An encouraging new study has found that the interference from exoplanets—planets that orbit stars outside our solar system—has been overestimated in searches for extraterrestrial signals.
The results from the study, released last week in The Astronomical Journal, mean that scientists can concentrate on finer frequency shifts, markedly improving the potential effectiveness of campaigns to sniff out alien technosignatures, namely radio signals.
The laudable quest to discover signals from alien civilisations, a campaign known as SETI (Search for Extraterrestrial Intelligence), hinges on sifting through potential signals and differentiating them from “noise,” that is, natural signals. An improved understanding of the noise produced by exoplanets as they orbit their stars can significantly refine the search for alien signals.
Indeed, the ability to pinpoint genuine alien transmissions amidst the vast cacophony of space is pivotal for the success of these search campaigns. Hence the importance of the new paper, led by Megan Grace Li, a PhD student at UCLA. Li conducted this research as a National Science Foundation intern for the Research Experience for Undergraduates at the Breakthrough Listen project at the Berkeley SETI Research Center.
“This work gives deeper insight into what extraterrestrially transmitted signals may look like if they come from exoplanets, informing not only the parameter space of technosignature searches but also possible interpretations of detected signals,” said Li in a SETI Institute press release.
Scientists must consider how exoplanets move relative to Earth while searching for alien signals; this helps them to pinpoint potential sources and determine if detected signals are genuine or just from celestial movements. Now, when scientists try to capture signals from distant exoplanets, the Doppler effect comes into play. And yes, this is the same phenomenon that causes the pitch of an ambulance siren to change as it zooms past.
In the case of SETI astronomy, the Doppler effect results in a shift in the frequency of signals due to the relative movement between the transmitting exoplanet and Earth. This variation in frequency, termed the “drift rate,” is influenced by both the Earth’s and the exoplanet’s orbits and rotations. A lower drift rate indicates a more stable signal, which is essential for distinguishing potential alien transmissions from natural interferences.
Previously, based on the research of Sofia Sheikh from the SETI Institute, it was believed that, in the most extreme cases, exoplanetary systems exhibited drift rates of up to 200 nHz, prompting Sheikh to propose this value as a threshold (thresholds help scientists prioritise signals that are more likely to be stable and potentially of interest). However, the new research, which took data from over 5,300 known exoplanets from the NASA Exoplanet Archive, found a surprising revelation. In 99% of cases, the drift rate caused by these exoplanets was only 53 nHz. Furthermore, for stars without any known orbiting planets, the drift rate plummeted to a mere 0.44 nHz.
In essence, these findings suggest that the previous threshold of 200 nHz may have significantly underestimated the potential stability of signals originating from extraterrestrial civilisations, potentially making it easier for us to detect deliberate transmissions from aliens; the lower drift rate threshold allows SETI researchers to focus on more stable signals, which are easier and faster to analyse. And as Sheikh explained in the press release: “These results imply that, in many cases, the drift rate will be so low that we can prioritise other parameters (such as covering more frequencies or analysing datasets faster) without worrying that we will miss true signals.”
The newly established limits, which include the majority of drift rates generated by stable radio signals from exoplanets, are poised to significantly reduce the time and computing expenses required for future searches, including the one planned by Breakthrough Listen using the MeerKAT telescope. This means that these searches could become almost a thousand times faster and more cost-effective, according to the paper.
This is amazing news for future SETI campaigns. The added precision should allow for a more efficient use of resources, akin to searching a specific corner of a haystack for a needle rather than having to scour the entire stack.