Harmonizing with yesterday’s post about a NASA grant to study technosignatures is word from Breakthrough Listen, which has released a catalog of what it calls ‘exotica’ or, to cite the accompanying paper: “an 865 entry collection of 737 distinct targets intended to include “one of everything” in astronomy.” The idea is to produce a general reference work that can guide astronomical surveys and, in the case of Breakthrough, widen the search for technosignatures.

Brian Lacki (UC-Berkeley), who is lead author of the new catalog, notes that it’s not meant to be restricted to SETI, though its uses there may prove interesting. Here are the four categories of exotica the catalog defines:

  • ‘Prototypes.’ Here the intent is to list one example, perhaps more, an archetype of every known type of non-transient object in the sky. According to the paper, “We emphasize the inclusion of many types of energetic and extreme objects like neutron stars…, but many quiescent examples are included too.”
  • ‘Superlatives.’ These are objects with the most extreme properties (“among the most extreme in at least one major physical property, the record-breakers”), including unusually metallic stars, or the fastest known pulsar, the stars with the highest metal content and those with the lowest, etc. Here the list “includes objects of known subtypes but that are on the tail ends of the distribution of some properties, to better span the range of objects in the Universe.”
  • ‘Anomalies.’ The enigmas go here, including objects like KIC 8462852 (Boyajian’s Star), whose odd lightcurve is still under examination and a long way from being explained, and ‘Oumuamua, the interstellar visitor that entered our system in 2017 and is now leaving it. We can also include phenomena that have triggered searches at both radio and optical frequencies — here I think of Fast Radio Bursts (FRBs), but stars with excess infrared radiation would also be on the list.
  • A fourth category is a control sample of “sources not expected to produce positive results.”

So what to make of this? It’s apparently introduced as an attempt to jog our preconceptions, at least according to what Lacki says:

“Many discoveries in astronomy were not planned. Sometimes a major new discovery was missed when nobody was looking in the right place, because they believed nothing could be found there. This happened with exoplanets, which might have been detected before the 1990s if astronomers looked for solar systems very different than ours. Are we looking in the wrong places for technosignatures? The Exotica catalog will help us answer that question.”

Lacki’s point is well taken with regard to exoplanets. We quickly learned at the beginning of our exoplanet detections that stellar systems come in a huge variety of configurations, so that our own Solar System can hardly be considered a common template. Every new system studied now seems to drive this point home. As the paper notes, everything from the cosmic microwave background to gamma-ray bursts has been found by scientists who were not explicitly looking for what they discovered, usually because new instruments and telescopes widen our capabilities.

From the paper:

Other discoveries – like the moons of Mars or Cepheid variables in external galaxies – were delayed because no thorough observations were carried out on the targets (Hall 1878; Dick 2013). The pattern persists to this day. Because ultracompact dwarf galaxies have characteristics that fall in the cracks between other galaxies and globular clusters, they were only recognized recently despite being easily visible on images for decades (Sandoval et al. 2015). Of relevance to SETI, hot Jupiters were speculated about in the 1950s (Struve 1952), but they were not discovered until 1995 in part because no one systematically looked for them (for further context, see Mayor & Queloz 2012; Walker 2012; Cenadelli & Bernagozzi 2015). This may have delayed by years the understanding that exoplanets are not extremely rare, one of the factors in the widely-used Drake Equation in SETI relating the number of ETIs to evolutionary probabilities and their lifespan (Drake 1962).

Are there SETI discoveries that could be made if we widen the range of targets? Breakthrough Listen has already upped the pace of both radio and optical SETI, being a 10-year program whose core is a search for artificial radio emission from over 1,000 nearby stars, although a million more stars in the Milky Way are targeted for related study. The organization is clearly not averse to trying new approaches, hence its interest in technosignatures of the kind once suggested by Freeman Dyson, and its intention to expand the parameter space.

Image: This is Figure 1 from the paper. Caption: A cartoon of the three directions of target selection and the relative advantages of Breakthrough Listen’s primary programs observing stars and galaxies (green), a survey of the Breakthrough Listen Exotica Catalog (blue), and some example campaigns. Previous SETI surveys have generally aimed for depth, achieving strong limits for a small number of similar targets, or high-count, achieving modest limits for a large number of similar targets. Other exotica efforts can be high-depth (red) or high-count (gold) campaigns, but observations of the Exotica Catalog will be broad, achieving modest limits on a small number each of a wide variety of targets. Future discoveries may be added to a later version of the catalog (pale blue), or prompt new campaigns that we cannot yet plan for (grey). Credit: Lacki et al.

Andrew Siemion (UC-Berkeley), who leads the Breakthrough Listen science team, notes that the few searches for technosignatures that have taken place have largely focused on stars thought to host planets in their liquid water habitable zone (although exceptions like Penn State’s Glimpsing Heat from AlienTechnologies, working at the galactic scale, are clear exceptions to this). What Siemion wants to do is expand the search. ‘Survey breadth’ — how wide a range of objects is studied in an observing program — is the operative term.

Or we might ask, are there objects we now consider natural that may in fact be artificial? And which natural objects — perhaps Boyajian’s Star, for instance, or some FRBs — mimic the kind of artificial signal that SETI researchers are looking for? Breakthrough Listen will spend 10 percent of its observing time on exotic objects. The 737 objects in the Exotica Catalog are sorted into different levels of priority for observation, with about a dozen considered high priority for SETI. Most entries are considered low priority and slated for observation as time allows.

The paper continues:

There are many reasons to search for technological intelligence in unconventional places. Unearthlike or nonbiological entities will not be constrained to live in Earthly habitats hospitable to lifeforms like us. It is also conceivable that some kinds of seemingly natural phenomena are the result of alien engineering. Yet there are good motivations for observing unusual objects even if ETIs cannot possibly live there. Extreme, energetic objects are more likely to produce unusual signals, particularly transients, that might be confused with artificial signals. Breakthrough Listen has unique instrumentation, and observation of a broad range of objects would benefit the general astronomy community. Finally, there could be unaccounted for systematic errors in our systems that give false positives. Observing exotic objects and empty regions on the sky allow us to constrain these possibilities.

You can find the Breakthrough Listen Exotica Catalog here. The paper is Lacki et al., “One of Everything: The Breakthrough Listen Exotica Catalog,” available in draft version online.