Many SETI efforts look for extremely narrow radio signals because they are unlikely to be produced by natural cosmic processes. However, researchers found that these signals may become distorted before they ever leave their home star system.

How Stellar Activity Can Distort Radio Signals

For decades, SETI scientists have searched for narrow spikes in radio frequencies that could indicate the presence of advanced technology. The assumption has been that an extraterrestrial transmitter would produce a highly concentrated signal that stands out from natural background noise.

The new research points to a potential problem with that approach. Even if a civilization sends a perfectly narrow radio signal, it may no longer appear that way after traveling through the environment surrounding its star.

Scientists already account for many of the effects that occur as radio waves cross interstellar space. This study instead examined what happens much closer to the signal's source. Fluctuations in plasma density within stellar winds, along with explosive events such as coronal mass ejections, can alter radio waves near their origin. These effects can spread a signal's energy across a broader range of frequencies, weakening the sharp peak that many search methods depend on.

"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 Dr. Vishal Gajjar, Astronomer at the SETI Institute and lead author of the paper.

Using Spacecraft Signals to Study the Effect

To measure how significant this broadening could be, the researchers turned to a source of data much closer to home: radio transmissions from spacecraft operating within our solar system.

Using observations from solar system probes, the team calibrated how turbulent plasma affects narrowband radio signals. They then applied those measurements to a variety of stellar environments to estimate how the same processes might influence signals originating around other stars.

The result is a practical framework that allows scientists to estimate how much signal broadening may occur around different types of stars and at different observing frequencies, particularly in the active "space weather" environments found around some stars.

Implications for the Search for Extraterrestrial Intelligence

The findings could influence both SETI target selection and search strategies. According to the study, M-dwarf stars,which make up roughly 75% of all stars in the Milky Way, are especially likely to broaden narrowband radio signals before they can escape the system.

Because of this, the researchers suggest that future searches should remain sensitive to signals that are wider than the ultra-narrow signatures traditionally targeted by SETI programs.

"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 C. Brown, co-author of the study and research assistant at the SETI Institute.

The project highlights the kind of high-risk, high-impact research supported by the SETI Institute's STRIDE program (Support Technology, Research, Innovation, Development, and Education). The initiative helps researchers investigate emerging scientific questions and develop new tools and techniques to address them. STRIDE is funded through the Franklin Antonio Bequest, which was established to advance breakthrough science and education efforts at the SETI Institute.