The real science behind SETI’s hunt for intelligent aliens


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In 1993, a team of scientists published a paper in the scientific journal Nature that announced the detection of a planet harboring life. Using instruments on the spacecraft Galileo, they imaged the planet’s surface and saw continents with colors “compatible with mineral soils” and agriculture, large expanses of ocean with “spectacular reflection,” and frozen water at the poles. An analysis of the planet’s chemistry revealed an atmosphere with oxygen and methane so abundant that they must come from biological sources. “Galileo found such profound departures from equilibrium that the presence of life seems the most probable cause,” the authors wrote.

But the most telltale sign of life was measured by Galileo’s spectrogram: radio transmissions from the planet’s surface. “Of all Galileo science measurements, these signals provide the only indication of intelligent, technological life,” wrote the authors.

The paper’s first author was Carl Sagan, the astronomer, author, and science communicator. The planet that he and his co-authors described was Earth.

Twenty years later, as far as we can tell, Earth remains the only planet in the Universe with any life, intelligent or otherwise. But that Galileo fly-by of Earth was a case study for future work. It confirmed that modern instruments can give us hints about the presence of life on other planets—including intelligent life. And since then, we’ve dedicated decades of funding and enthusiasm to look for life elsewhere in the Universe.

But one component of this quest has, for the most part, been overlooked: the Search for Extraterrestrial Intelligence (SETI). This is the field of astronomical research that looks for alien civilizations by searching for indicators of technology called “technosignatures.” Despite strong support from Sagan himself (he even made SETI the focus of his 1985 science-fiction novel Contact, which was turned into a hit movie in 1997 starring Jodie Foster and Matthew McConaughey), funding and support for SETI have been paltry compared to the search for extraterrestrial life in general.

Throughout SETI’s 60-year history, a stalwart group of astronomers has managed to keep the search alive. Today, this cohort is stronger than ever, though they are mostly ignored by the research community, largely unfunded by NASA, and dismissed by some astronomers as a campy fringe pursuit. After decades of interest and funding dedicated toward the search for biological life, there are tentative signs that SETI is making a resurgence.

At a time when we’re in the process of building hardware that should be capable of finding signatures of life (intelligent or otherwise) in the atmospheres of other planets, SETI astronomers simply want a seat at the table. The stakes are nothing less than the question of our place in the Universe.

The Arecibo Radio Telescope on Puerto Rico receives interplanetary signals and transmissions. And it was in the movie Contact!
Enlarge / The Arecibo Radio Telescope on Puerto Rico receives interplanetary signals and transmissions. And it was in the movie Contact!

Getty Images/Stephanie Maze

How to search for life on other worlds

You may have heard of searching for life on other planets by looking for “biosignatures”—molecules or phenomena that would only occur or persist if life were present. These could be microbes discovered by directly sampling material from the planet (known as “in-situ sampling”) or using spectroscopic biosignatures, like chemical disequilibria in the atmosphere and images of water and agriculture, like those detected by the Galileo probe in 1990.

The biosignature search is happening now, but it comes with limitations. In-situ sampling requires sending a spacecraft to another planet; we’ve done this, for example, with rovers sent to Mars and the Cassini spacecraft that sampled plumes of water erupting from Saturn’s moon Enceladus. And while in-situ sampling is the ideal option for planets in the Solar System, with our current technology, it will take millennia to get a vehicle to a planet orbiting a different star—and these exoplanets are far, far more numerous.

To detect spectroscopic biosignatures we will need telescopes like the James Webb Space Telescope (JWST) or the ground-based Extremely Large Telescope, both currently under construction. To directly image an exoplanet and obtain more definitive spectra will require future missions like LUVOIR (Large Ultraviolet Optical Infrared Surveyor) or the Habitable Exoplanet Imaging Mission. But all of these lie a number of years in the future.

SETI researchers, however, are interested in “technosignatures”—biosignatures that indicate intelligent life. They are signals that could only come from technology, including TV and radio transmitters—like the radio transmission detected by the Galileo spacecraft—planetary radar systems, or high-power lasers.

The first earnest call to search for technosignatures—and SETI’s formal beginning—came in 1959. That was the year that Cornell University physicists Giuseppe Cocconi and Philip Morrison published a landmark paper in Nature outlining the most likely characteristics of alien communication. It would make the most sense, they postulated, for aliens to communicate across interstellar distances using electromagnetic waves since they are the only media known to travel fast enough to conceivably reach us across vast distances of space. Within the electromagnetic spectrum, Cocconi and Morrison determined that it would be most promising to look for radio waves because they are less likely to be absorbed by planetary atmospheres and require less energy to transmit. Specifically, they proposed a narrowband signal around the frequency at which hydrogen atoms emit radiation—a frequency that should be familiar to any civilization with advanced radio technology.

What’s special about these signals is that they exhibit high degrees of coherence, meaning there is a large amount of electromagnetic energy in just one frequency or a very small instance of time—not something nature typically does.

“As far as we know, these kinds of [radio] signals would be unmistakable indicators of technology,” says Andrew Siemion, professor of astronomy at the University of California, Berkeley. “We don’t know of any natural source that produces them.”

Such a signal was detected on August 18, 1977 by the Ohio State University Radio Observatory, known as “Big Ear.” Astronomy professor Jerry Ehman was analyzing Big Ear data in the form of printouts that, to the untrained eye, looked like someone had simply smashed the number row of a typewriter with a preference for lower digits. Numbers and letters in the Big Ear data indicated, essentially, the intensity of the electromagnetic signal picked up by the telescope, starting at 1 and moving up to letters in the double-digits (A was 10, B was 11, and so on). Most of the page was covered in 1s and 2s, with a stray 6 or 7 sprinkled in.

But that day, Ehman found an anomaly: 6EQUJ5. This signal had started out at an intensity of 6—already an outlier on the page—climbed to E, then Q, peaked at U—the highest power signal Big Ear had ever seen—then decreased again. Ehman circled the sequence in red pen and wrote “Wow!” next to it.

Alas, SETI researchers have never been able to detect the so-called “Wow! Signal” again, despite many tries with radio telescopes around the world. To this day, no one knows the source of the Wow! Signal, and it remains one of the strongest candidates for alien transmission ever detected.

NASA began funding SETI studies in 1975, a time when the idea of extraterrestrial life was still unthinkable, according to former NASA Chief Historian Steven J. Dick. After all, no one then knew if there were even other planets outside our Solar System, much less life.

Jill Tarter

In 1992, NASA made its strongest-ever commitment to SETI, pledging $100 million over ten years to fund the High Resolution Microwave Survey (HRMS), an expansive SETI project led by astrophysicist Jill Tarter. One of today’s most prominent SETI researchers, Tarter was the inspiration for the protagonist of Sagan’s Contact, Eleanor Arroway.

But less than a year after HRMS got underway, Congress abruptly canceled the project. “The Great Martian Chase may finally come to an end,” said Senator Richard Bryan of Nevada, one of its most vocal detractors. “As of today, millions have been spent and we have yet to bag a single little green fellow. Not a single Martian has said take me to your leader, and not a single flying saucer has applied for FAA approval.”

The whole ordeal was “incredibly traumatic,” says Tarter. “It [the removal of funding] was so vindictive that, in fact, we became the four-letter S-word that you couldn’t say at NASA headquarters for decades.”

Since that humiliating public reprimand by Congress, NASA’s astrobiology division has been largely focused on searching for biosignatures. And it has made sure to distinguish its current work from SETI, going so far as to say in a 2015 report that “the traditional Search for Extraterrestrial Intelligence… is not a part of astrobiology.”

Despite or because of this, the SETI community quickly regrouped and headed to the private sector for funding. Out of those efforts came Project Phoenix, rising from the ashes of the HRMS. From February 1995 to March 2004, Phoenix scanned about 800 nearby candidate stars for microwave transmission in three separate campaigns with the Parkes Observatory in New South Wales, Australia; the National Radio Astronomy Observatory in Green Bank, West Virginia; and Arecibo Observatory in Puerto Rico. The project did not find any signs of E.T., but it was considered the most comprehensive and sensitive SETI program ever conducted.

At the same time, other projects run by the Planetary Society and UC Berkeley (including a project called SERENDIP, which is still active) carried out SETI experiments and found a handful of anomalous radio signals, but none showed up a second time.

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