If intelligent aliens visit the Earth, it would be one of the most profound events in human history.

Surveys show that nearly half of Americans believe that aliens have visited the Earth, either in the ancient past or recently. That percentage has been increasing. Belief in alien visitation is greater than belief that Bigfoot is a real creature, but less than belief that places can be haunted by spirits.

Scientists dismiss these beliefs as not representing real physical phenomena. They don’t deny the existence of intelligent aliens. But they set a high bar for proof that we’ve been visited by creatures from another star system. As Carl Sagan said, “Extraordinary claims require extraordinary evidence.”

I’m a professor of astronomy who has written extensively on the search for life in the universe. I also teach a free online class on astrobiology. Full disclosure: I have not personally seen a UFO.

Unidentified flying objects

UFO means unidentified flying object. Nothing more, nothing less.

There’s a long history of UFO sightings. Air Force studies of UFOs have been going on since the 1940s. In the United States, “ground zero” for UFOs occurred in 1947 in Roswell, New Mexico. The fact that the Roswell incident was soon explained as the crash landing of a military high-altitude balloon didn’t stem a tide of new sightings. The majority of UFOs appear to people in the United States. It’s curious that Asia and Africa have so few sightings despite their large populations, and even more surprising that the sightings stop at the Canadian and Mexican borders.

Most UFOs have mundane explanations. Over half can be attributed to meteors, fireballs and the planet Venus. Such bright objects are familiar to astronomers but are often not recognized by members of the public. Reports of visits from UFOs inexplicably peaked about six years ago.

Many people who say they have seen UFOs are either dog walkers or smokers. Why? Because they’re outside the most. Sightings concentrate in evening hours, particularly on Fridays, when many people are relaxing with one or more drinks.

A few people, like former NASA employee James Oberg, have the fortitude to track down and find conventional explanations for decades of UFO sightings. Most astronomers find the hypothesis of alien visits implausible, so they concentrate their energy on the exciting scientific search for life beyond the Earth.