Alien Day is good time to reflect on the possible appearances of UFOs. Getty

The origins of Alien Day, which is recognized on April 26, go back to 2009 as a promotional event for 20th Century Fox. Alien Day might be as good a time as any to reflect on some of the "controversies" and "conspiracies" dealing with extraterrestrial life visiting Earth.

First off, does life exist outside of Earth? The probability is high, according to scientists. "You can't think we're alone once you study how common our ingredients of life are in the universe," said famed astrophysicist Neil deGrasse Tyson on CBS's "Late Late Show" in September 2016. "The atoms of our body—hydrogen, oxygen, carbon—they are the three most common chemically active ingredients in the universe. We are not made of special stuff."

But whether they have visited humans on Earth is another matter. Regardless of the probability, there are a few stories dating back 60 years over unidentified flying objects sightings.

Roswell, New Mexico, 1947

One of the most publicized stories of a UFO sighting involves a rancher who said that he and his son found metal debris from a crash, prompting an investigation from the Roswell Army Air Field. Following an RAAF press release, the Roswell Daily Record newspaper put out a front-page article with the headline, "RAAF Captures Flying Saucer On Ranch in Roswell Region." Military officials later reported a link between the incident and a top-secret atomic espionage project called "Project Mogul."

McMinnville, Oregon, 1950

A farm couple photographed a "metallic disk-shaped object" in the sky. Life Magazine and newspapers printed the photographs, which are considered to be among the most famous claims of a UFO.

Broad Haven, Wales, 1977

Fourteen children playing soccer in the seaside resort of Broad Haven claimed to have seen a yellow cigar-shaped craft in a field near a school. Witnesses later claimed a weird silver creature clamber out of the UFO. The incident became known as "The Broad Haven Triangle."