One of the most historically significant moments in the search for alien life came in 1976 with NASA’s Viking Mars Landers. The twin spacecraft carried a series of life-detection experiments designed to search for microbial activity in the Martian soil. Results from one experiment showed gas was released after nutrients were added to unheated soil samples, which was interpreted by some scientists as a possible sign of microorganisms metabolizing the nutrients.

However, other instruments on the landers failed to detect organic molecules, leading many scientists to doubt the findings. Some argued that reactive chemicals in the soil, like hydrogen peroxide, caused the results without involving life. This sparked decades of debate, with the experiment’s lead scientist, Gilbert Levin, maintaining it was evidence of life until his death in 2021, at the age of 97.

“I'll never forget studying the data detected by these missions during my university years,” says Musilova. “It was the first time that a tangible search for extraterrestrial life was being conducted on another planetary body beyond Earth.” 

“The results continue to be debated to this day,” she adds.

On August 18, 1977, astronomer Jerry Ehman sat at his kitchen table, sorting through computer printouts detailing data collected a few days earlier by the Big Ear Radio Telescope in Ohio. On one page, he spotted a string of six letters and numbers, 6EQUJ5—a measurement of signal intensity that shot off the charts. He grabbed a red pen, circled the sequence, and scribbled a single word in the margin: “Wow!”

"That's the nice thing about the word 'wow.' I was, uh ... I was astonished," Ehman told NPR in 2010. 

Eighteen years earlier, astronomers had theorized that radio waves were the most logical way for advanced civilizations to communicate across interstellar distances—they are inexpensive to produce, require little energy, and can travel vast distances through space with minimal interference.

The radio burst received by Big Ear, which lasted 72 seconds, was unusually strong—30 times louder than typical deep space radio noise. It fell within a frequency range commonly used by human technology and matched the natural emission frequency of hydrogen, the most abundant element in the universe. Scientists had long speculated that an intelligent civilization might use this frequency as a cosmic calling card. So, if someone—or something—was trying to reach out, this was exactly the kind of signal astronomers had anticipated.

“No conclusion was ever possible other than it certainly had the potential of being a signal from extraterrestrial intelligence,” Ehman told Astronomy Magazine in 2016.

And yet, the signal was never heard again. 

Scientists traced the signal’s apparent origin to a relatively empty patch of sky in the constellation Sagittarius with no known stars or planets that could explain the bizarre signal. In 2017, one theory proposed the signal might have come from a hydrogen-emitting comet passing through that region of sky, while other experts have proposed that the signal could have been caused by a rare cosmic phenomenon, though no consensus has been reached.

To this day, the signal has never been definitively linked to intelligent life—but that possibility has never been fully ruled out either. 

Long before spacecraft delivered close-up images of the Martian surface, 19th-century astronomers were already speculating about life on the Red Planet—thanks mostly to a mistranslated word and some vivid imagination.

In 1877, Italian astronomer Giovanni Schiaparelli used the term canali—Italian for “channels”—to describe the dark, linear features he observed on Mars through his telescope. But when his notes were translated into English shortly after, canali became “canals,” a word that suggested humanmade waterways. This mistranslation, combined with the limited optical resolution of the time, fueled widespread belief that an intelligent civilization might be constructing vast waterways on Mars.

The idea was championed by Percival Lowell, a wealthy American astronomer who devoted much of his career to promoting the notion that Martians were building canals to transport water from the polar ice caps to their equatorial cities. In three books he wrote, Lowell argued that these features were evidence of a dying but technologically advanced civilization managing scarce resources.

Lowell’s theories captivated the public imagination, inspiring early science fiction like H.G. Wells’ The War of the Worlds and helping cement Mars’ reputation as a world potentially teeming with life.

However, as telescopic technology advanced and spacecraft began exploring the planet, the so-called canals were revealed to be an optical illusion—patterns created by the human eye connecting faint surface features into lines. NASA’s Mariner missions in the 1960s definitively debunked the canal theory, showing Mars to be a rugged, cratered world with no signs of artificial structures.

Decades later, in 1996, the discovery of Martian meteorite ALH84001 reignited interest in the possibility of life on Mars. Scientists found microscopic structures and chemical signatures in the rock that some interpreted as evidence of ancient microbial life. However, the claim remains controversial, with many researchers attributing the features to non-biological processes. Although ALH84001 did not conclusively prove that life once existed on Mars, it played a crucial role in establishing astrobiology as a serious scientific discipline, encouraging researchers to explore how life might originate and be detected beyond Earth.

"Without this paper, the field of astrobiology may never have come to exist," astrobiologist Andrew Steele of the Carnegie Institution for Science in Washington, D.C. told Space.com. 

Around 2015, astronomers analyzing data from the Kepler Space Telescope noticed Tabby’s Star (officially KIC 8462852) in the constellation Cygnus was behaving mysteriously. Unlike typical stars that shine steadily or show predictable variations, Tabby’s Star exhibited irregular and dramatic dips in brightness, sometimes fading by as much as 20 percent for days or even weeks. This unusual behavior baffled scientists, as no known natural phenomenon could fully explain such erratic dimming.

The strange flickering sparked widespread excitement and speculation, including the tantalizing hypothesis that the star might be surrounded by a massive alien megastructure—perhaps an enormous artificial swarm of satellites or a Dyson sphere built by an advanced civilization to harness the star’s energy.

However, a subsequent $107,000 Kickstarter-funded study involving over 200 scientists pointed toward clouds of dust obscuring the star. But the origin of the dust itself remains a mystery, leaving astronomers with new questions to explore.

In September 2020, a team of scientists announced the detection of phosphine gas in the upper atmosphere of Venus. On Earth, phosphine is a foul-smelling, toxic gas typically produced by decaying organic matter or certain types of bacteria.

“Now, astronomers will think of all the ways to justify phosphine without life, and I welcome that,” Clara Sousa-Silva, a research scientist at the Massachusetts Institute of Technology, who was involved with the discovery, said at the time. “Please do, because we are at the end of our possibilities to show abiotic processes that can make phosphine.”

However, subsequent observations failed to reproduce the findings, and scientists questioned the reliability of the original data and analysis. They suggested the signal might have been a processing artifact or due to sulfur dioxide, not phosphine. Others pointed out that unknown chemical or geological processes on Venus might explain the gas without requiring biological origins.

Later studies yielded mixed results—some appeared to confirm a weak signal, while others found none—leaving both the existence of phosphine and its possible sources an open question.

These episodes highlight both the excitement and the complexity of the quest to find life beyond Earth, says Musilova. 

“We learned so much about the limits of life as we know it,” she says. “It is challenging not only to try to detect alien life, but especially to prove that the finding truly is related to extraterrestrial lifeforms.”