2. SETI in 2020
2020 was the first year of the pandemic, the lost year, the year of COVID. Even as I write this in 2021, many facilities are closed, and much of the world does not have access to the vaccines that have allowed many nations to begin re-opening and returning to normal ways of research. It is likely this review would have more papers to cite without this global tragedy.
Many opportunities for SETI practitioners to meet and interact were lost. For instance, the First Penn State SETI symposium, intended to be a major, annual, international meeting in the field, was postponed to 2021 (and then later to 2022). That symposium was also intended to inaugurate the Penn State Extraterrestrial Intelligence Center, dedicated to research, training, curriculum, and fundraising for the field.
But, being forced to interact remotely also made the world a little smaller. The Technoclimes conference in August 2020 explored non-radio technosignatures and produced many new collaborations. Two talk series on technosignatures, the NASA Goddard Technosignatures Seminar (led by Ravi Kopparapu) and the PSETI Seminar, joined the IAA community meeting to create a trio of worldwide "SETI colloquia" that anyone could attend.
There was also movement on the funding front. The US Congress continued to indicate that technosignatures were properly part of NASA's portfolio (although no explicit funding was allocated), the Technoclimes workshop was supported by a NASA Exobiology grant, and two NASA Exoplanet Research Program grants were awarded to researchers studying technosignatures (Jean-Luc Margot at UCLA and Ann Marie Cody at the SETI Institute). Finally, the Characterizing Atmospheric Technosignatures (CATS) effort led by Adam Frank at the University of Rochester was announced, and its members joined the NExSS research coordination network at NASA.
2020 is also a notable landmark for SETI because it marks the target for which the "SETI 2020" roadmap was written. We have probably made less progress than that work aspired to since then, but that work nonetheless proved influential in directing radio SETI efforts, for instance in the development of the Allen Telescope Array, but also for other efforts such as the Square Kilometer Array.
2020 had more than its share of SETI-specific tragedy: in February we lost Freeman Dyson, one of the pioneers of the field, at the age of 96; and in December the instrument platform suspended above the 300-meter dish of the Arecibo radio telescope collapsed, destroying not only one of the most powerful and storied radio telescopes in the world, but an icon of the field.
3. Results from searches (7 papers)
The heart of SETI is the actual searches, and 2020 brought several new important results and upper limits.
The recent resurgence of the field is due in no small part to the Breakthrough Listen Initiative, which has injected funding, training, community, and energy into SETI. The team and its collaborators published four new upper limits in the radio, making use of the extremely high bandwidth backends at Green Bank and Parkes. The first was by Price et al., reporting null detections for over 1300 stars over the entire sky in L and S bands (1–3.5 GHz). Following the methodology suggested by Heller and Pudritz, Sheikh et al. reported null results in C band (4–8 GHz) for stars in the ''restricted Earth Transit Zone'', i.e. the region of the sky that ''sees'' Earth transit the Sun. Wlodarczyk-Sroka et al. extended the upper limit calculations of previous searches to include all of the sources in the beam (not just the target star at the center). And finally, Perez et al. reported null results for the Kepler-160 system.
Overall, this represents both a substantial increase in the ''Cosmic Haystack'' fraction searched, and the deployment of important new search strategies in the radio.
The Allen Telescope Array (ATA) continued its survey of the skies, publishing in 2020 a deep survey and null result of the Wow! Signal field. This work, by Harp et al., scanned the entire field consistent with the original signal, and included a 10 MHz bandwidth and ∼100 h of observation, the longest follow-up yet performed. It is good to see the field followed up in this manner to search for potential repetitions.
Tremblay and Tingay reported on the deepest narrowband SETI search yet with the Murchison Widefield Array (MWA), this time of the Vela region. These low-frequency searches explore very large regions of parameter space in the Wright et al. Cosmic Haystack formalism, thanks to their sensitivity and very large fields of view.
Perhaps the most offbeat and unconventional search of 2020 was that of Hippke, who followed a suggestion by Hsu and Zee that there could be a message embedded in the cosmic microwave background, presumably put there intentionally at the observable universe's beginning. Not finding anything meaningful in the power spectrum, Hippke helpfully provides a 1000-bit representation of the CMB temperature spectrum for others to scrutinize for any messages that were not apparent to him.
4. Search methods and instrumentation (16 papers)
4.1. New instrumentation
One of the most exciting projects on the horizon is PanoSETI, an all-sky optical/NIR fast transient project designed to search for pulsed laser emission. 2020 saw significant progress on the PanoSETI prototype at Palomar Observatory, as described in a trio of SPIE papers by Brown et al. (describing the overall mechanical system design) Liu et al. (describing the focal plane electronics and protocols), and Maire et al. (showing on-sky results and instrument design).
In addition to being a SETI program, this project promises to open up new avenues for research in cosmic rays, as well as the potential for serendipitous discoveries in almost completely unexplored regions of parameter space for fast optical and NIR transients.
Another exciting new SETI instrument is the Five-hundred-meter Aperture Spherical radio Telescope (FAST) and its SERENDIP VI multi-beam spectrometer, which was declared fully operational in January. Li et al. described FAST's capabilities and its partnership with Breakthrough Listen, and Zhang et al. described the first observations and RFI mitigation strategies they employ. FAST is shaping up to be a worthy successor to Arecibo.
4.2. Anomaly detection
Following up on suggestions by Albrecht and Balazs, Djorgovski, and others to search for highly unusual events in large sky surveys, many SETI practitioners have begun work on anomaly-detection or outlier analysis as a way to identify unexpected, potentially technological phenomena. Singam et al. provided an overview of searches for such ''non-canonical astrophysical phenomena'' and made suggestions for them.
Villarroel et al. described the Vanishing and Appearing Sources during a Century of Observations project (VASCO), which leverages photographic and digital sky surveys across many decades to search for star-like sources that seem to disappear or appear. The survey has many components, including hypothesis-driven searches (for instance for failed supernovae) and serendipitous searches driven by machine learning. Villarroel et al. describe the citizen science component of the project.
De la Torre explored the challenges of serendipitous detection with neural networks, and the role of human cognition and bias in such searches. He compares the results of a neural network to those of human volunteers identifying features in imagery, and uses the bright spots on Ceres as an example.
Brzycki et al. described using convolutional neural networks to identify and characterize narrow band signals in noisy data with an eye towards radio SETI. Lesnikowski et al. described an unsupervised neural network to analyze imagery of the moon to look for artifacts and other anomalies, and shows they can recover the obviously artificial Apollo 15 landing site with it.
Finally, Giles and Walkowicz discussed an outlier scoring technique for lightcurve data as a way to search for anomalies and new phenomena, and produce such scores for millions of light curves from the Kepler mission. Such work is useful for finding anomalies such as Boyajian's star, which have proven interesting both for SETI and natural astrophysics purposes.
4.3. Other methodology
Michaud et al. reviewed the case for preserving the lunar farside for radio astronomy and radio SETI, to take advantage of what is (today) a region free from terrestrial radio frequency interference.
Sandora and Silk outlined a Bayesian framework for designing and interpreting results from searches for biosignatures and technosignatures. Haqq-Misra et al. argued that the relative abundance of technosignatures versus biosignatures detected in future astronomical surveys with space telescopes will help determine whether the ''Great Filter'' (which they define as the ''hardest step in planetary evolution'') is in our past or future.
Finally, Seto and Kashiyama presented an analysis of stellar motions and their uncertainties from Gaia for use in targeting METI signals, which might be useful for SETI as well.
5. Target and frequency selection (8 papers)
The Cosmic Haystack is large, and so narrowing down the best targets and frequencies to check is essential.
This problem of finding someone also looking for you is a game introduced to game theory by Thomas Schelling, who actually used Cocconi and Morrison as an example of his point. In [36] I described and formalized this connection of target and frequency selection as a hunt for ''Schelling points'' and applied the idea to the suggestion of Planck that his eponymous units would be truly universal, known even to alien species. The resulting lists of frequencies to check (across the electromagnetic spectrum) are arguably still too anthropocentric and arbitrary, but are nonetheless small enough in number to add to the lists of ''magic frequencies'' to search at.
Another Schelling point is when to look. Gray argued that an efficient transmission strategy is to broadcast continuously from a fixed point and direction on a planetary surface. The result would be that signals would repeat every planetary rotation. Gray's application here is to the Wow! Signal (and indeed this motivated Harp et al. above) but more generally is that similar ''one-off'' signals should be prioritized for re-observation.
We also need to know where to look. Caballero attempted to identify the most likely stars in the Wow! Signal region of the sky to host life. Kaltenegger and Pepper used the Gaia DR2 catalog to extend the work of Heller and Pudritz to identify new Earth Transit Zone stars.
Arguments for searches for artifacts within the solar system are as old as modern SETI, but the idea seems to have new resonance these days. Gertz made one such argument, and includes some suggestions for how to search.
Developing ideas by Wolfe, Turnbull and Tarter, and others, Gertz made a good argument that white dwarfs, evolved stars, and similar objects, while presumably poor targets for searches for biosignatures, are actually good targets for technosignature searches because they have given rise to a technological species that has outlived its host planet.
Ćirković and Vukotić argued that M dwarfs are poor targets for radio SETI, because their frequent flaring would interfere with the development of radio technology on their planets (although I wonder whether the presence of a powerful and important radio source like a flaring star nearby would not increase the likelihood of the development of such technology).
Finally, Lacki et al. produced a remarkable catalog of ''one of everything'', a list of an example of every major kind of astronomical object and superlative. Interesting in its own right beyond SETI, this ''exotica catalog'' for Breakthrough Listen also provides a target list for investigation into whether any of these presumably natural kinds of objects is actually an example of or influenced by technology on large scales.
6. Development of technosignatures (15 papers)
6.1. The axes of merit
Many technosignatures across a range of scales and detection strategies have been proposed, and it remains unclear which are the most promising.
The literature has long seen papers arguing for various criteria of merit when weighing technosignatures. Indeed, within two years of Cocconi and Morrison's foundational article arguing for radio communication as an ideal technosignature, Dyson, Bracewell, and Schwartz and Townes all argued for others on various grounds. For a good example, see the discussion of Shostak, who made the case for artifacts over radio transmissions.
At the NASA Technosignatures Workshop in 2018, Sofia Sheikh compiled many of these criteria into a single diagram she called the ''axes of merit'' for technosignature search. This helpfully compiled various arguments about detectability, contrivance, longevity, and so on into a single, qualitative, somewhat subjective framework.