Scientists hunting for radio signals from any form of advanced extraterrestrial life that might be out there trying to contact us are now starting to wonder if something has been messing with their work.
A new study published by researchers at the SETI Institute, short for the Search for Extraterrestrial Intelligence, has tested the possibility that “space weather” could render strong premeditated alien broadcasts into the kind of fainter radio signals that SETI typically ignores.
“SETI searches are often optimized for extremely narrow signals,” according to Vishal Gajjar, an astronomer at the SETI Institute and lead author on the new work, published this past Thursday in the American Astronomical Society’s Astrophysical Journal.
Natural astrophysical events—including varying densities of the energetic plasmas that flow within a given star’s stellar wind or, more dramatically, any highly charged stellar eruptions, like a coronal mass ejection—could “smear” a strong narrow radio signal across multiple frequencies, according to the paper.
“If a signal gets broadened by its own star’s environment,” Gajjar continued in a press statement, “it can slip below our detection thresholds, even if it’s there.” Gajjar and his colleagues now suspect that this might explain some of the eerie “radio silence” they have documented in their past scans for alien radio technosignatures.
To test their hypothesis, the SETI team turned to data from some of humanity’s own far-ranging spacecraft, tracking how broadcasts from older probes, including the Pioneer series, Helios 1/2, the Viking program, and others, have all been impacted by our own solar system’s local space weather.
Recalibrating with humanity’s own technosignatures
Early data from the Mariner IV and Pioneer 6 spacecraft, launched in 1964 and 1965, respectively, show that their broadcasts along the roughly 2.3-gigahertz S-band range showed noticeable smearing across frequencies when beaming as far as 3.9 million miles (6.26 million kilometers) away from the Sun. Further analysis of the Pioneer 6 signals, in particular, confirmed that this “spectral broadening” effect, as it’s termed in the paper, was even more pronounced during solar storms.
Data from the later Helios 1 and 2 probes—launched in 1974 and 1976, respectively, and bound for orbital missions around the Sun—helped to confirm that this smearing of radio signals across the frequency spectrum increased as the craft broadcast from closer to our Sun. The fact that both Helios probes were broadcasting these signals during a solar minimum (the lowest ebb in the cyclical flow of energy from the Sun) only strengthened the case for this space weather proximity effect.
Broadcasts from NASA’s Martian Viking probes, both launched in 1975, alongside a broad dataset of many other probe transmissions, revealed that this effect dissipates steeply between the Sun itself and a distance of about 1.3 million miles (2 million km) away. The effect then dissipates at a new, less steep rate before flattening out into a much, much more gradual loss of this solar weather effect starting at about 4.32 million miles (6.95 million km) from the Sun.
Ultimately, all this information on how the “spectral broadening” effect shifts based on a broadcast’s distance from the Sun—Earth’s nearby sample star—can now be used to better calibrate SETI’s expectations. In other words, it provides a rough guide for just how clear or how smeared any given extraterrestrial radio broadcast might look when broadcast from a given spot in another solar system.
To further refine its model, the SETI team segregated this data into broadcasts taken either closer to the solar maximum or the solar minimum of energy output. And they have begun the complex task of translating these results into estimates for other sizes and densities of stars—specifically red M dwarf stars, which are fainter, smaller, and cooler than the Sun at the center of our solar system.
“By quantifying how stellar activity can reshape narrowband signals, we can design searches that are better matched to what actually arrives at Earth,” SETI Institute researcher Grayce C. Brown, a co-author on the study, said in a statement, “not just what might be transmitted.”
