The Navy tracks UFO sightings. Scientists explain what’s really going on.

The Navy caused a bit of a sensation this spring when it implemented a formal process for pilots to report unexplained aerial phenomena — what most people call UFOs — after being accused in the past of not taking such reports seriously.

Alas for those who might be tempted to make the leap, such sightings are not evidence of life on other planets.

No one doubts that the pilots are seeing something, but psychologists and specialists in aviation medicine say there are plenty of reasonable explanations for such sightings other than extraterrestrial beings. Earthly sources of light reflected by clouds or haze, for example, or optical illusions wrought by fatigue after staring through a cockpit window for hours on end.

Another possibility is that the pilots were seeing some sort of experimental drone or other advanced technology about which they had not been briefed. Or, the objects were simply satellites, such as those launched in May by the Elon Musk-founded company SpaceX, which prompted a flurry of UFO reports from puzzled observers, the news agency AFP reported.

That does not mean scientists doubt the existence of life elsewhere in the universe. On the contrary, they say there is a good chance we are not alone. That is the view of astronomers who search for other planets that might have water — an essential substance for life as we know it — and geologists who study the conditions on Earth when life arose more than 3.5 billion years ago.

But it would not be the sort of complex organism that could communicate with us, much less send a spaceship our way. Think instead of microbe-like organisms — possibly something that derives energy from a chemical process other than photosynthesis, said Alexandra Davatzes, an associate professor of earth and environmental science at Temple University.

The odds of extraterrestrial life are reasonably good even in our own solar system — given the ample evidence that liquid water has existed on Mars and is present today beneath the icy surface of Europa, one of Jupiter’s moons, Davatzes said.

“I am optimistic,” she said.

But first, some explanations for what those pilots might be seeing.

Though we treat our sense of vision as if it provides an exact representation of our surroundings, it is far from perfect in many respects, said Alan Stocker, a University of Pennsylvania associate professor of psychology who studies the neuroscience of perception.

First, most objects do not emit light, so we see them only because they are reflecting light from another source.

“It’s an indirect signal,” Stocker said. “It’s not the object itself.”

Additional uncertainty arises from the fact that the brain must process and interpret the two-dimensional signals that are projected onto the retina, converting them into a representation of the actual three-dimensional world.

Humans get by with their imperfect eyesight pretty well, using knowledge of prior experiences and context from other senses to fill in the gaps, but there are times when it breaks down.

Among the earliest to document that fact was Aristotle, who found that if he stared at a moving stream for a while, then shifted his gaze to some rocks nearby, they appeared to move in the opposite direction. This phenomenon, now called motion aftereffect, occurs because the neurons that detect motion in a certain direction will adapt after being stimulated for a period of time, temporarily slowing their firing rate, Stocker said. That response results in the illusion of stationary objects moving in the reverse direction.

“It’s very clear that people don’t see the world the way it is in all kinds of ways,” Stocker said.

Another biological factor that can skew the vision of airborne personnel is fatigue — no surprise given that some long-range crews stay aloft more than 24 hours at a time, said Brian S. Pinkston, director of the aerospace medicine center at University of Texas Medical Branch in Galveston. Extreme fatigue can cause the brain’s visual cortex to register something that is not there, he said.

Equally potent is a phenomenon called autokinesis: the illusion that a stationary point of light is moving when it is viewed against a dark, featureless background.

“Your eyes have some inherent movement in them, and it will make it appear as if the object is moving,” said Pinkston, a former Air Force flight surgeon. “You can have stars in the sky, and it will appear as if they’re moving.”

Harder to explain are the occasions when similar sightings are experienced by multiple people, as in the case of Navy pilots interviewed by the New York Times. But there is some evidence that the culprit is advanced government technology of which the pilots were unaware, according to the Drive, a media outlet focusing on military affairs.

Whatever the true explanation, the key to analyzing new phenomena is a scientific principle called parsimony, said Scott Engle, an assistant professor of astrophysics and planetary science at Villanova University. In everyday English, parsimonious means being frugal with money. To a scientist, it means being frugal in making unwarranted assumptions.

“The explanation that requires the fewest assumptions or modifications to your understanding is usually going to be true,” said Engle, who studies the habitability of planets outside the solar system.

In the case of unexplained aerial sightings, that means — absent some extraordinary evidence — that the notion of alien spaceships simply does not hold water. There are so many questions. Among them: Wouldn’t we detect communication signals or some other sign of advanced beings before they traveled the vast distances needed to get here? And how would such a spacecraft even work?

Proxima b, a planet Engle has studied, is considered one of the nearest where conditions might be right for liquid water to exist on the surface. Yet it is 25 trillion miles away, a distance that would take thousands of years to travel with available technology. There is no reason to think some other life form has cracked the code of interplanetary travel.

“So they have the type of technology to visit us and are doing so in secret, but they accidentally slip up every once in a while and get spotted and then turn away?" Engle said. "Versus humans are imperfect and our eyes are imperfect and our optical processing is imperfect.”

No surprise: he goes for door No. 2.

Still, that has not stopped scientists from looking.

The modern quest for signs of extraterrestrial life is said to have been launched about 1950, when Nobel Prize-winning physicist Enrico Fermi posed a provocative question to some colleagues at Los Alamos National Laboratory.

“Where is everybody?” he asked.

Others at the table immediately understood what he meant. Given the sheer size of the Milky Way and the billions of years that have elapsed since its formation, surely there must be someone else out there. Why are we so special?

A decade later, astronomer Frank Drake expanded Fermi’s observation into something called the Drake equation, which seeks to calculate the probability that other advanced civilizations exist in the galaxy. Among the variables it includes are the rate at which stars are formed, the estimated fraction of those stars that have orbiting planets, and the still-smaller fraction with conditions that might harbor an ecosystem.

It is a thought experiment, tantalizing but fraught with lots of uncertainty.

Davatzes focuses on a more concrete problem: the conditions that gave rise to life on Earth.

In her office at Temple, she has a 3.5 billion-year-old hunk of rock called a stromatolite — a rusty-red specimen from western Australia that contains fossils of some of the earliest known forms of life on Earth.

The rock is interspersed with wavy layers of pinkish sediment revealing the activity of long-ago microbial “mats” — communities of microbes. The rock was formed as layers of sand were deposited on top of the mats. The microbes grew upward through each successive layer, giving them their wavy, “crinkly” appearance, she said.

Conditions on Earth were far different back then. The atmosphere contained little or no oxygen, for one thing, and it would not for a billion years more.

Yet life began. Who’s to say it can’t happen somewhere else?