How scientists decide if they’ve actually found signals of alien life

When searching for extraterrestrial intelligence, being certain can be tricky business.
Silver radio telescopes with mountains in the background.
The Allen Telescope Array in California, where many small radio dishes listen for ET signals. SETI Institute

In 2014, a meteor entered Earth’s atmosphere and burst apart in the air above the ocean near the Pacific island nation of Papua New Guinea, probably scattering tiny fragments along the seabed. Meteors that burn up in the atmosphere and leave small traces are not unusual— NASA estimates nearly 50 tons of space rock falls on the Earth every day—but Harvard astrophysicist Avi Leob recently made headlines when he suggested this particular meteor, dubbed CNEOS 2014-01-08 or IM1, may actually have been a piece of an alien spacecraft. 

Many of Loeb’s colleagues in the fields of astrophysics and the Search for ExtraTerrestrial Life, or SETI, are highly skeptical of his claims. They’ve also cast doubt on the evidence that CNEOS 2014-01-08 is truly an interstellar object. But Loeb’s claim—and the responding criticism—raise important questions: Just how do you decide whether you’ve found evidence of alien life when the data are often so small, far away, or just ambiguous? And how do you share your findings?

”It’s a big problem,” Jason Wright, a professor of astronomy at Penn State. “We call these the post-detection protocols in SETI.”

Scientists working on SETI in the 1960s and 1970s, including Carl Sagan and Frank Drake in the US, and Nicolai Kardashev and Iosif Shklovsky in the Soviet Union, created a set of protocols for how they would assess potential radio signals of extraterrestrial origin.

The first step was keeping the claims to a small group. “When you thought you might have found something, you would be able to share it only with other scientists without making it public,” he says. That might sound “incredibly naive today,” he notes, but it made pre-internet sense. After analyzing the signal and ensuring they were not mistaking Earthly radio signatures for aliens, “then you would make a big announcement—you would go to the UN and you would go to the governments.”

What the Cold War Era SETI post-detection protocols didn’t anticipate, Wright says, were more ambiguous signals or evidence. But those began cropping up as early as the 1970s, with a set of experiments on board the NASA Viking missions to Mars.

The tests, which were meant to detect the presence of organic compounds and possibly alien life on the Red Planet, had unclear and conflicting results. A biology experiment on the Viking 1 spacecraft showed one positive result for the presence of organic compounds, one negative result, and a third that was undetermined. The lead scientist for the experiment, the late Gilbert Levin, who died in 2021, argued as recently as 2012 that the experiment had, in fact, found signs of life on Mars. 

Then, in 1996, a team of scientists led by NASA Johnson Space Center’s David McKay began investigating a meteorite of Martian origin, known as Alan Hills 84001. Members of the team became so convinced they’d found evidence of fossilized Martian life in the space rock that it reached President Bill Clinton, who said “it speaks of the possibility of life” in an address to the nation. 

Though the scientific community came to believe McKay and his team were mistaken, they “were reasonably responsible [in the first article they published about it], even if they clearly believed they had something,” Arizona State University astrophysicist Steven Desch says. 

Since the 1996 announcement, scientists have put even more thought into how to gauge the levels of evidence for signs of alien life in different circumstances. The European Space Agency’s ExoMars mission Rosalind Franklin, a rover scheduled to launch to the Red Planet in 2028, will use a complex “biosignature score” rubric, ranking the confidence experiments have found signs of aliens. 

Key to any such evaluation of evidence of alien life is understanding what your confounders are, Wright says. Put another way: What might you detect that you would mistake as what you are looking for? 

For astronomers looking for signs of alien telecommunications, if you’re hoping to eavesdrop on alien radio, you need to rule out radiation signals from Earth. “When they do a SETI search these days, millions and millions and millions of hits, they call them, get detected, and they’re all from terrestrial transmitters,” Wright says. “It’s extremely challenging to rule those out to get rid of them. It’s like being in a crowded room and everyone’s talking at once, and you’re trying to hear the one voice.”

For signs of technological origin, or signs of fossilized life in meteorites, confounders are processes that could produce those objects without an appeal to alien life. Most scientists ultimately concluded that what looked like fossil microbial life in Alan Hills 84001 could be produced by other chemical or geological processes. 

[Related: Alien civilizations could send us messages by 2029]

Off the coast of Papua New Guinea, Loeb and his research team used a magnetic sled to drag the sea bed along the expected trajectory of the space rock. They collected small metal spheres. (Authorities from the island nation have suggested the material may have been illegally acquired.) The astronomer announced on his blog that his team had recovered unusual magnetic material consisting of an alloy of iron, titanium and magnesium that “does not resemble known human-made alloys or familiar asteroids.” He asked whether the asteroid might have been manufactured by some alien technology.

But he will also need to rule out that these spherules didn’t originate from the many other sources that create tiny metallic bits on the ocean floor, SETI experts say. 

“You’d have to match them against what are the more mundane possibilities including spherules from [non-intersterstellar] asteroid material hitting the Earth,” Desch says, noting that the seabed is covered in tiny pieces of ordinary meteorites. Then there is volcanic ash and artificial spherules—“stuff comes out of coal fired plants and lands on the seafloor too.” 

And while comparing a possible sign of life to more mundane alternatives, it’s also important to acquire that most essential form of comparison in science—a control sample. Since it landed on Mars in 2021, for example, NASA’s Perseverance rover has been collecting tubes of rock and soil that will be returned to Earth for analysis in the early 2030s. 

To help ensure any signs of life are not actually contamination brought to Mars from Earth, the rover carries five “witness tubes” containing Earth materials that could, in theory, contaminate the rover samples. Briefly opening the witness tubes on Mars will give scientists a pattern of what terrestrial contamination of a Mars sample would look like. 

The same type of measurement is even easier to make when trolling the seabed for signs of interstellar meteorites, according to Desch. “Go 100 miles away and collect the stuff from there and see if it’s any different,” he says. “If you find the same mix of stuff everywhere, it’s not all aliens, it’s just natural.”

For his part, Loeb says his team believes the spheres they found are in fact from the meteor, and not other sources. “The composition of the spherules along the meteor path is different from that of volcanic ash,” he writes in an email to Popular Science. “Our control samples were obtained tens of kilometers away from the meteor path, and revealed an abundance of spherules lower by a factor of 10 from the meteor path.”

Loeb plans to do further laboratory analysis of the recovered materials at the Harvard College Observatory. If history is any guide, that analysis will need to be extremely thorough, as confirming ambiguous signs of alien life has so far proven to be a big and incomplete task. 

[Related: Astronomers want to wield a tiny laser to look for life on neighboring worlds]

But that’s not to say there are no conditions that would point indisputably to the existence of extraterrestrial life. If intelligent, space-faring aliens really are capable of visiting Earth, they could, of course, fulfill a 1950s sci-fi stereotype and land on the White House lawn to ask to see the president. 

“Another example is finding a technological gadget as the relic of an interstellar meteor,” Leob writes. “Such an object could have components that are unfamiliar, including a label: ‘Made on an exo-planet.’”

More convincing than that, however, might be the detection of a radio signal that could not be produced by natural means, according to Wright. “Only technology can produce narrowband radio emission,” Wright says, referring to radio transmissions encoded in a narrow range of frequencies that efficiently use bandwidth to communicate data. He envisions “plenty of scenarios where we’re sure it’s technology and space, and we’re sure it’s not ours, because it’s not local.” The SETI Institute’s Allen Telescope Array, several dishes in California, are designed to hunt for such a signal. 

But even a detection of an alien radio transmitter might set up a whole new level of analysis. Just because you receive the signal, it doesn’t mean it’s for you, that you can decipher it, or that the sender would respond if you tried to talk to them. “We say, ‘Well, that star’s got radio transmissions.’ Sometimes you see them, sometimes you don’t. They’re definitely technological,” Wright says. “Yes, they have radio transmitters. And that’s all we know.”