How Will We the Find First Signs of Alien Life — and When?

When will we find evidence for life beyond Earth? And where will that evidence be found? University of Arizona astronomer Chris Impey, the author of a book called “Worlds Without End,” is betting that the first evidence will come to light within the next decade or so.

But don’t expect to see little green men or pointy-eared Vulcans. And don’t expect to get radio signals from a far-off planetary system, as depicted in the 1992 movie “Contact.”

Instead, Impey expects that NASA’s James Webb Space Telescope — or one of the giant Earth-based telescopes that’s gearing up for observations — will detect the spectroscopic signature of biological activity in the atmosphere of a planet that’s light-years away from us.

“Spectroscopic data is not as appealing to the general public,” Impey admits in the latest episode of the Fiction Science podcast. “People like pictures, and so spectroscopy never gets its fair due in the general talk about astronomy or science, because it’s slightly more esoteric. But it is the tool of choice here.”

“Worlds Without End” is the 14th book written by Impey, who has been an author for 200 peer-reviewed publications focused on observational cosmology, galaxies and quasars. His previous books have taken on subjects ranging from the universe’s origin (“How It Began”) to its expected demise (“How It Ends”).

Impey says it’s the right time for a book about the search for worlds beyond Earth, and the search for life on those worlds.

“It’s still less than 30 years since the first exoplanet was detected, and now we have over 5,000,” he says. The raw numbers of newfound planets are no longer all that interesting, but Impey is intrigued by the different strategies that are being used to look for life on those planets.

Some researchers are putting their money on SETI, the search for extraterrestrial intelligence. That quest involves looking for radio signals from faraway star systems that may carry messages from alien civilizations. Twenty years ago, a couple of SETI astronomers predicted that such signals would be detected by 2025 — but although there are still a couple of years of shelf life left for that prediction, scientists aren’t counting on that timetable as a sure thing.

There was a time when Impey thought the first signs of alien life would be found on Mars, thanks to NASA’s plans to bring samples back to Earth for detailed study. But those plans have run into a budgetary buzzsaw, and Impey suspects it will take longer to nail down the evidence of biological activity on the Red Planet, if it exists.

Life may also lurk in the hidden oceans of a Jovian moon called Europa and a Saturnian moon called Enceladus. But the next mission designed to study the outer solar system’s icy frontier — NASA’s Europa Clipper — won’t start looking for evidence until the 2030s.

By then, Impey expects scientists to be analyzing the spectroscopic signs of life they’ll pick up from telescopic observations of exoplanets.

Those observations won’t be easy to come by: Teams of researchers will have to identify the chemical signatures of compounds detected in exoplanet atmospheres. But scientists have already shown that the James Webb Space Telescope is up to the job. Other telescopes that are due to come online within the next seven years — including the Rubin Observatory, the Giant Magellan Telescope and the Extremely Large Telescope — could do an even better job of sniffing alien air.

If an exoplanet is found to have, say, a higher-than-expected percentage of oxygen in its atmosphere, that could be a clue pointing to biological activity. Other types of detections might represent technosignatures rather than biosignatures. For example, if scientists see elevated levels of chlorofluorocarbons or anomalous heat emissions, that could signal that an alien civilization is facing the same environmental challenges we’ve been going through.

In any case, it’ll take more than one promising data point.

“It’ll be a set of spectra,” Impey says. “It’ll maybe be half a dozen, or a dozen planets, saying the same thing. Or maybe saying different things. Maybe it turns out that a lot of the habitable planets are dead, and that’s a possibility everyone has to be prepared for.”

Everyone will also have to be prepared for a debate over whether the spectroscopic evidence of biological activity is sufficiently persuasive.

“It’s a hard experiment, and so the data will be ambiguous,” Impey acknowledges. “It won’t be enough to convince some people, maybe. And it depends of what the data says, of course. So, beyond that, it is hard to do much better.”

What about the idea that the aliens are already here? This week, a House subcommittee heard sworn testimony from a witness claiming that the U.S. government has been aware of extraterrestrial connections to UFOs (now known as unidentified anomalous phenomena, or UAPs) going as far back as the 1930s. Impey doesn’t buy it, however.

“That’s something that most astronomers just don’t subscribe to, because of the Saganism of ‘extraordinary claims require extraordinary evidence,’” he says. “That bar has never been cleared. Extraordinary evidence is not anecdotal evidence. It’s not video or images, because, well, you know how they can be altered. And even if they’re not altered, even if it’s video from an Air Force pilot, the interpretation of that data is very ambiguous.”

Impey is a bit more intrigued by Harvard astronomer Avi Loeb’s claims that an interstellar object known as ‘Oumuamua might have been an alien artifact, or that metallic spherules recovered from the seafloor might be the remains of an extraterrestrial alloy.

“Worlds Without End” by Chris Impey (MIT Press)

“These particular cases that he’s picking out are interesting, because you learn something either way,” he says. “You either learn something new about planetary science, for example, or you learn a dramatic thing about interstellar visitors and artifacts that come into the solar system from elsewhere. But I think the strong bet is still that these are natural phenomena.”

In the latter chapters of his book, Impey turns his attention to future trends in space exploration. Will humanity make a second home on Mars, as SpaceX founder Elon Musk hopes?

“I don’t think it’s the future of humanity,” Impey says. “It’s almost a deflection or a distraction to think of us leaving the Earth, because we’re never going to do that in large numbers or sufficiently to make it a Plan B. I just think it’s coming from a different place.”

The way Impey sees it, the drive to explore space comes from the same place that has driven humans to set up outposts in Antarctica. But if humans decide to push outward from Earth someday, Impey says it would mark the start of “an extraordinary biological experiment.”

“The bottleneck effect in genetics means they will diverge as a population, more rapidly than normal,” he says. “And so you’ll speciate, and at some point they won’t be humans anymore. They’ll be the next thing. And that’s extraordinary.”

Impey goes into detail about what could happen over the decades to come in his book, and in our podcast. Perhaps the biggest question for the next 50 years will be whether intelligent life — as opposed to mere microbes — exists beyond Earth.

This month, a nationwide survey suggested that 40 percent of Americans think intelligent life will be discovered on another planet by the year 2073. “I probably will go in on a bet like that,” Impey says.

And if we don’t find intelligent life in the next 50 years?

“This is very tricky, because the methods you use involve technologies like radio and lasers and so on,” Impey says. “What if they don’t have radio or lasers? Well, some of them will, you imagine. But the sensitivity of those methods is going to be so good by then, and there’ll have been so much searching and listening by then, that if any of our types of technologies exist on millions and millions of planets around millions and millions of stars, then we would see them.”

If we don’t see them by 2073, that may have to serve as the answer to one of humanity’s ultimate questions: Are we alone?

“At some point, the null result becomes meaningful,” Impey says. “It just says they’re either not there, or they’re exceptionally rare. And that’s what could happen on a 50-year timescale.”

Check out the original version of this report on Cosmic Log for reading recommendations from the Cosmic Log Used Book Club — and stay tuned for future episodes of the Fiction Science podcast via Apple, Google, Overcast, Spotify, Player.fm, Pocket Casts, Radio Public and Podvine. Alan Boyle’s co-host for the Fiction Science podcast is Dominica Phetteplace, an award-winning science-fiction writer who currently lives in San Francisco. To learn more about Phetteplace, visit her website, DominicaPhetteplace.com.