The search for extraterrestrial intelligence (SETI) aims to detect engineered signals, or technosignatures, indicating technologically advanced life beyond Earth. The discovery of Proxima Centauri b (Prox Cen b), an exoplanet orbiting Proxima Centauri, has heightened interest in this nearby star system as a potential location for life. While Prox Cen b's proximity to its star presents challenges to habitability due to factors such as stellar flares and ionizing radiation, the possibility of liquid water and its overall Earth-like characteristics remain intriguing. The Breakthrough Listen (BL) project, a 10-year program to search for technosignatures at radio and optical wavelengths, has included Proxima Centauri in its survey of nearby stars. Previous SETI programs largely overlooked Proxima Centauri due to its faintness as an M-dwarf star. This study utilizes data from the Parkes radio telescope to conduct a sensitive search for radio technosignatures emanating from the Proxima Centauri system, addressing the relative lack of previous observations targeting this potentially habitable exoplanet.

Several studies have investigated the habitability of Prox Cen b, considering factors like its atmospheric water cycle and the potential effects of stellar flares and coronal mass ejections. These studies presented conflicting conclusions on the likelihood of life on Prox Cen b, with some suggesting that M-dwarf stars are viable hosts for life while others point to challenges posed by the harsh stellar environment. Previous SETI programs, such as Project Phoenix, focused primarily on solar-like stars and thus didn't extensively survey Proxima Centauri. A recent search for optical technosignatures using archival data from the High Accuracy Radial Velocity Planet Searcher spectrometer also yielded no results. This lack of previous targeted searches for technosignatures from Proxima Centauri underscores the novelty and importance of this current investigation.

This research employed the CSIRO Parkes radio telescope (Murriyang) as part of the Breakthrough Listen initiative. The Parkes Ultra-Wideband Low receiver (UWL), with its 3.3 GHz bandwidth (704 to 4,032 MHz) and low system noise temperature (22 K), was used to observe Proxima Centauri for a total of 26 hours between April 29 and May 4, 2019. The observations consisted of alternating on-source and off-source pointings to facilitate the rejection of radio frequency interference (RFI). Observation lengths were approximately 30 minutes on-source and 5 minutes off-source, differing from previous BL observations. This longer on-source time maximized the chances of detecting signals that last for extended periods, while the shorter off-source intervals minimize the time wasted on calibrations. Furthermore, the researchers used a longer cadence (12-17 pointings) than typical for BL searches to enhance their ability to identify and characterize RFI. The data were processed using the BLDRP pipeline, generating filterbank files for analysis. The turboSETI Doppler search code was applied to search for narrowband signals with a Doppler drift, considering the expected drift rate based on the rotation and orbital motion of the Earth and Proxima Centauri. Candidate events were identified as narrowband signals appearing in on-source but not off-source pointings, subject to filtering processes involving visual inspection, consideration of frequency ranges of known transmitters and comparison with other similar events. The analysis pipeline also employed several stages of filtering to reduce false positives.

The search yielded 4,172,702 total hits above the signal-to-noise ratio (SNR) threshold. Out of 5,160 events initially identified, only one, designated BLC1 (Breakthrough Listen Candidate 1), passed all filtering and visual inspection steps. BLC1 was detected at 982.002571 MHz, with a drift rate of −0.238 Hz/s, and persisted for over two hours. BLC1's frequency falls within a band reserved for aeronautical radio navigation; however, no registered transmitters operating at that frequency were located within 1,000 km of the observatory. The signal's narrow bandwidth, Doppler drift, and apparent lack of association with known terrestrial sources led to its initial characterization as a potential technosignature. Detailed analysis presented in a companion paper ultimately attributed BLC1 to an unusual, but ultimately terrestrial, source of interference. Despite this, the study highlights the high sensitivity achieved in the search, with a calculated minimum detectable effective isotropic radiated power (EIRP) of 1.9 GW, which is lower than previously reported values for similar observations due to factors such as the lower system noise temperature of the UWL receiver and longer observation times employed in this study. The improved sensitivity underscores the effectiveness of the methods used in detecting even weak signals over extended periods.

The detection and subsequent identification of BLC1 as terrestrial interference underscores the challenges and complexities of searching for technosignatures. Although BLC1 proved to be not of extraterrestrial origin, its characteristics initially suggested potential extraterrestrial technology. The improved sensitivity of this search, compared to previous studies, illustrates the potential of current technologies to detect weaker and more subtle signals. The results of this study highlight the importance of careful consideration of potential interference sources and thorough analysis in SETI research, as even unusual local interference can mimic technosignatures. The fact that a single observation of a potentially habitable exoplanet, Proxima Centauri b, with current equipment could yield a possible candidate emphasizes the potential for future progress in the search for extraterrestrial life. This emphasizes the ongoing need for improved instruments, techniques and expanded observations to rigorously search a broader range of frequencies and wavelengths.

This research conducted a sensitive search for radio technosignatures towards Proxima Centauri using the Parkes UWL receiver. While the initial signal of interest, BLC1, was ultimately identified as terrestrial interference, this study showcases the high sensitivity achieved. The minimum detectable EIRP was improved compared to previous observations. The research highlights both the potential and the challenges of searching for technosignatures, emphasizing the ongoing need for further observational efforts using improved instruments and methodologies across diverse spectral ranges. Future studies should continue to explore Proxima Centauri and other potentially habitable exoplanet systems.

The study focused solely on a radio frequency search, neglecting other wavelengths that could potentially harbor technosignatures. The observation period covered only a limited timeframe. The ultimate attribution of BLC1 to terrestrial interference emphasizes the difficulty in distinguishing between technosignatures and terrestrial interference. Furthermore, the analysis relies on the availability of registered transmitters near the observatory.