We've always been fascinated by aliens, and that's putting it mildly. And mathematically speaking, aliens should exist.

"There are two trillion other galaxies we can see, each with a 100 billion Earth-like worlds," said Seth Shostak , the senior astronomer at the SETI Institute. [SETI stands for the Search for Extraterrestrial Intelligence.] "100 billion, OK? It's hard to believe they're all sterile."

Correspondent David Pogue asked, "I happen to know that this is sort of a trick question when people say, 'Do UFOs exist?'"

"Well, I mean, obviously UFOs exist, 'cause that just means unidentified flying objects," Shostak said. "But the real issue here is not that you see things in the sky, it's what you claim they might be."

Author and investigative journalist Leslie Kean said, "You just can't assume if you see something in the sky that looks a little weird that it's some kind of alien thing, which is what a lot of people assume. Most sightings that people have – Oh, I see something in the sky! – those kinds of sightings can usually be explained: The planet Venus, airplanes, comets, shooting stars, birds. Let's say five to ten percent are the cases that any conventional explanation can be ruled out; those are the cases that are of interest."

In 2017, Kean coauthored a New York Times story that revealed the existence of, believe it or not, a secret Pentagon program devoted to studying UFO sightings.

"It was a huge revelation to know that the government took it that seriously," Kean said. "There's really been a sea change of attitude towards this topic. It's not to say that there's no more ridicule, but it's way less."

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