Sheikh and Gerrard use game theory for antisolar SETI survey

Isabel Gerrard of Breakthrough Listen at the University of Oxford and SETI Institute research scientist Sofia Z. Sheikh presented a paper at the 77th International Astronautical Congress proposing that the Allen Telescope Array target the antisolar point, where an agent using the transit method (a planet passing in front of its star, slightly dimming it) would detect Earth against the Sun as habitable.

Sheikh says the central problem is deciding when another civilization might transmit, as well as where. “The challenge in SETI isn’t simply deciding where to point a telescope,” she says. Game theory inspires the new effort, which tries to anticipate another player’s moves based on expectations of your own next move. The authors liken the other player to an intelligent civilization running its own SETI-like effort.

They use the game-theory idea of Schelling points, or focal points, to suggest coordinates where humans and an alien agent might both send and search for a signal without talking first. The antisolar point is an abstract coordinate marked by a line from the Sun through a viewer on Earth and out the far side, shifting as Earth circles the Sun.

From the antisolar perspective, an extraterrestrial agent using the transit method would detect Earth against the Sun as habitable once each year. At the midpoint, Earth would sit directly between the observer and the Sun. Sheikh says another civilization would almost certainly use the transit method because it presents itself naturally.

Researchers call the Allen Telescope Array antisolar point search the ATA-ASP survey. The campaign is the array’s largest survey effort and will gather 11 petabytes over four years. The campaign is intermittent rather than constant because the antisolar point changes as Earth orbits, and the array is not solely occupied with the survey.

It takes at least two years of observing sessions to cover every possible point along the ecliptic because daylight blocks half of it each year. Because Earth orbits the Sun roughly every 365.25 days, the accessible nighttime part of the ecliptic shifts slightly year to year, so observers must stagger sessions over four years for full coverage.

The initial pilot ran from Aug. 30, 2024, to Aug. 25, 2026. During the pilot, the array picked up about 1.9 million radio signals in 17 hours of observations, and every one came from human sources. Since that pilot, the campaign has added 121 nights of observations, though the campaign has not shared those results yet. The idea of scanning the antisolar point dates back decades, and this pilot is the first time anyone has carried it out.

The project tries to predict when and where humans might detect a deliberate alien transmission and reply, while also anticipating when and where another civilization might do the same. The authors say limited resources mean effective use of time and observing capacity improves the odds of success for humans and any alien civilization sending signals.

Kerins says game theory can resolve the Fermi Paradox by determining how best to search for and communicate with extraterrestrial civilizations. He works at the University of Manchester in the U.K. His approach centers on mutual detectability, which means targeting places where an observer would stand about the same chance of spotting us.

He says mutual detectability is not always symmetric, since each side holds different information such as details about the parent star’s brightness and the exoplanet’s orbit. He says the best strategy limits searches to exoplanets where a civilization could look for humans the same way humans look for them, an idea he calls common denominator information. Kerins says the party with the greater amount of shared information about the other civilization should take the initiative to send a signal.

He applied this approach to the NASA Exoplanet Archive, which lists 74 exoplanets in the habitable zones of their parent stars. The most common method for spotting exoplanets watches for the slight dimming of a parent star when a planet passes in front of it, and only some exoplanets line up to see Earth transit the Sun. Astronomers know of only one potentially habitable exoplanet in that aligned region, the Earth Transit Zone: K2-155d, a super-Earth orbiting in the habitable zone of K2-155 in Taurus. The Kepler Space Telescope found it in 2018 along with two sister planets outside the habitable zone.

Kerins says humans bear the responsibility to send the first message to K2-155d because that world is more visible to Earth than Earth is to it. “The onus to transmit falls on the party with superior [common denominator information] about the other party,” Kerins says.

The approach has not yet turned up a candidate with an onus to contact humans, since the lower luminosity stars most likely to host such candidates have not been well surveyed. He estimates the Milky Way probably contains thousands of exoplanets in both the habitable zone and Earth’s Transit Zone that meet the conditions for potential contact. That does not mean a civilization exists on any of these worlds; it only means they offer the best chance of contact if one does.

SETI Institute associate researcher Lewis J. Pinault co-authored a paper submitted to the International Journal of Astrobiology and posted as an arXiv preprint. It proposes searching lunar regolith (lunar soil) for physical artifacts of alien civilizations. The paper defines Arkhipov particles, named for Ukrainian astronomer Alexei Arkhipov, as debris from an alien Dyson swarm’s destruction (a hypothetical megastructure built around a star to capture its energy), and defines Bracewell particles, named for physicist Ronald Bracewell, as smart dust sent on purpose that might have reached the moon over its roughly 4.5-billion-year history. The moon preserves particles better than Earth since it has no atmosphere, plate tectonics or water cycle.

A single cubic meter of lunar regolith equals 1.5 metric tons of dirt and contains over a trillion micrometer-sized grains, too many for manual checking. Researchers plan to scan electron microscope images of regolith with an AI vision model called YOLO-ET. The model would flag abnormal particles for closer inspection. A null result from one cubic meter would rule out cases where sunlike stars spread more than 0.1 Earth masses of artificial dust across the galaxy over time, and would show that no civilization has sent probes at a rate above 0.4 kg per billion years.

Manuel Scherf and Helmut Lammer of the Austrian Academy of Sciences presented findings at the EPSC-DPS2025 Joint Meeting in Helsinki on Oct. 12, 2025. Their estimate puts the closest technological society about 33,000 light years from Earth. They say a different technological civilization would need to survive for at least 280,000 years, or possibly millions of years, to coexist with humanity. Scherf estimates that ten simultaneous civilizations would require an average lifetime of more than 10 million years, and because the Sun sits roughly 27,000 light years from the galactic center, the nearest civilization might lie on the Milky Way’s far side.

UCLA astronomer emeritus Dr. Ben Zuckerman discussed his broadband alien civilizations paper in a SETI Live conversation hosted by Dr. Lauren Sgro. He says an advanced civilization might already know where Earth is because space telescopes and interferometers can detect oceans, an atmosphere and seasons. Broadband radio surveys aimed at distant galaxies also gathered data from many nearby Sun-like stars, and AI could search those archives for narrowband technosignatures that were previously overlooked. Roughly two million Sun-like stars have come within 100 light-years of Earth over two billion years, compared with about 20 billion comparable stars across the galaxy.