly put UFOs under impossible probably due to the light barrier, even though there are ways, ways PLURAL, of passing it making it only improbable, not impossible.
Back on track, this is a common scenario a UFO allegedly lands in a small community, behind a barn. The very next day, there's scorch marks on the ground with impressions of what can only be described as landing pads.
Can that be faked without anyone noticing? Unlikely, but I cannot say impossible.
Next step. The soil over the scorched area was tested to have shown signs of being subjected to high levels of radiation AND nothing radioactive was found. And the radiation exposure was directly under the burn mark for several feet sterilizing everything, not around. Backed up by the data collected.
Can that be hoaxed? So highly unlikely that it's safe to say it's impossible.
Could it be a natural phenomenon? Also highly unlikely as the hoaxers just happen to find a section of soil that was naturally sterilized straight down without anyone noticing, it's safe to say impossible.
Once you eliminate earthly, man made one natural occurences, then all that's left is extraterrestrial.