One of the longest-standing techniques in humanity’s search for life beyond Earth may be causing scientists to miss alien signals entirely, a new study finds.
Since the very beginnings of the search for extraterrestrial intelligence (SETI), narrowband radio signals have been the focus of these searches. Narrowband radio signals are considered ideal technosignatures — signs of technology that could indicate intelligent life — because they travel long distances, require low power, and stand out from the broader frequency ranges that dominate natural cosmic noise. The problem, according to a SETI Institute study published March 5 in The Astrophysical Journal, is that space weather near a transmitting star may be diluting those signals before they leave their home system, making them harder to detect. The culprit is turbulent plasma in stellar winds and, in some cases, violent eruptions from the host star.
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“SETI searches are often optimized for extremely narrow signals. If a signal gets broadened by its own star’s environment, it can slip below our detection thresholds, even if it’s there, potentially helping explain some of the radio silence we’ve seen in technosignature searches,” said Vishal Gajjar, lead author and SETI Institute astronomer, in a press release.
Decades of silence
The idea that searchers should look for narrowband signals dates back to the origins of SETI. In 1959, Giuseppe Cocconi and Philip Morrison, physicists at Cornell University, published their paper “Searching for Interstellar Communications,” kicking off SETI as a legitimate field of science. They suggested the hydrogen line at 1.4 GHz, a frequency an advanced civilization would likely recognize, as the logical place to start. Since that initial paper, astronomers have searched the skies for a sign that anyone is out there and have come up empty-handed.
Cocconi and Morrison weren’t wrong — narrowband signals remain the most logical candidate for an intentional interstellar transmission. What their landmark paper couldn’t account for was the chaotic environment surrounding distant stars. If an alien civilization were to transmit a narrowband signal towards Earth, it would first have to pass through this so-called interplanetary medium (IPM).
Every star, including our own Sun, is surrounded by an IPM — a region of plasma and magnetic fields constantly shaped by stellar winds, flares, and occasional violent eruptions called coronal mass ejections (CMEs). It is this environment, the study argues, that can intercept and distort a narrowband signal before it ever reaches outer space.
Plasma turbulence near a star can broaden a narrowband signal across a wider range of frequencies, dulling its peak the way fog scatters a flashlight beam. This would render the signal invisible to instruments looking for a sharp, narrow spike.
The extent of the broadening
To measure how severe that effect might be, the team turned to radio transmissions from spacecraft in our own solar system. The team collected data on how narrowband communications from probes like Mariner, Helios, Cassini, and Voyager broadened as they passed behind the Sun relative to Earth. They then extrapolated that model across a simulated survey of 1 million nearby stars, varying stellar types, orbital configurations, and space weather conditions.
For a signal being transmitted by alien technology at 1 GHz — the frequency range where SETI searches are concentrated — roughly 70% of nearby stellar systems would broaden the signal by around 1 Hz — enough to weaken a passing signal, though it might still be detectable with the right instruments. For about 30% of systems, the broadening exceeds 10 Hz, degrading roughly 94% of the signal, meaning current searches would likely miss it entirely.
At lower frequencies, where next-generation telescopes like SKA-Low and LOFAR are designed to search, the problem is significantly worse — for a signal being transmitted at 100 MHz, more than 60% of the simulated stellar systems produce broadening severe enough to push signals below detection thresholds. And in the rare case that a CME passes across the line of sight during a search, the signal is effectively wiped out altogether. The problem is worst around M-dwarf stars — small, dim red stars that make up about three-quarters of all stars in the galaxy — where stronger stellar winds and more violent space weather make distortion especially severe.
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Implications for future searches
The findings have potential consequences for how SETI searches are designed. Current searches already account for Doppler shifts — the frequency shifts caused by the motion of a distant planet relative to Earth. What they don’t account for is the additional broadening caused by the IPM. The researchers recommend that future surveys consider both effects into their search filters — looking not just for signals that have drifted in frequency, but for signals that have also been broadened by their home star’s environment. The researchers also recommend that next-generation facilities like SKA-Low — a low-frequency radio telescope currently under construction — build these considerations in from the start.
“By quantifying how stellar activity can reshape narrowband signals, we can design searches that are better matched to what actually arrives at Earth, not just what might be transmitted,” said Grayce Brown, co-author and research assistant at the SETI Institute.
The search for alien signals has always been a long shot — a needle in an infinite number of haystacks. Cocconi and Morrison acknowledged as much in that very first paper. “The probability of success is difficult to estimate,” they wrote. “But if we never search, the chance of success is zero.” Sixty-six years later, the search continues — hopefully with a sharper understanding of what we might have been missing.
