"Breakthrough Listening": Mysterious signals from Proxima Centauri are most likely not alien civilizations

Scientists' in-depth analysis of an interesting candidate signal discovered by the Breakthrough Listen project last year showed that it was unlikely to come from Proxima Centauri. The Breakthrough Initiatives team announced on Friday that it appears to be the product of human technology interfering with Earth. Two research papers published in Nature-Astronomy discuss the detection of candidate signals and an advanced data analysis process that finely distinguishes "false positives."

Yuri Milner, founder of the Breakthrough Project, said: "The significance of this result is that the search for civilizations beyond our planet is now a mature, rigorous field of experimental science."

Breakthrough Listening (one of the Project Breakthroughs) is an astronomical science project looking for technical signals — signs of technology that may have been developed by an alien intelligent civilization. The Breakthrough Listening scientific team, led by Dr. Andrew Siemion of the University of California, Berkeley, uses some of the world's largest radio telescopes, equipped with the most powerful digital processing systems, to capture data across the radio spectrum in the direction of a wide range of celestial targets. The search is challenging because the planet is flooded with radio signals from human technology — radar, satellites, TV transmitters, and so on. Searching for a faint signal from a distant star is like finding a needle in a haystack — and it's a "needle" that changes over time.

The Parkes Telescope (one of the largest telescopes in the Southern Hemisphere) of the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in New South Wales, Australia, is one of the facilities involved in the Breakthrough Listening search. One of the targets being monitored by the Parkes telescope is Proxima Centauri, which is the closest star to the solar system and is just over 4 light-years away. The star is a red dwarf star orbited by two known exoplanets. The Breakthrough Listening team scanned the target in the frequency range of 700MHz to 4GHz at a resolution of 3.81Hz — in other words, performing the equivalent of tuning to more than 800 million radio channels at once, with superb detection sensitivity.

Shane Smith, an undergraduate researcher who works with Breakthrough Listening project scientist Dr. Danny Price, in the Summer 2020 Breakthrough Listening internship program, runs these observations through a search "pipeline" of Breakthrough Listening. He detected more than 4 million "hits" — frequency ranges that show signs of radio emission. This is actually quite typical for the observations of the Breakthrough Listening project.

Like all observations of Breakthrough Listening, the "pipeline" filters out signals that appear unlikely to come from transmitters far from Earth based on two main criteria.

First, does the frequency of the signal change steadily over time? A transmitter on a distant planet is expected to be in motion relative to the telescope, resulting in a Doppler Shift, similar to the change in pitch of an ambulance siren as it moves relative to an observer. Rejecting "hits" that show no signs of such movement reduced the number of "hits" for this particular dataset from 4 million to around 1 million.

Second, for the remaining "hit" rates, do they appear to come from the direction of the target? To determine this, the telescope points in the direction of Proxima Centauri and then into the distance, repeating this "on-off" pattern several times. Local sources of interference are expected to affect "on" and "off" observations, and a candidate technical feature should only appear in "on" observations.

Even after applying both data filters, there are still a few candidate signals that must be visually inspected. Sometimes, a faint signal is actually visible in OFF observations, but not strong enough to be discovered by automated algorithms. Sometimes, similar signals appear in neighboring observations, indicating that the source of interference may be turned on and off at the wrong time period, or that the team can track satellite signals that are usually broadcast in certain frequency bands.

Occasionally an intriguing signal remains and further examination is necessary. Smith found such a signal of interest in his observations using the Parkes telescope against neighbors. A narrow-band, Doppler-shifted signal, which lasted 5 hours of observation, appeared only in the "on" observations of the target star, not in the interspersed "off" observations, and it had some of the characteristics that a candidate for technical features should have.

Dr. Sofia Sheikh, currently a postdoctoral fellow on the UC Berkeley Listen team, delved into larger datasets of observations conducted at other times. She found about 60 signals that had many of the characteristics of the candidates, but were also seen in their respective OFF observations.

Sheikh said: "Therefore, we can confidently say that these other signals are local to the telescope and are artificially generated." These signals appear in regular frequency intervals in the data, which seem to correspond to multiples of the frequencies used by oscillators commonly used in various electronic devices. Taken together, the evidence suggests that the signal is interference from human technology, although we cannot determine its specific source. The original signal shane Smith found wasn't apparently detected when the telescope was far away from Proxima Centauri — but given the millions of signals, the most likely explanation is still that it came from a transmission of human technology that happened to 'strangely' fool our filter in the right way. ”

Dr. S. Pete Worden, executive director of the Breakthrough Program, said: "While we cannot come up with a real technical signal, we are increasingly convinced that if such a signal exists, we have the necessary tools to detect and verify it."

The Breakthrough Listen project team is making all the data from Parkes scans available to the public for them to examine for themselves. The team has also just published two papers outlining the details of data acquisition and analysis, as well as a study note describing follow-up observations of Centaurus with the Parkes telescope in April 2021. Breakthrough Listening will continue to monitor Proxima Centauri, and it remains a compelling target for technical feature searches, using a suite of telescopes around the world. And, the team will continue to refine the algorithm, including as part of a recent crowdsourced data processing competition completed in collaboration with kaggle.com.