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“Exotica” Catalog As the Centerpiece of Our Efforts to Expand the Diversity of Targets Surveyed in the Search for Extraterrestrial Intelligence (SETI)

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“Exotica” Catalog As the Centerpiece of Our Efforts to Expand the Diversity of Targets Surveyed in the Search for Extraterrestrial Intelligence (SETI) Draft version June 19, 2020 Typeset using LATEX twocolumn style in AASTeX63 One of Everything: The Breakthrough Listen Exotica Catalog Brian C. Lacki,1 Bryan Brzycki,2 Steve Croft,2 Daniel Czech,2 David DeBoer,2 Julia DeMarines,2 Vishal Gajjar,2 Howard Isaacson,2, 3 Matt Lebofsky,2 David H. E. MacMahon,4 Danny C. Price,2, 5 Sofia Z. Sheikh,2 Andrew P. V. Siemion,2, 6, 7, 8 Jamie Drew,9 and S. Pete Worden9 1Breakthrough Listen, Department of Astronomy, University of California Berkeley, Berkeley CA 94720 2Department of Astronomy, University of California Berkeley, Berkeley CA 94720 3University of Southern Queensland, Toowoomba, QLD 4350, Australia 4Radio Astronomy Laboratory, University of California, Berkeley, CA 94720, USA 5Centre for Astrophysics & Supercomputing, Swinburne University of Technology, Hawthorn, VIC 3122, Australia 6SETI Institute, Mountain View, California 7University of Manchester, Department of Physics and Astronomy 8University of Malta, Institute of Space Sciences and Astronomy 9The Breakthrough Initiatives, NASA Research Park, Bld. 18, Moffett Field, CA, 94035, USA ABSTRACT We present Breakthrough Listen's \Exotica" Catalog as the centerpiece of our efforts to expand the diversity of targets surveyed in the Search for Extraterrestrial Intelligence (SETI). As motivation, we introduce the concept of survey breadth, the diversity of objects observed during a program. Several reasons for pursuing a broad program are given, including increasing the chance of a positive result in SETI, commensal astrophysics, and characterizing systematics. The Exotica Catalog is an 865 entry collection of 737 distinct targets intended to include \one of everything" in astronomy. It contains four samples: the Prototype sample, with an archetype of every known major type of non-transient celestial object; the Superlative sample of objects with the most extreme properties; the Anomaly sample of enigmatic targets that are in some way unexplained; and the Control sample with sources not expected to produce positive results. As far as we are aware, this is the first object list in recent times with the purpose of spanning the breadth of astrophysics. We share it with the community in hopes that it can guide treasury surveys and as a general reference work. Accompanying the catalog is extensive discussion of classification of objects and a new classification system for anomalies. We discuss how we intend to proceed with observations in the catalog, contrast it with our extant Exotica efforts, and suggest similar tactics may be applied to other programs. Keywords: Search for extraterrestrial intelligence | Classification systems | Celestial objects catalogs | Philosophy of astronomy | Astrobiology 1. INTRODUCTION (Worden et al. 2017). It joins other programs in the Breakthrough Listen is a ten year program to con- Search for Extraterrestrial Intelligence (SETI), most of duct the deepest surveys for extraterrestrial intelligence which have also focused on nearby stars (Tarter 2001). (ETI) in the radio and optical domains (Worden et al. But where should we look for ETIs? Indeed, how should 2017). The core of the program is a deep search for ar- we look for new phenomena of any kind? tificial radio emission from over a thousand nearby stars Serendipity is a key ingredient in the discovery of most and galaxies (Isaacson et al. 2017, hereafter I17; see also new types of phenomena and extraordinary new ob- Enriquez et al. 2017; Price et al. 2020 for results), and jects (Harwit 1981; Dick 2013; Wilkinson 2016). From commensal studies of a million more stars in the Galaxy Corresponding author: Brian C. Lacki [email protected] 2 Lacki et al. Ceres1 to pulsars, from the cosmic microwave back- lence (Brin 1983). If at least some ETIs are willing and ground (CMB) to gamma-ray bursts (GRBs), the ma- capable of expanding across interstellar space, a bolder jority of unknown phenomena have been found by ob- interpretation of the null results is popularly referred servers that were not explicitly looking for them.2 His- to as the Fermi Paradox, the unexpected lack of any torically, theory has rarely driven these findings.3 In- obvious technosignatures in the Solar System (Cirkovic stead, they frequently come about by new regions of 2009).4 Although the simplest resolution may be that parameter space being opened by new instruments and we are alone in the local Universe (Hart 1975; Wesson telescopes (Harwit 1981). 1990), and others question whether we should expect to Other discoveries { like the moons of Mars or Cepheid have detected technosignatures yet (Tarter 2001; Wright variables in external galaxies { were delayed because no et al. 2018), many have suggested that ETIs are actu- thorough observations were carried out on the targets ally abundant but we are simply looking in the wrong (Hall 1878; Dick 2013). The pattern persists to this places for them (e.g., Corbet 1997; Cirkovi´c&´ Brad- day. Because ultracompact dwarf galaxies have char- bury 2006; Davies 2010; Di Stefano & Ray 2016; Benford acteristics that fall in the cracks between other galaxies 2019; Gertz 2019). It is very difficult to detect a society and globular clusters, they were only recognized recently of similar power and technology as our own through the despite being easily visible on images for decades (San- traditional methods of narrowband radio searches unless doval et al. 2015). Of relevance to SETI, hot Jupiters it makes intentional broadcasts (Forgan & Nichol 2011). were speculated about in the 1950s (Struve 1952), but But like hot Jupiters, might there be easy discoveries in they were not discovered until 1995 in part because no SETI that we keep missing because we keep looking in one systematically looked for them (for further context, the wrong ways or at the wrong places? see Mayor & Queloz 2012; Walker 2012; Cenadelli & Considerations like these in astrophysics have inspired Bernagozzi 2015). This may have delayed by years the efforts to accelerate serendipity, by expanding the region understanding that exoplanets are not extremely rare, of parameter space explored by instruments (c.f., Har- one of the factors in the widely-used Drake Equation wit 1984; Djorgovski et al. 2001; Cordes 2006; Djorgov- in SETI relating the number of ETIs to evolutionary ski et al. 2013). This approach has been highlighted in probabilities and their lifespan (Drake 1962). SETI to gauge the progress of the search (Wright et al. Despite searches spanning several decades, no com- 2018; Davenport 2019; see also Sheikh 2019).5 Break- pelling evidence for ETIs has been found by the SETI through Listen harnesses expanding capabilities in sev- community to date (e.g., Horowitz & Sagan 1993; Grif- eral dimensions. In radio, Breakthrough Listen has de- fith et al. 2015; Pinchuk et al. 2019; Lipman et al. 2019; veloped a unique backend, already implemented on the Price et al. 2020; Sheikh et al. 2020). The continuing Green Bank Telescope (MacMahon et al. 2018) and the lack of a discovery among SETI efforts looking for var- CSIRO Parkes telescope (Price et al. 2018), and more ious technosignatures is sometimes called the Great Si- are being installed on MeerKAT (an array described in Jonas 2009). These allow for an unprecedented fre- quency coverage at high spectral and temporal resolu- 1 A planet between Mars and Jupiter was \predicted" by the Titius-Bode Law. Interestingly, in September 1800, a group tion. In optical, Breakthrough Listen continues to use of astronomers colloquially known as the \Cosmic Police" chose the Automated Planet Finder (APF; Vogt et al. 2014) twenty-four astronomers to search for this planet. Giuseppe Pi- for high spectral resolution observations of stars in hopes azzi was among the twenty-four selected, but did not know this of spotting laser emission (e.g., Lipman et al. 2019), and when he discovered Ceres serendipitously during the construction of a star catalog in January 1801 (Cunningham et al. 2011). we have partnered with the VERITAS gamma-ray tele- 2 For discussion of the discovery of Ceres, Cunningham et al. 2011; the discovery of pulsars, reported in Hewish et al. 1968, is re- counted in Bell Burnell 1977; the CMB is reported as unexpected 4 The accuracy of the name \Fermi Paradox" is disputed by Gray noise in Penzias & Wilson 1965; Klebesadel et al. 1973 presents (2015); Cirkovic(2018) on the other hand applies the term to all the discovery of GRBs by the Vela satellites, designed to watch of the Great Silence. for for nuclear weapon tests in violation of treaty. 5 We should be careful not to equate parameter space volume 3 Among the rare exceptions are the discovery of radio emission to survey value or the probability of discovery, however. The from interstellar HI (Ewen & Purcell 1951) and molecules (Wein- volume depends on parameterization (for example, vastly dif- reb et al. 1963), small Kuiper belt objects (Jewitt & Luu 1993), ferent volumes are found when substituting wavelength for fre- and binary black hole mergers (Abbott et al. 2016). The CMB quency). The more general notion of measure on parameter space was almost found by a dedicated experiment (Dicke et al. 1965), is more appropriate; these include Bayesian probability distribu- but Penzias & Wilson(1965) discovered it instead before the re- tions (c.f., Lacki 2016a). A suitable measure avoids the apparent −20 sults came in. The discovery of Neptune { not a new type of problem that current SETI efforts are worth ∼ 10 the value object but certainly significant { was driven by theoretical calcu- of an ETI discovery noted by Wright et al. 2018; current and past lations of its perturbations on Uranus (Galle 1846; Airy 1846). SETI efforts do have significant value. Breakthrough Listen Exotica Catalog 3 scope (Very Energetic Radiation Imaging Telescope Ar- 2.1. Breadth, depth, and count ray System; Weekes et al. 2002) for its sensitivity to Each astronomical survey on a given instrument extremely short optical pulses (Abeysekara et al.
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