Kamal Ghorui
For more than six decades, astronomers have listened for possible signals from extraterrestrial civilisations, with no confirmed detection to date. A new statistical study considers a narrower and more uncomfortable possibility. It asks whether one or more alien signals may already have reached Earth since the early 1960s but gone unnoticed. Using a Bayesian framework, researchers model how missed contacts would affect present-day expectations of detectability. The analysis focuses on the period since the first modern SETI experiment, which took place in 1960. It concludes that if past signals were real but undetected, the number of transmitting civilisations required to explain today’s silence would be unexpectedly high, in some cases exceeding reasonable estimates of habitable planets within several hundred light years of Earth.
If nearby alien civilisations are transmitting signals to Earth, why is there no evidence
The research,“Undetected Past Contacts with Technological Species: Implications for Technosignature Science”, is asking a simple but uncomfortable question. If alien signals have already reached Earth at some point in the past 65 years, why have we not detected them?
Scientists working in SETI, the Search for Extraterrestrial Intelligence, often argue that we have only explored a tiny fraction of the possible “search space”. That search space includes:
- Different distances in the Milky Way
- Different wavelengths such as radio, microwave, optical or infrared
- Different signal strengths
- Different time windows
Because this search space is so large, one explanation for the silence is that we simply have not looked in the right place, at the right time, with the right sensitivity. The new study tests a stronger idea. What if signals have already reached Earth, but we missed them?
What the researchers did
They used a Bayesian statistical model. In plain terms, they asked:If at least one alien signal reached Earth in the last 65 years but went undetected, what does that imply about how many civilisations are transmitting?They found something surprising. To make today’s non-detections consistent with past missed signals, you would need an implausibly large number of emitting civilisations nearby. In some scenarios, the number required would exceed the number of habitable planets within a few hundred light-years. That creates tension. The numbers do not sit comfortably together.
Why detectability matters
The study practically defines detectability. A signal is considered detectable if its source lies within a distance R from Earth. That distance depends on two things:
- How powerful the signal is
- How sensitive our telescopes are
For radio signals, stronger transmitters can be detected from farther away. The minimum detectable flux depends on telescope sensitivity, bandwidth and integration time.Projects such as Breakthrough Listen have dramatically expanded the range of frequencies and targets searched since 2016. Future facilities like the Square Kilometre Array and the Next Generation Very Large Array are expected to increase sensitivity even further.Yet even with these improvements, the model shows that high present-day detectability of nearby civilisations would require unrealistically high numbers of transmitters.
What this suggests
If undetected alien signals really have reached Earth before, then they were likely rare and distant. The best chances of detection may lie thousands of light-years away, not just in our local neighbourhood. Even then, the study suggests we should expect only a few detectable signals, not a sky full of them.
