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A Catalog of Celestial Exotica

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.

{ 14 comments… add one }
  • Alex Tolley June 24, 2020, 11:39

    An interesting and welcome addition to SETI methodology. While most of the catalog is extra-solar and looking for a civilization, some effort is also made for signs of ETI within the solar system, with specific reference to Benford’s Lurkers.

    My analogy to this idea is to place ourselves in the position of an ant colony living in an anthill. The colony wants to detect other colonies of organisms and starts by assuming that they must live in some sort of structure, however different from their anthill. So those strange flat-sided structures (buildings) could also be colonies. Because they cannot assume this is the only sign they include transients. These could be the flash of light from an opening window reflecting the sun, or the smell of a BBQ on the wind. Perhaps it is those moving structures that swiftly pass by on that flat ribbon. Individual ants are asked to collect non-organic material they find while foraging too (The Roadside Picnic analogy).

    But would these ants lookout for transients like passing airliners, or drones? What about less material things like a water spray from a lawn sprinkler, or a firework, or the airflow from an electric fan? These seem like transients that the catalog does not include, and for the stated reason that they are hard to confirm.

    Having said that, we have to start somewhere, and I think this catalog is a useful guide, allowing different people and groups to focus on targets they are most interested in, or to be aware that their object or phenomenon of interest might just be artificial rather than natural.

    Our own backyard, the solar system, has barely been explored beyond a Lewis and Clark mapping expedition. There is a myriad of places that alien technology could be located, even deliberately hiding. If only we could dedicate swarms of cheap probes to monitor our system and improve our chances of serendipitously detecting such technology if it is present.

  • randomengineer June 24, 2020, 19:44

    The idea of lurkers and sentinels and so on hanging about in the solar system awaiting discovery seems like this:


    I’m afraid I no longer quite understand the purpose of SETI. There are no low hanging fruit radio signals and there seems to be no obvious optical ones either, and if aliens existed and really wanted us to ring them up then these things ought to be readily found. If not this devolves quickly to the pony story.

    • ljk June 26, 2020, 10:36

      The “old” versions of SETI may now be seen as limited, but the field should and must grow as our knowledge and technologies improve. We are finally seeing signs of this now that the original Radio SETI gang is falling away.

      And radio is still a very plausible method for interstellar communication, so we should not eliminate it as a possibility, just don’t make it the only way to go circa 1960.

      I have brought up this thesis before but it clearly needs to be done on a regular basis – please read this history of SETI that goes well beyond the usual official sources by the very folks who wrote their own histories of the field. You should find it most enlightening and be less than surprised afterwards why we have yet to find alien minds – and it is not due to any real faults of the aliens:


  • ljk June 24, 2020, 20:52


    Life in the Galaxy: Is This as Good as It Gets?

    Press Release – Source: Goldschmidt Conference

    Posted June 23, 2020 11:04 PM

    Researchers have found that rocky exoplanets which formed early in the life of the galaxy seem to have had a greater chance of developing a magnetic field and plate tectonics than planets which formed later.

    As both these conditions are considered favourable to the development of life, this means that if life exists in the galaxy, it may have developed earlier than later, and that planets formed more recently may have less chance of developing life.

    As lead scientist, planetary researcher Craig O’Neill said, “Plate tectonics is important for habitability, and it looks like the optimum conditions plate tectonics existed for planets forming early in the galaxy’s lifespan, and may be unlikely to easily recur. For life, maybe that was as good as it gets.”

    Exoplanets — planets in orbit around distant stars — have been attracting great interest because of the possibility that some of them may harbour life. Presenting the results at the Goldschmidt geochemistry conference, Professor Craig O’Neill (Director of the Macquarie Planetary Research Centre, Macquarie University) continued: “Because of the great distances involved, we have a limited amount of information on these exoplanets, but we can understand some factors, such as position, temperature, and some idea of the geochemistry of the exoplanets. This allows us to model how they develop.”

    Using huge simulations involving hundreds of processors on the Australian National Computing Infrastructure, the team ran the parameters through the ASPECT* geodynamics code, which simulates the development of the interior of planets. O’Neill’s group was able to show that many early planets would have tended to develop plate tectonics, which is favourable to the development of life.

    He commented: “Plate tectonics act as a kind of thermostat for the Earth creating the conditions which allow life to evolve. The Earth has a lot of iron in its core, and we had assumed that this would be necessary for tectonic development. However we found that even planets with little iron may develop plate tectonics if the timing is right. This was completely unexpected.”

    The development of plate tectonics has a major knock-on effect. “Planets which formed later may not have developed plate tectonics, which means that they don’t have this built in thermostat. This doesn’t just affect the surface temperature, this means that the core stays hot, which inhibits the development of a magnetic field. If there’s no magnetic field, the planet is not shielded from solar radiation, and will tend to lose its atmosphere. So life becomes difficult to sustain. A planet needs to be lucky to have the right position and the right geochemistry at the right time if it’s going to sustain life,” said Professor O’Neill.

    Researchers know that the overall chemical balance of the galaxy has changed over time for diverse reasons, such as material coalescing into stars and planetary bodies, or being expelled through supernova. This means that the interstellar material available to form planets is significantly different to that available in the early galaxy.

    “So the planets which formed earlier did so in conditions favourable to allow the development of life” said Craig O’Neill, “These conditions are becoming increasingly rarer in our galaxy.”

    Commenting, Professor Sara Russell said: “Over the last few years, amazing projects such as the NASA Kepler mission have located thousands of planets orbiting around other stars. However, these exoplanet observations alone provide very basic information. It is so important to combine observing campaigns with large simulation projects like this, that really tell us something about the geological evolution of planets formed at different stages of galactic evolution. This enables us to build a picture of what these strange worlds might look like, and how habitable they may be.”

    Sara Russell is a member of the Scientific Committee of the Geochemical Society. She is Professor of Planetary Sciences and leader of the Planetary Materials Group at the Natural History Museum, London. She was not involved in this work; this is an independent comment.

    *Advanced Solver for Problems in Earth’s ConvecTion (https://aspect.geodynamics.org). ASPECT was developed by the University of California (Davis) with funding from the National Science Foundation.

    Reference (Goldschmidt conference abstract):

    “How Does Galactic Chemical Evolution Affect Terrestrial Planet Composition and Tectonics?” Craig O’Neill (Macquarie Planetary Research Centre, Macquarie University, Sydney, Australia) — The exoplanet catalogue of terrestrial planets has grown enormously in the past decade, but our ability to characterise these bodies had not. A key unknown is the compositional variation in Earth-scale planets, and how factors such as the evolution of galactic reservoirs may have systematically impacted the composition of planets. Galactic chemical evolution models (GCEs) have been used to calculate the evolution of the interstellar media over time, and have suggested systematic trends in the availability of geophysically critical elements, such as Fe (relative to Si), and heat producing elements (HPEs) U, Th And K, suggesting a systematic diluting of heat producing elements over time, and an increase in Fe/Si due to increased Type Ia supernova activity since galaxy formation. Here we test the consequences of these trends for terrestrial planet behaviour, assuming solar:Earth element partitioning for terrestrial Earth-sized exoplanets. The concentration of the geophysically critical elements determine core size, gravity, and internal temperatures, and govern the geodynamics of these systems. We have varied the Fe/Si ratios/core size of simulated planets, and use a mineral physics package to calculate internal structures, physical properties, and gravity. We then use the mantle convection code Aspect to simulate their evolution. Planets forming early in the Milky Way’s history tend to have low Fe/Si ratios, and thus small cores, although elevated HPE budgets. The convection configuration and lower temperature in planets that have large mantle fractions relative to the Earth tends to promote high stress in the lithosphere and cause tectonic activity. Together with weaker surface faults due to lower gravity, such small-core planets show enhanced plate tectonic behaviour. In contrast, currently forming planets have large Fe/Si, core size, higher gravity, and a lower propensity to plate tectonics. Our results suggest a strong tendency towards plate tectonics on Earth-sized planets early in galactic history, with the tendency for tectonics diminishing as the galaxy has evolved.

    Please mention the Goldschmidt Conference in any article resulting from this press release.


    Please follow Astrobiology on Twitter.

    • Michael Fidler June 25, 2020, 10:39

      Another interesting article to be presented tomorrow at the Goldschmidt Conference in Singapore:

      Goldschmidt2020 Abstract

      Lost world of complex life:
      Molecular traces of our deep
      eukaryotic ancestors.
      1Research School of Earth Sciences, Australian National
      University (jochen.Brocks@anu.edu.au); 2Max-PlanckInstitute for Biogeochemistry (bnett@bgc-jena.mpg.de).

      Little is known about our deep eukaryotic roots.
      Eukaryotes are organisms with complex cells that possess
      organelles such as a nucleus and mitochondria. The group
      includes animals, plants, fungi, algae and single celled
      protists. Based on molecular clock estimates, the last common
      ancestor of all living eukaryotes (LECA) roughly originated
      some 1 to 2 billion years ago. However, going deeper back in
      time, past LECA, the history of eukaryotes becomes obscure
      as there are no fossils that can be attributed to pre-LECA
      eukaryotes with confidence. A second line of enquiry to study
      ancient eukaryotes are steranes, the molecular fossils of
      eukaryotic sterols. However, the oldest clearly indigenous
      eukaryotic steranes only date back to ~800 Ma. In this
      presentation we describe the hunt for elusive eukaryotic
      steranes in yet older sediments.
      In 1964, Konrad Bloch won the Nobel Prize for
      deciphering the biosynthetic pathway of the sterol cholesterol.
      He found that cholesterol is produced from epoxysqualene in
      no less than 13 enzymatic steps. The biosynthesis of sterols is
      so intricate and energy intensive that their precise structure
      must be crucial for the eukaryotic cell. This insight led Bloch
      to the famous hypothesis that the biosynthetic pathway
      towards cholesterol was perfected in a Darwinian process step
      by step over hundreds of millions of years, and that each of
      the intermediates was an improvement over its precursor. At
      some point in deep time, each intermediate must have been a
      functional end-product in the membrane of a pre-LECA
      ancestor. Bloch’s hypothesis predicts that the geological
      record may hide an array of fossil sterols (steranes) that
      record the sequence of assembly of the original biosynthetic
      pathway in deep time.
      Contrary to the current paradigm, we show that fossil
      sterols are abundant in the sedimentary record between 1.6
      and 1.0 Ga. The molecules had remained obscure as they
      possess unexpected structures. They comprise two successive
      biosynthetic steps in Bloch’s pathway and may thus represent
      the first observation of evolutionary biosynthetic
      intermediates in the geological record, potential witnesses of
      Proterozoic oceans that were teaming with our most distant
      eukaryotic ancestors.

    • Michelangelo Pantaleoni June 25, 2020, 19:58

      This is something amazing. Indeed the Rare Earth hypothesis is gaining terrain as we look Closer. Absolutely fascinating and unreal. Would love to read any papers on the subject discussed here but I guess that since this was presented in a conference there’s nothing yet to reference :(

      • Paul Gilster June 25, 2020, 21:08

        Re ‘Rare Earth,’ I think you’ll be interested in tomorrow’s post.

      • Harold Shaw June 26, 2020, 8:57

        Both articles erode the Rare Earth hypothesis. One demonstrates that plate tectonics won’t be rare. The other makes the case that biosynthesis becomes increasingly complex and productive because each evolution of function is stable. Biosynthesis and endosymbiosis wouldn’t be products of a miraculous coincidence but instead common and repeatable. If true, complex life becomes more likely.

  • Gary Wilson June 25, 2020, 12:59

    Thanks for that very helpful link and comment ljk. Anyone who believes we can answer the question “Where are all the aliens?” should really be a little more patient. It may take thousands of years to even begin to answer it. There is a lot of work to do and we will only ever have a time modified version of the answer based on the laws of physics and the speed of light. Unless there are ways to send signals via some form of quantum tunneling or other exotic unknown method of course. Maybe the communications from ET’s are passing by us as we speak. I don’t believe in lurkers personally but by being active in space we may find them if they exist. Curiosity can never be quenched. Keep on keepin’ on :).

  • Thomas R Mazanec June 25, 2020, 13:01

    A decade or two ago I read an article trying to estimate how many kinds of celestial objects and phenomena there are, as opposed to how many we had discovered. It used the analogy of collecting baseball cards…if you know how many players you have, and how many times you have bought a duplicate player card you can estimate the total number of players. A duplicate in this case would be an independent rediscovery…like we detected neutron stars first in radio, and then in X-rays.
    IIRC we had found about half the total catalog.

  • Wojciech J June 25, 2020, 14:33

    Very impressive step into what I would say is an anticipated direction.
    If you study SETI efforts and speculation it does dawn on you that directed signals seem to be implausible due to time differences and resources that would need to be invested.

    I wonder if the catalog includes:
    Przybylski’s Star-discussed here on Centauri Dreams.

    Locations of 37 radio events observed by META

    The possible Dyson Sphere candidates

    And the interesting red dwarf galaxies

    • Alex Tolley June 26, 2020, 9:07

      If you go to the website you can download the list of objects as a CSV file. The paper also has a complete list.

      And yes, Przybylski’s star is in the list.

  • ljk June 26, 2020, 11:11

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