A 28-hour search for radio signals from a star-planetary system about 40 light years from Earth has failed to detect alien signals but demonstrated a new eavesdropping strategy to bolster the search for extraterrestrial intelligence (SETI).

Scientists announced in a research paper published on Wednesday the results of the longest single-target radio search directed at TRAPPIST-1, a star orbited by seven Earth-sized rocky planets all of which could potentially have water on the surface.

“We report no detection of signals of non-human origin,” Nick Tusay, a researcher at the Pennsylvania State University and his coauthors wrote in the paper in The Astrophysical Journal. However, their work provides SETI “a powerful new observational tool and search strategy.”

The research team focussed on a phenomenon called planet-planet occultations (PPOs) which occur when one planet moves in front of another. During PPOs, if an intelligent civilisation from one of the planets sends a radio signal to another planet, the transmission could be detected on Earth.

The TRAPPIST-1 system — a star with seven planets, some of them in the habitable zone around the star where the temperature is expected to be just right for liquid water on the surface — is a good laboratory to predict and observe PPOs.

Tusay and his colleagues used the Allen Telescope Array, a set of 42 radio telescopes located about 480km northeast of San Francisco dedicated to both astronomical studies and SETI. From a set of some six million signals, they identified a set of 11,127 candidate signals of which 2,264 occurred during PPOs.

The scientists reasoned that just as engineers on Earth use deep space radio signals to communicate with spacecraft on Mars or elsewhere in the solar system, intelligent beings on one of the TRAPPIST-1 planets might send radio signals to a neighbouring planet.

“Earth is favourably aligned to receive powerful signals transmitted from one planetary body and wash over another during a PPO event,” Tusay and his colleagues wrote. The transmission source planet for two of the seven PPO events during their study was within the system’s habitable zone.

The US researchers did not find any signal that could be interpreted as coming from alien intelligence. However, they plan to continue the search with bigger and more powerful telescopes designed to find weaker signals.

The signals could also be aimed from one planet to orbiters or rovers on other planets in the system.

“Detection of leaked radio transmission from nearby planetary systems may be on the horizon,” the researchers said.

The PPO strategy appears designed to eavesdrop on radio signals from one planet to another when they are aligned in a straight line with Earth, said Yogesh Wadadekar, an astronomer at the National Centre for Radio Astrophysics, Pune, who was not associated with the US study.

The PPO study included several undergraduate students. "The students looked for signals from human-made orbiters around Mars to check if the system could detect signals correctly," Sofia Sheikh, a researcher at the SETI Institute, said in a media release from the institute. "It was an exciting way to involve students in cutting-edge SETI research."