Centauri Dreams

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.

What a pleasure to see — after three decades — a grant from NASA for a SETI project, and on technosignatures at that. NASA’s history with SETI has been a challenging one given the subject’s reception in Congress. It was in 1971 that the agency funded Barney Oliver’s study on the huge array called Project Cyclops, whose price-tag would have been astronomical, but the report in which it was described provided numerous insights into the SETI effort. NASA’s engagement with SETI later came under fire from William Proxmire in the Senate, resulting in the termination of SETI funding in 1982. Proxmire would later change his mind on SETI’s value.

Even so, the NASA Microwave Observing Program (MOP) planned as a search of 800 nearby stars in the early 1990’s was again targeted in Congress and canceled shortly thereafter. The SETI effort developed in the ensuing years without government funding through efforts like Project Phoenix, which picked up the Mobile Observing Program under the guidance of Jill Tarter and the SETI Institute. With a number of interesting projects along the way, SETI has remained an ad hoc, privately funded effort in the US, with ongoing work through Breakthrough Listen, the Allen Telescope Array, the Berkeley SETI Research Center (SERENDIP) and others. The SETI League continues an energetic grassroots effort under the guidance of Paul Shuch. It’s interesting to note that in China, SETI is explicitly folded into the FAST radio observatory’s charter.

I haven’t gotten into optical SETI or other SETI projects in the US and abroad, but the point is that a new NASA grant makes a welcome return of NASA’s willingness to look at SETI, and with an eye toward the cutting-edge question of technosignatures as opposed to the reception of radio or optical signals. Here we’re talking about what might be created by advanced cultures, everything from vast Dyson spheres exploiting the host star’s energies to industrial pollution in exoplanet atmospheres, or even city lights. Exoplanet research motivates the current attention, says Adam Frank (University of Rochester), the primary recipient of the grant. Says Frank:

“The Search for Extraterrestrial Intelligence (SETI) has always faced the challenge of figuring out where to look. Which stars do you point your telescope at and look for signals? Now we know where to look. We have thousands of exoplanets including planets in the habitable zone where life can form. The game has changed.”

Image: Artist’s impression of the exoplanet LHS 1140b, which orbits its star within the “habitable zone” where liquid water might exist on the surface. The LHS 1140 system is only about 40 light-years from Earth, making it a possible target for studying the atmosphere of the planet if it has one. Credit: M. Weiss/CfA.

Called “Characterizing Atmospheric Technosignatures,” the new study will focus on solar panels and pollutants as indicators of technological activities. Avi Loeb (Harvard University) explains:

“The nearest star to Earth, Proxima Centauri, hosts a habitable planet, Proxima b. The planet is thought to be tidally locked with permanent day and night sides. If a civilization wants to illuminate or warm up the night side, they would place photovoltaic cells on the day side and transfer the electric power gained to the night side.”

What the study probes is how something like this would be detected if present, as a way of developing the mathematical and observational tools that can extend technosignature searches to various nearby stars. We are developing the ground- and space-based observatories with which this work can be conducted, but telescope time is precious, and knowing how and where to look is a critical issue. As we begin to be able to probe the atmospheres of rocky planets, we’ll have the potential for detecting not just biosignatures but evidence of artificially produced molecules that, like chlorofluorocarbons (CFCs), are unlikely to appear naturally.

Along with Harvard’s Loeb, Frank is joined in the effort by Penn State’s Jason Wright, Mansavi Lingam of the Florida Institute of Technology, and Jacob Haqq-Misra of Blue Marble Space. Out of the study, we can hope, will come quantitative information about how the project of technosignature detection can proceed. Let me recommend Frank’s The Light of the Stars (W. W. Norton & Company, 2018), along with Jason Wright’s Astrowright site for continuing insights into technosignatures. Also have a look at Wright’s Glimpsing Heat from Alien Technologies, published in these pages several years ago.

Yesterday we looked at a new paper from Robert Gray on the possibility — even likelihood — that the kind of signal SETI is looking for would be intermittent in nature rather than continuous. The numbers tell the story: In Gray’s calculations, an isotropic transmission with a range of 1,000 light years — i.e., a continuous beacon broadcasting in all directions — requires on the order of 10¹⁵ W to produce the kind of signal-to-noise ratio that would allow us to pick it up with facilities like those used in current SETI searches.

10¹⁵ is a big number, going beyond the current terrestrial power consumption of 10¹³ W by orders of magnitude and reaching 1 percent of the total power received by Earth from the Sun. Reduce the desired range of the signal to 100 light years and the requirement for isotropic broadcasts is still daunting, demanding something like 10¹³ W, or 10,000 1,000 MW power plants. As Gray puts it:

The large power required for continuous isotropic broadcasts could conceivably be available to some very technologically advanced civilizations (Kardashev 1964, 1967), but assuming very advanced civilizations seems very optimistic.

Indeed. Hence the need to ponder alternatives. Consider the savings gained, for example, in using high-gain antenna systems to target single stars. Gray describes an Arecibo-class transmitting antenna following this strategy. Now the power requirement begins to fall to recognizable levels. An Arecibo-like transmitting antenna punching out a signal to a star 1,000 light years away needs 10⁸ W, dropping to 10⁶ W for a range of 100 light years. In other words, this we can do today with the actual Arecibo planetary radar.

Transmitting is one thing, reception quite another. If we think in terms of sending a signal to more than one target star, broadcasting to each in succession in a repetitive pattern, we are sending a signal that would obviously appear intermittent to any receiving station, and factors that are entirely unknown to us come into play. How often would such signals repeat? We would need to know the number of targets the transmitting civilization had chosen and the dwell time devoted to each. Any calculations we run fall victim to the depth of the imponderables here.

Image: Australia Telescope Compact Array (ATCA) antennas at night. Credit: Sascha Schediwy.

But there are a few numbers we might plug in to give us a sense of the possibilities. Gray’s paper looks at planetary rotation periods as an indicator. The thinking is that transmission from the surface of planets could make rotation a factor, rendering some signals periodic. In our own Solar System, we find a median day of 23.9 hours (dropping to 21.1 hours if we leave Pluto out). Three quarters of the eight planets have days in the range of 10-25 hours. Our knowledge of exoplanet days will grow with time, allowing us to get a sense of day length for the planets we are most interested in: Rocky planets in or near the habitable zone of stars other than our own.

Thus the ‘interstellar lighthouse,’ a directional transmission from a fixed antenna on a rotating planet producing an intermittent signal with the period of the planet’s sidereal day:

In the case of a source planet with the median day in our planetary system and a rotating 1° lune, distant observers would be illuminated for 23.9/360 = 0.0664 hours or 3.9 minutes every 23.9 hours. Such a signaling strategy would have the isotropic broadcasting advantage of illuminating the entire sky although not constantly, and the directional transmission advantage of much lower power requirements than isotropic, and with no need for tracking. A transmission from a rotating antenna system might display a signature Gaussian rise and fall as it swept across a detector, and that might suggest re-observation efforts scaled to a planetary day.

A daily cadence for both radio and optical SETI is thus a possibility, and as Gray notes, most of our searches (and transmissions) have been conducted from single sites on the surface. Planetary days would be a known factor in specifically targeted transmissions. Obviously, there are other options here, including using multiple scattered sites like the Deep Space Network on Earth, or operating a transmitter from space, so this is only one factor to consider.

A distant civilization detecting our transmissions would note the periodicity of the signal based on the terrestrial sidereal day, and conceivably might use the same timing to return a signal to us. Other time intervals studied in this paper include pulse periodicities — are there, in other words, preferred periodicities in signaling just as there may be preferred frequencies? 21 possible time intervals, some defined by pulsar periods, have been suggested in the literature. Orbital periods are an obvious interval for targeted signaling, while some recent papers have suggested synchronization between astronomical events like the conjunction of two exoplanets along the line of sight from Earth or the opposition of planets in other planetary systems.

Thus the assumption of continuous signals and very brief observation times becomes problematic if the signals we are looking for are intermittent. Historically, the longest observation of a single field is 100 hours in work at the Allen Telescope Array, although Gray also notes Frank Drake’s work at Project Ozma, which studied Tau Ceti and Epsilon Eridani for approximately 100 hours each. Intermittent signals would demand long dwell times, but a consideration of a planetary day time scale might prove a useful guide to operations.

A planetary day time scale might be useful in searching for interstellar signals, because planetary rotation would have physical effects on both transmissions and searches operating on the surface of planets. Observations over a planetary day would off course cover many possible shorter repetition rates; observations extending over approximately 25 hours would include signal repetition rates up to the 66th percentile of days in our solar system. That is a much longer observing time than is typical in SETI, but techniques such as radio imaging can be used to observe many stars in a wide field simultaneously. Observations over less than 10 hours would not cover even the shortest planetary days in our solar system.

Writing about interesting papers is frustrating when I come up against time limitations as severe as those I’m under today. I haven’t had time, for example, to discuss the other ways in which signals might be intermittent, but Gray discusses variations in power at the source, propagation effects, variable power cost/availability, interstellar scintillation, variable frequency effects and more (including, and this interested me, variable polarization). Obviously, if these things intrigue you, track down the paper for the full treatment.

The paper is Gray, “Intermittent Signals and Planetary Days in SETI,” International Journal of Astrobiology 4 April 2020 (abstract).

My guess is that most people think of SETI as doing a ‘long stare’ at a given star, on the theory that it may take time to acquire a possible signal from an extraterrestrial civilization. But in reality observations take place over short time periods. The Mega-channel ExtraTerrestrial Assay, known by its acronym as META, led by Harvard’s Paul Horowitz and aided by The Planetary Society, could only devote a few minutes to any particular star.

The same was true of the follow-on BETA (Billion-channel Extraterrestrial Assay), while targeted searches like Phoenix, led by Jill Tarter and using facilities at Green Bank (West Virginia), the Parkes 64-meter dish in Australia and the 300 meter radio telescope at Arecibo, still observed targets for less than an hour. The problem with this is that there are numerous reasons why an extraterrestrial signal might be intermittent.

We’ve looked at this issue before, particularly in terms of ‘Benford beacons,’ as discussed by Greg and Jim Benford in these pages (see, for example, SETI: Figuring Out the Beacon Builders for an introduction to the discussion).

Robert Gray takes on the matter in a new paper in the International Journal of Astrobiology. Gray’s will be a familiar name to anyone tracking SETI closely, as the author of The Elusive Wow: Searching for Extraterrestrial Intelligence (Palmer Square Press, 2011) and the man most associated with the unusual reception at Ohio State’s ‘Big Ear’ observatory in 1977. The name comes from the ‘Wow!’ that radio astronomer Jerry Ehman inscribed on the computer printout of the event, which had characteristics of a genuine signal rather than noise..

Image: Independent SETI researcher and data analyst Robert Gray. Credit: Sharon Hoogstraten CC BY-SA 3.0.

Gray became fascinated with the signal when he learned about it several years after it was received. Since those days, he has led his own search for the signal, both with his own 12-foot dish and professional installations like the Harvard/Smithsonian META radio telescope at Harvard’s Oak Ridge Observatory as well as the Very Large Array in New Mexico. He would later extend the search to the Mount Pleasant Radio Observatory in Hobart, Tasmania. As you can see, Gray’s observations are the antithesis of the short dwell times that are the norm in SETI due to the desire to widen the search to as many stars as possible. The Wow! Signal has now had over 100 hours of follow-up, but no signals resembling the original one were ever detected.

What would happen if we could somehow extend SETI to long, fixed stares on high-interest stars? What Gray’s new paper points out is that signals that repeat at intervals of hours or more could be detected with such a capability, whereas current methods would be unlikely to find them. It’s interesting to speculate on why it might, in fact, be far more likely for signals to be intermittent than continuous. The Benfords have analyzed the issue in terms of cost, a thought Gray echoes in the paper by noting the power demanded by a continuous isotropic beacon.

Lower the duty cycle and the average power drain is sharply diminished, producing a possibly strong but intermittent signal. Thus a beacon might be operating at repetition rates that would be invisible to our current methods, as would directed transmissions, whether optical or radio, that targeted specific stars. We’ve wondered before in these pages about planetary radars like Arecibo, perhaps detectable at considerable distance, but appearing in an alien sky only momentarily as the signal swept by chance past the receiving apparatus. ETI’s astronomical community would be left with an interesting transient and no real hope of confirmation.

Image: The famous ‘Wow!” signal.

Here’s Gray on moving away from continuous isotropic broadcasting. I quote this passage because it places current SETI efforts in context:

…reducing the duty cycle to 1% could provide a 100-fold reduction in average power required, perhaps radiating for 1 s out of every 100 s. Searches observing targets for a matter of minutes might detect such signals, such as the Ohio State and META transit surveys which observed objects for 72 s and 120 s respectively, or Breakthrough Listen observing targets for three five minute periods…, or a targeted search such as Phoenix observing objects for 1,000 s in each of several spectral windows…, or the ATA observing for 30 minutes… Reducing duty cycle further yields further savings—for example a 10^-4 duty cycle with a 10⁴ reduction in average power might result in a 1 s signal every three hours, but most searches to date would be likely to miss such signals. Assuming longer signal duration does not help much; a 1-hour signal present every 100 or 10,000 hours would be very unlikely to be found by most current search strategies unless the population of such signals is large.

There are other ways of reducing transmission power that make sense, with repercussions for how SETI might detect them. And there are reasons for intermittency that go beyond broadcast strategies, all of which must be considered. Are there strategies that can help us here, given that our knowledge of signal duration — if indeed such signals exist — is nil? Gray suggests some possibilities that I want to look at tomorrow, as we continue to ponder intermittent signals and their possible reception.

The paper is Gray, “Intermittent Signals and Planetary Days in SETI,” International Journal of Astrobiology 4 April 2020 (abstract).