The premier single-dish radio telescope in the world is the Robert C. Byrd Green Bank Telescope (GBT), which is a part of the Green Bank Observatory in West Virginia. The GBT offers unparalleled sensitivity in the millimeter to meter wavelengths very high to extremely high frequency (VHF to EHF) thanks to its 100-meter dish (328-foot), unobstructed aperture, and good surface accuracy.
It also became one of the primary tools employed by Breakthrough Listen and other organizations involved in the search for extraterrestrial intelligence (SETI) starting in 2017.
Recently, an international team of researchers from the SETI Institute, Breakthrough Listen, and multiple universities scanned twelve exoplanets for signs of technological activity (aka “technosignatures”). Their observations were timed to coincide with the planets passing in front of their sun relative to the observer (i.e., making a transit).
Although the scan found no conclusive proof of technosignatures, it did find two radio signals that are worth further investigation. The discipline of SETI could be greatly expanded by this new method, opening up a variety of new research prospects.
Sofia Z. Sheikh, a graduate student with the SETI Institute and Berkeley SETI Research Center (UC Berkeley), led the team and other members of a graduate SETI program run by the Penn State Extraterrestrial Intelligence Center (PSETIC).
Teams from Breakthrough Listen, the Center for Exoplanets and Habitable Worlds (CEHW), the International Center for Radio Astronomy Research (ICRAR), and multiple universities and research institutes, joined them. The paper that details their research is scheduled to be published in The Astronomical Journal.
The search for radio signals has been an established SETI convention since the first survey was conducted in 1961. This was Project Ozma, led by the late and legendary Cornell astrophysicist Frank Drake, for whom the Drake Equation is named.
Traditional radio SETI is expanding exponentially, with new sources of funding and new radio telescopes coming on-sky (for example, MeerKAT). In addition, many new scientists are getting involved in the field, including both students and experts in other sub-fields who are applying their skills to the techno signature challenge. It’s very exciting to be a part of SETI at such a dynamic point in time!
Sofia Z. Sheikh
With the development of next-generation radio telescopes and novel data processing methods (many of which incorporate machine learning), the field of SETI has significantly expanded in recent years. Technosignatures that researchers might be on the lookout for are also expanding, with suggestions ranging from gravitational waves and directed energy to neutrinos.
However, radio broadcasts continue to be the most sought-after technosignature, and radio surveys have significantly improved thanks to newly created technology and cutting-edge computational methods. As Sheikh told Universe Today via email:
“Traditional radio SETI is expanding exponentially, with new sources of funding and new radio telescopes coming on-sky (for example, MeerKAT),” she said. “In addition, many new scientists are getting involved in the field, including both students and experts in other sub-fields who are applying their skills to the techno signature challenge. It’s very exciting to be a part of SETI at such a dynamic point in time!”
Finding proof of man-made radio signals is still difficult, needing strong arrays, a lot of observation time, and a lot of dedication and persistence from research teams.
Additionally, SETI researchers are becoming increasingly concerned that the majority of the search space both in terms of actual physical space and potential categories of technological signatures remains undiscovered. According to Sheikh and her team, this creates the opportunity to mount new projects that could fill the unexplored regions of “parameter space”:
“The classic problem with SETI is the “needle in a haystack” issue even if someone is trying their hardest to get our attention, space is big, and there are so many forms that a message could take (even if you just restrict the possibility space, or parameter space, to the radio spectrum). So it helps if we try to figure out special places, times, or frequencies that might be more likely locations for messages than any random point.”
These are referred to as “Schelling points,” a term from game theory where two or more persons naturally and without conversation arrive at the same solution. Examples include places like the Galactic Center, where some SETI researchers believe civilizations are most likely to be found, or frequencies like 1420 MHz.
Also known as the “hydrogen” or “21-centimeter line,” this frequency corresponds to the change in the energy state of neutral hydrogen. Radio waves at this frequency are considered favorable by SETI researchers since they can penetrate large clouds of dust in the interstellar medium (ISM).
Sheikh and her colleagues used information on 12 exoplanets discovered by the Kepler Space Telescope in their investigation. These planets were detected using Transit Photometry (aka. Transit Photometry), where periodic dips in a star’s luminosity are used to confirm the presence of exoplanets and constrain their size and orbital periods.
The GBT gathered data on these Kepler exoplanets as they made transits of their respective stars on March 25th, 2018. The objective was to determine whether radio transmissions a definite indicator of an advanced civilization trying to communicate coincided with these transits.
“For this particular project, we used the centers of planetary transits as Schelling points. In other words, we timed our observations such that the exoplanet-of-interest was lined up with its host star and the solar system. That’s a time that we know (by observing the dip in brightness as the planet passes in front of its star), and it’s a time that any potential life on the exoplanet would also know therefore, it is ‘mutually derivable,’” said Sheikh.
While sending a continual message consumes a lot of energy, this technique limits the transmission to a particular mutually-derivable timeframe. As a result, the price of sending messages into space is drastically reduced while the likelihood of message detection is greatly increased.
Sheikh and her colleagues are the first to use this technique in a radio technosignature search. Although they did not find any technosignatures, their groundbreaking study devised a method that would make future iterations of similar surveys much simpler to carry out.
“This was a pilot study for the transits-as-Schelling-points idea, and future research will expand the sample to include significantly more exoplanets,” she concluded. “This is especially relevant with upcoming commensal projects like COSMIC on the ngVLA, which will be surveying the sky almost constantly with so much new data on the way, knowing exactly when and where to look will help us prioritize potentially real signals over the growing background of radio frequency interference from Earth.”
