A narrowband radio signal transmitted from an alien planet (left, white) begins as a sharp spike – the kind that SETI searches are designed to detect. But as it passes through the plasma-filled atmosphere surrounding its host star, turbulence expands it into a broad, flat shape (right, green) that current instruments would completely miss. Credit: Vishal Gajjar
One of humanity’s longest-running techniques in the search for life beyond Earth is causing scientists to completely ignore alien signals, a new study has revealed.
Since the beginning of the search for extraterrestrial intelligence (SETI), narrowband radio signals have been the focus of these searches. Narrowband radio signals are considered ideal technological signatures – signals of technology that may indicate intelligent life – because they travel long distances, require low power, and differ from the wide frequency range that dominates natural cosmic noise. That’s the problem, according to a study by the SETI Institute published March 5. The Astrophysical JournalThis is because space weather near a transmitting star can weaken those signals before they leave their home system, making them harder to detect. Turbulent plasma in stellar winds and, in some cases, violent explosions from the host star are responsible.
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“SETI searches are often optimized for extremely narrow signals. If a signal gets wider than its own star’s atmosphere, it can slip below our detection limit even if it is there, potentially helping to explain some of the radio silence we have seen in technosignature searches,” lead author and SETI Institute astronomer Vishal Gajjar said in a press release.
decades of silence
The idea of researchers looking for narrowband signals goes back to the origins of SETI. In 1959, Cornell University physicists Giuseppe Cocconi and Philip Morrison published their paper “Searching for Interstellar Communications”, beginning SETI as a legitimate field of science. He suggested the hydrogen line at 1.4 GHz, a frequency that an advanced civilization would probably recognize as a logical place to start. Since that initial paper, astronomers have searched the skies for signs that someone is out there and have come up empty-handed.
Cocconi and Morrison were not wrong – narrowband signals remain the most logical candidate for intentional interstellar transmission. Their landmark paper could not account for the chaotic environment around distant stars. If an alien civilization were to transmit a narrowband signal toward 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 that is constantly shaped by stellar winds, flares, and sometimes violent explosions called coronal mass ejections (CMEs). The study argues that it is this environment that can intercept and distort a narrowband signal before it reaches outer space.
Plasma turbulence near a star can amplify a narrowband signal over a wide range of frequencies, reducing its peak range in the same way fog scatters a flashlight beam. This will make the signal invisible to devices looking for a sharp, narrow spike.
range of expansion
To measure how severe this effect might be, the team turned to radio transmissions from spacecraft in our own solar system. The team collected data on how narrow-band communications from probes such as Mariner, Helios, Cassini and Voyager broadened as they passed behind the Sun relative to Earth. They then extrapolated that model to a simulated survey of 1 million nearby stars, varying stellar types, orbital configurations, and space weather conditions.
For signals transmitted by alien technology at 1 GHz – the frequency range where the SETI search is focused – about 70% of nearby stellar systems will amplify the signal by about 1 Hz – enough to weaken the passing signal, although it may still be detectable with the right instruments. For about 30% of systems, the broadening exceeds 10 Hz, which degrades about 94% of the signal, meaning it would disappear completely from current searches.
At lower frequencies, where next-generation telescopes such as SKA-Lo and LOFAR are designed to make discoveries, the problem is significantly worse – for signals transmitted at 100 MHz, more than 60% of simulated stellar systems are broadened so severely as to push the signal below detection threshold. And in the rare case that a CME passes beyond the line of sight during the search, the signal is effectively wiped out completely. The problem is worst around M-dwarf stars – small, dim red stars that make up about three-quarters of all the stars in the galaxy – where stronger stellar winds and more violent space weather make distortion particularly severe.
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Implications for future discoveries
The findings have potential consequences for how SETI searches are designed. Current discoveries already account for the Doppler shift – a frequency shift caused by the motion of a distant planet relative to Earth. They do not take into account the additional expansion caused by IPM. The researchers suggest that future surveys consider both effects in their search filters – looking not only for signals that have changed in frequency, but also for signals that have been broadened by the atmosphere of their home star. The researchers also recommend that next-generation facilities such as SKA-Lo – a low-frequency radio telescope currently under construction – build in these ideas from the start.
“By quantifying how stellar activity can reshape narrowband signals, we can design searches that better match what actually comes to Earth, not just what may be transmitted,” said Gracie Brown, co-author and research assistant at the SETI Institute.
The search for alien signals has always been a long endeavor – a needle in an infinite number of haystacks. Cocconi and Morrison had already acknowledged this in the paper. “It is difficult to estimate the probability of success,” he wrote. “But if we never discover, the chances of success are zero.” Sixty-six years later, the search continues – hopefully with a sharper understanding of what we’re missing.
