It has been over sixty years since the first Search for Extraterrestrial Intelligence (SETI) survey occurred. This was Project Ozma, a survey led by Dr. Frank Drake (who devised the Drake Equation) that used the National Radio Astronomy Observatory (NRAO) in Green Bank, West Virginia, to listen for radio transmissions from Epsilon Eridani and Tau Ceti. While the search revealed nothing of interest, it paved the way for decades of research, theory, and attempts to find evidence of technological activity (aka. “technosignatures”).
The search continues today, with researchers using next-generation instruments and analytical methods to find the “needle in the cosmic haystack.” This is the purpose behind Breakthrough Listen Investigation for Periodic Spectral Signals (BLIPSS), a collaborative SETI project led by Cornell graduate student Akshay Suresh to look for technosignatures at the center of the Milky Way. In a recent paper, Suresh and his team shared their initial findings, which were made possible thanks to data obtained by the Greenbank Observatory and a proprietary algorithm they developed.
Suresh is a Ph.D. candidate at the Cornell Center for Astrophysics and Planetary Science who leads the BLIPPS campaign, a collaboration between Cornell University, the SETI Institute, and Breakthrough Listen. He and his colleagues were joined by astrophysicists from the Cahill Center for Astronomy and Astrophysics, the Institute for Mathematics, Astrophysics, and Particle Physics (IMAPP), the Institute of Space Sciences and Astronomy, and the International Centre for Radio Astronomy Research (ICRAR). Their paper, “A 4–8 GHz Galactic Center Search for Periodic Technosignatures,” appeared on May 30th in The Astronomical Journal.
To date, all SETI surveys have been dedicated to looking for evidence of artificial radio transmissions. The accepted theory is that radio signatures would fall into one of two categories: narrowband intentional beacon emissions and broadband radiation leakage from radio transmitters. Of the two, the spectrotemporal characteristics (frequency over time) of radiation leakage are much harder to speculate about and likely to be weaker. For this reason, most modern SETI efforts have focused on looking for wideband searches for narrowband beacons from planetary systems or neighboring galaxies.
In particular, a rotating beacon near Galactic Center (GC) is considered a promising technosignature to SETI researchers. For an advanced species, such a beacon would provide a means for communicating with the entire galaxy without the need for direct contact. For species dying to know if they are alone in the Universe but not so eager as to advertise their location, a beacon is doubly attractive because it would also allow some anonymity to be maintained. As Suresh told Universe Today via email:
“From a game theory perspective, the core of the Milky Way is a likely “Schelling point” by which different alien worlds may establish communication without prior contact. For instance, intelligent aliens may choose to transmit beacons toward the center of the Milky Way to reach a maximum number of targets. Equivalently, such aliens may also transmit directly away from the center of the Milky Way, knowing that societies like ours will look towards the core of the galaxy.”
For their search, the team employed a fast folding algorithm (FFA), an open-source machine learning software designed to detect periodic events within time series data. They first tested this algorithm on known pulsars, successfully detecting the expected periodic emissions. They then consulted datasets obtained by the 100-meter Green Bank Telescope (GBT) – part of the Breakthrough Listen’s network – on a region at the center of the Milky Way during a 4.5-hour observing period. This region measures 50 light-years in diameter and encompasses over half a million stars.
Unlike pulsars, which emit signals across a broad range of radio frequencies, BLIPSS narrowed its search to look for regularly-spaced sequences of pulses (11 to 100 seconds apart) across a signal range of a few kilohertz – similar to radar communications. “Unfortunately, our searches did not reveal any pulsating signals that may be of extraterrestrial origin,” said Suresh. “Our results suggest that from the nearly 600,000 stars surveyed at the center of our galaxy, beacons with repetition rates between 11 to 100 seconds were either off or too faint to be detected during our observations.”
While their search did not turn up any clear indications of periodic radio signals, it accomplished several firsts. Before BLISS, radio SETI was primarily dedicated to looking for continuous signals, whereas a galactic beacon would rely on pulsed radio bursts. The search conducted by Suresh and his colleagues is the first-ever comprehensive, in-depth search for these signals. It is also the first time that FFA algorithms have been used to mine data for possible indications of pulsed signals.
In addition, the methodology employed by Suresh and his colleagues was novel in its combination of narrow bandwidths with periodic patterns that could be technosignatures. And by searching for timed sequences in a specific frequency range, this unprecedented survey has established constraints that future studies can build on. Said Suresh:
“Constraints from our study help fine-tune future surveys by excluding parameter spaces (e.g., pulse repetition rate, pulse bandwidth, and brightness) and regions of the sky from which we have a non-detection of pulsating extraterrestrial signals of artificial origin. We hope to expand our searches for pulsating signals from the Milky Way and beyond using multiple radio telescopes distributed across the globe. In doing so, we aspire to mine vast unexplored parameter spaces of pulsating signals.
Further Reading: SETI Institute, The Astronomical Journal