There is no convincing evidence that any UFO accounts, modern or pre-modern, are associated with an intelligent species visiting Earth from another planet.
At the end of the day, the more we have learned about physics, the distances between us and any nearby likely habitable worlds, the travel limits imposed by the speed of light etc, we know that most likely any other spacefaring species is likely from outside our solar system. We also have a good amount of evidence to suggest none of the star systems near us is likely to have planets where intelligent life could have developed, based on our best knowledge of conditions required for such things to occur. The Alpha Centauri triple star system does have a roughly earth sized planet in the “habitable zone” of Proxima Centauri, but other things we know about the conditions there suggest it is unlikely life or a civilization could have developed in that location. For example that world is subjected to stellar winds 2000 times stronger than Earth, meaning the world would need a significantly strong magnetic field or its atmosphere would likely be blown away. It is also possible (but not certain) that this world is tidally locked to its star, which would be another major hurdle for life to develop.
This means within the bounds of known science, if there is a spacefaring civilization out there it is most likely more than 5 light years away. This is a vast distance.
I also believe the more we have learned about the unique circumstances that lead to life on earth, and then the evolutionary processes and cataclysmic events that lead to the evolution of tree dwelling apes, and then the series of events that lead to one species of those apes developing intelligence and societies, and eventually a civilization capable of spacefaring, suggests that such life developing is likely a very rare event. The odds that any hypothetical other spacefaring civilizations are remotely near us, or that if they were, they would have interest in reaching us, is low. Additionally within the bounds of our knowledge nothing suggests this spacefaring civilization would be able to solve the fundamental, physical problems of distance, energy, and speed that make interstellar travel of crewed vessels seem fundamentally impossible. Now is it impossible? We haven’t proven that. We do know it is a very very hard problem, and it stands to reason not every civilization could hope to solve it if it could be solved.
Now all that being said, that means just the baseline likelihood that aliens are visiting is very, very low. For something that is so unlikely, very solid evidence should be demanded as proof. If someone comes into work on Monday and tells you he was shooting hoops on his garage hoop, and landed 4 baskets in a row without missing…you don’t really need proof for that. That is not a particularly impressive scenario. If he said he landed 1000 in a row, you would likely demand some evidence. The aliens visiting here thing is a lot less likely than someone landing 1000 baskets in a row, and thus should demand proof of the highest level.
Instead we have events that all fall into a few categories:
- Sightings of genuine craft which are simply human made terrestrial craft, be it weather balloons (which actually can look really weird), standard aircraft seen at a weird angle or in a weird lighting, or experimental craft being worked on by the military
- Sightings of natural phenomena that seem freaky and weird but are akin to mirages or effects like the “Fata Morgana”
- Simple human eye failures of perception or memory
- Failure of instrumentation that supposedly detected the craft
- After all of that we have genuine “incidents with no clear explanation”, a great many of these incidents could fall into one of the first 5 types, but likely at least some do not. But just the existence of unexplainable incidents is not anywhere close to the evidence needed to assume we have alien visitors.