During a Democratic presidential debate last week, the topic of UFOs came up. As an astronomer, I am very often asked about UFOs, aliens, and other such things. So, here are my answers to these questions. And they are all opinion, not proven scientific fact.
Do I believe that people see UFOs?. Yes, without a doubt, but UFO means "Unidentified Flying Object", and not "alien spacecraft." People see things in the sky all the time that they can't identify -- this is especially true at night. Our eyes did not evolve to see well in the dark, and can play tricks on us. Airplanes, satellites, meteors, even planets and bright stars are often called "UFOs" because the people looking at them did not know what they were looking at. Even trained astronomers and pilots accustomed to flying at night see weird things that, in retrospect, can be explained. The number of UFO sightings that are truly unexplained is a very tiny fraction of all total reports.
Do I believe that alien beings visit the Earth? No. The reason is that space is amazingly huge and hostile, much more so than almost anybody recognizes. At current best speeds, it takes our space probes tens of thousands of years to reach the nearest stars! And, even if there is technology that allows these distances to be crossed easily, why would aliens seek us out yet remain so secretive? With all do deference to people who think they have seen alien spacecraft, the evidence is exceedingly flimsy. Even among those handful of reliable UFO reports that cannot yet be otherwise explained, there are no high-quality photographs or videos that stand up to rigorous scientific scrutiny. And finally, if there are lots of alien civilizations out there, we are not very advanced, and could not possibly be very interesting. If aliens are just curious about us (like us watching meerkats on TV), then why take all the personal risks of space travel, when a robot ship could do all sorts of testing, filming, and so on?
And please don't accuse me of being close-minded; I'd very much like to know if alien civilizations exist, and, if they are scooting about the stars, how they do it. But there has to be very rigorous evidence, and nothing of what I've seen approaches rigorous.
Again, I do not question that people who have seen UFOs have indeed seen something. I just don't believe these people have seen alien spacecraft.
What about Roswell? Read this article. I think that alien spacecraft made a great cover story for what was really a defense experiment, and so the government was willing to let the story linger (and perhaps even encourage it).
Finally, do I think alien life exists? I suspect that it does. As we have started to find planets in the galaxy, we are finding them everywhere, in every shape and size. And on Earth, we are finding life just about anywhere it can exist, and many places where we think it shouldn't. But we still have very little idea why life arose on Earth, and how narrow of conditions a planet has to experience to develop life. And, for three and a half billion years, life on Earth was nothing more than mats of bacteria and algae -- we don't know why that suddenly changed 500 million years ago. So, as we look for life elsewhere, I think the first things we are most likely to find are bacteria. But until we do discover that, Earth remains a very special place.