A better way to search for extraterrestrial intelligence

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When you're looking for signs of extraterrestrial intelligence, it helps to know what you're looking for and to go about it in the most efficient way. But work so far has generally not done so, writes Benjamin Zuckerman, an astrophysicist and emeritus professor in the Department of Physics & Astronomy at the University of California at Los Angeles (UCLA).

Zuckerman suggests our searches via the electromagnetic spectrum should be redesigned, making use of astronomical surveys done independently of the search for extraterrestrial intelligence (SETI), and he offers an improved search technique suggested by his research.

Because there are apparently no alien probes in our solar system, his methodology allows the conclusion that "no alien civilization has passed within 100 lt-yr [light-years] of Earth during the past few billion years." His work has been published in The Astrophysical Journal.

Most published work (by humans at least) has assumed that the alien extraterrestrial intelligence (ET) will be power-limited—the electrical power available to aliens will be increasingly difficult to generate and is ultimately capped. To optimize the signal-to-noise ratio at the receiving end, it's been thought the ETs will employ very narrow transmission and reception bandwidths.

And so, human searches of received radio transmissions use algorithms that look at very small frequency bands of only a few Hertz. Even using sophisticated hardware and software, the total bandwidth covered so far is only a "modest percentage of the full radio/microwave window," writes Zuckerman.

In 1964, the Soviet astrophysicist Nikolai Kardashev, known for his Kardashev scale of civilizational status based on their energy capture, proposed that any transmitted signal sent out by an ET would be isotropic, of equal intensity in all directions, and not via highly directional antennas. In any particular direction, an isotropic signal will be weak, making them power limited, which has influenced SETI science ever since.

Why beamed signals change everything

Zuckerman proposes the exact opposite—that any ET will choose to instead send highly directional signals, so limited power won't be much of an issue. In this case, he writes, the most uncertain factor for us will be the signal's wavelength (or alternatively, frequency), so we should look much wider across the electromagnetic spectrum—not just as radio waves, but also infrared and optical light at least. But this is "very difficult" to do with currently designed radio SETI search programs.

For nearby technological civilizations communicating with a purpose, doing their "technological best to establish communications," power limitations, and thus filtering for very narrow bandwidths at Earth, are "irrelevant" writes Zuckerman.

This scenario makes it possible for the searching, receiving species to have a "serendipitous detection of an alien transmitter in electromagnetic sky surveys undertaken for reasons that have nothing to do with SETI." Any such signals would be screaming at us if we're in just the right direction, if we're simply doing routine astronomical observations with modest telescopes.

In this case, the fact that about 100 years of solid astronomical study can be used to put limits on the number of purposeful technical species within about 100 light-years, "in the solar neighborhood." Primitive or unsophisticated civilizations would go undetected by this methodology, but they mostly would anyway.

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Narrowing the hunt to sun-like stars

Zuckerman considered five relevant observation parameters: distance, direction, sensitivity (power received per square meter per hertz), wavelength (or frequency) and time.

He assumed any detectable extraterrestrial life would, like us, be water-based, meaning their planet must be in their star's habitable zone, where there will be liquid water on some part of the surface. A planet's star would need to be sun-like; only stars less than 1.25 solar masses live long enough (4.5 billion years) for such a technological species to evolve on the planet.

Zuckerman calculates that a target SETI search program may have to observe as many as 300,000 stars within 200 parsecs (650 light-years) to include all the old ones, and that about two million such stars have passed within 100 ly of our solar system in the last two billion years, the time oxygen has existed in Earth's atmosphere due to the Great Oxidation Event.

Through many such astronomical, astrophysical and SETI considerations, and given that many non-SETI star surveys in the radio and optical spectrum covering the entire sky could have detected the continuous, strong signals sent by technological civilizations purposely (via highly directional transmissions) trying to attract attention, Zuckerman obtains the limit mentioned above: "no alien civilization has passed within 100 light-years of Earth during the past few billion years."

What future surveys could reveal

While parts of the radio and infrared bands remain to be explored, what's needed, Zuckerman says, is a broadband survey of old, sun-like stars, from radio frequencies through to visible frequencies, which would extend the limit above and allow the first numerical estimation of the number of communicative ETIs in the Milky Way galaxy, with an upper limit of about 100,000 and perhaps even 10,000.

Zuckerman told Phys.org that he has communicated with some top SETI astronomers, and "none have criticized what I have to say." The same was true for other astronomers who are not SETI experts but qualified to evaluate his work and claims. But, he adds, "although the jury has not yet reported in, various evidence suggests that the verdict will be that we are alone in our little corner of our Milky Way galaxy." Just don't tell Steven Spielberg.

Written for you by our author David Appell, edited by Sadie Harley, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive. If this reporting matters to you, please consider a donation (especially monthly). You'll get an ad-free account as a thank-you.

Publication details

B. Zuckerman, Broadband Searches for Extraterrestrial Technological Intelligence: A New Strategy to Find Nearby Alien Civilizations, The Astrophysical Journal (2026). DOI: 10.3847/1538-4357/ae4c38

Journal information: Astrophysical Journal

Key concepts

Electromagnetic radiation astronomyHabitable zoneOptical astronomySearch for extraterrestrial intelligence

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