Editor’s Note: Don Lincoln is a senior scientist at the Fermi National Accelerator Laboratory. He is the author of several science books for general audiences, including the bestselling audio book “The Theory of Everything: The Quest to Explain All Reality.” He also produces a series of science education videos. Follow him on Facebook. The opinions expressed in this commentary are solely his. View more opinion articles on CNN.

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For centuries, people have witnessed unexplained lights in the sky and thought that perhaps they might be ghosts or angels. However, it was in the summer of 1947 when a different explanation became popular. Following a widely reported incident over Mt. Rainier in Washington state, people began to believe that these unidentified flying objects (UFOs) are actually alien spacecraft prowling the Earth.

Don Lincoln

Over the past 70 years, more than ten thousand similar reports have been made. Most reports were eventually debunked as weather balloons, the planet Venus or even oddly shaped clouds. Some accounts simply arose from nothing more than the fevered imaginations of UFO enthusiasts. But not all reports could be dismissed so easily.

In 2004, Navy fighter jet pilots operating from the USS Nimitz aircraft carrier reported seeing UFOs off the coast of San Diego. And, more recently, other military pilots flying with the USS Theodore Roosevelt in the Atlantic made similar claims. The news of those accounts became public knowledge from a story in the New York Times and a new miniseries on the History Channel. These media and entertainment reports brought the incidents to the attention of government leaders.

The question that comes to mind is: “Are these aliens?” Sadly for anyone who is a fan of the television show “The X-Files,” it is simply far, far, more plausible that what these pilots were seeing is something with a more ordinary explanation, whether it be an instrumental glitch or some other unexplained artifact.

Given the professionalism of the pilots who reported the sightings, I am fairly certain that they did indeed see a UFO. The problem is that many people jump directly from the “unidentified” in “UFO” to “flying saucer.” And that’s just too large a jump to be reasonable. There is simply no credible evidence that the Earth is being visited by aliens. There are no artifacts, no clear photographs, no captured aliens, no alien bodies – nothing.

The reports of UFOs are from eyewitnesses or poor resolution photographs or videos. Ask a criminal prosecutor about the value of eyewitness reports: they have contributed to a majority of the convictions that were later overturned by DNA evidence. An eyewitness can be an unreliable source of information and, in the case of something as extraordinary as the observation of alien spacecraft, pedestrian evidence simply won’t do. As Carl Sagan often said, extraordinary claims require extraordinary evidence.

The reality is that unidentified means just that. The pilots could have seen a real object that they couldn’t explain, or they could have experienced an instrumental artifact, which is basically just a glitch in the electronics. In the San Diego incident, there were earlier reports of high-altitude radar contacts from surface ships. And just prior to when an unidentified object was recorded on the fighter jets’ infrared camera, there were reports of something in or just below the water. Even though there were multiple observations of multiple phenomena, what is missing is multiple reports of the same phenomenon. It would be hasty to link these independent observations together.

While alien visitation is unlikely, as long as there remains even the faintest chance that there exist aerial objects with capabilities beyond those humanity can achieve, we should continue to investigate the possibility. After all, if these objects are real and actually move in the ways that the pilots reported, it’s something that any military would want to know about, as being aware of credible threats is one of their key responsibilities.

There was a time in history when the working explanation of UFOs didn’t involve extraterrestrials. During World War II and before the flying saucer craze, Allied pilots reported UFOs that they called “foo fighters,” which were thought to possibly be a new weapon employed by German Luftwaffe. By the same token, these modern accounts should certainly be investigated in the event that they actually exist and are of earthly origin. However, I would be very surprised if the reports turned out to be anything other than ordinary.

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    I’ve been fascinated by the idea of extraterrestrial life since I was a small boy and I think it is very likely that it exists throughout the universe. The tagline of the television show “The X-Files” resonates with me. I want to believe.

    But I’m going to need much better evidence than what we have so far. In the meantime, let’s keep an eye on these reports. You know… just in case.