Scientists looking for extraterrestrial signals are testing an unusual idea: Another civilization could choose a predictable interaction between the Earth and the Sun as the moment to contact us. That could turn a huge search into a precise observation strategy.
The approach does not assume that aliens exist or want to send messages – but it does give researchers a clear opportunity to test with a radio telescope.
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The team at the SETI Institute describes the survey and the first results in The Astronomical Journal. Using the Allen Telescope Array in Northern California, they follow the point in the sky opposite the Sun.
The reasoning is based on game theory
Game theory studies how decisions depend on what others might do. A common focal point can help two parties coordinate – even if they haven’t agreed on a plan beforehand.
To a distant observer in the right place, the Earth periodically passes over the Sun’s face. Such a transit can reveal the planet’s presence and orbital timing, and provide a potential transmitter with a recognizable event to organize a message around.
At the transit’s midpoint, the Earth lies between the Sun and the observer. From our perspective, the possible transmitter would therefore appear near the anti-solar point – the direction opposite to the Sun in our sky.
A civilization could conceivably use this adjustment to send a tightly focused signal instead of sending continuously. The idea is based on assumptions about the sender’s knowledge, place and choice – but provides a practical reason to investigate this changed direction.
Must come back throughout the year
As the Earth moves around the Sun, the anti-solar point moves towards the background stars. The survey must therefore return repeatedly throughout the year, with plans for observations two or three nights a week over about four years.
The pilot phase used 20 antennas over the course of three nights in the summer of 2023. Around 17 hours of observations yielded about 32 terabytes of data and about 1.9 million initial signal detections that required further screening.
None of them provided evidence of extraterrestrial technology. After filtering and reviewing, the detections can be explained by interference from human sources – illustrating why a radio signal is only a starting point, not a discovery.
Useful null results
The team compares signals from slightly different directions and tracks how their frequencies change to reject interference. The pilot also established sensitivity limits under specific transmitter assumptions, making the absence of detection scientifically useful.
Under certain conditions, the pilot could have detected a transmitter comparable in strength to the previous Arecibo planetary radar as far away as about 124 light-years. Different signal strengths, directions, or timing would change that range.
The big campaign is underway
The larger campaign has already collected more than 121 observation nets, with more results planned. Whether it finds a signal or not, the survey will help define which possible transmissions have been tested – and show how timing can sharpen the search for intelligence beyond Earth.
If you’re interested in this topic, you can read articles about hidden molecular patterns that could reveal extraterrestrial life on Mars and Europa, and about whether life could begin on Titan – the NASA study that finds clues in Saturn’s mysterious moon.
