Time Markers for SETI in Binary Systems: History and Prospects

Abstract

Contemporary surveys in the search for extraterrestrial intelligence (SETI) typically make one-off “spot scans” across the sky to search planetary systems for narrow-band radio signals that would indicate the presence of intelligent life. Spot scans may span a duration of seconds to minutes in order to observe a large number of targets with limited resources, but such a strategy does not necessarily consider the timing of exactly when to listen for extraterrestrial signals. Several ideas for possible time markers were suggested in the first few decades of SETI, such as the use of recurrent supernovae, gamma ray bursts, or pulsars as a way of establishing directionality and attracting attention toward an extraterrestrial beacon. Civilizations in binary systems might even choose the points of periastron and apastron in its host system to send transmissions to other single-star civilizations. However, all of these timing considerations were developed prior to the age of exoplanets, which enables a more detailed assessment of targets suitable for SETI. This paper suggests SETI strategies for circumbinary and circumprimary planets based upon the timing of orbital events in such systems. Events such as orbital extremes could represent a logical time marker for extraterrestrial civilizations to transmit, if they desire to be detected. Likewise, a transiting binary pair with inhabited planets around each star could yield maximum detectability of leakage radiation when both stars eclipse within our field of view. As planets in binary systems continue to be discovered, limited-duration SETI surveys should selectively target such systems based upon the occurrence of reasonable time markers.

1. Binary Orbits as SETI Time Markers

The search for extraterrestrial intelligence (SETI) is currently underway with regular surveys that include the Breakthrough Listen initiative (e.g., [1]) and nightly surveys by the SETI Institute with the Allen Telescope Array (e.g., [2]). Such surveys produce spot scans of stellar systems, with targets that might be informed by habitability parameters—such as the known presence of planets within the stellar habitable zone or the likelihood of habitable conditions based upon observed stellar properties [3,4,5,6]. However, modern SETI surveys rarely consider the optimal timing of exactly when to listen.

A paper published in Nature by Pace and Walker [7], titled “Time Markers in Interstellar Communication,” suggested that any civilizations within binary star systems could take advantage of the points of periastron and apastron to send signals, and likewise to listen for transmissions from others:

Single star civilizations would logically transmit signals to binaries at the observation of periastron and apastron. Binary star civilizations would scan single stars at a time $2T$ after periastron and apastron, where $2T$ is twice the signal propagation time between the stars. The length of scan need be no longer than twice the uncertainty in T. Contact signals from different stars would arrive at different times at the binary system because of differences in the value of T for each candidate. This strategy would enable a binary star civilization to complete a search of nearby stars with a small number of telescopes after only a few orbital revolutions.

[7]

Pace and Walker [7], with further elaboration by Pace [8], suggested that binary star civilizations would likewise be motivated to choose the points of periastron and apastron in their host system to send transmissions to other single-star civilizations. The idea that inhabited binary systems might contain a natural time marker represented the first published idea (to the author’s knowledge) of a method for conducting SETI searches in coordination with time-varying astronomical events.

2. The SETI Ellipsoid

A flurry of other suggestions followed this idea to use binary orbits as a time marker for SETI. Tang [9] first proposed that the occurrence of a supernova would provide a more universal time marker, and later suggested [10] that a recurrent supernova would provide even greater chances to establish communication. McLaughlin [11] and Makovetskii [12,13] calculated the timing of interstellar messaging events to several nearby stars by using Nova Cygni 1975 as a time marker. This concept of using a supernova or other astrophysical time marker to guide SETI observations was named the “SETI Ellipsoid” by Hilton [14], a term that has continued to remain in use by the SETI community.

In a 1981 interview, SETI pioneer Bernard Oliver acknowledged the importance of the timing problem:

There is a problem with SETI, which, if solved, would change the whole picture, and that is the question of determining from some natural event or synchronization factor where and when to look. People have given that a lot of thought. There’s a fellow who keeps calling me all the time with his proposal. Whenever a supernova or some other cosmic event occurs, we radiate. Other intelligent beings elsewhere in the universe will also be looking in that direction, so we radiate signals in the opposite direction.

[15]

Oliver seemed to indicate no knowledge of the timing idea suggested by Pace and Walker [7] several years prior to the interview and instead refered to “a fellow who keeps calling” as his source of the idea. The author (J.H.M.) was contacted by a Mr. W. J. Siebrand who claims to be the unnamed “fellow” that Oliver mentions. Siebrand writes the following:

[Dr. Oliver] misstates this proposal. We, or they, would not radiate a radio wave contact message signal when the event “occurs,” but, rather, when the explosive event is then detected, exactly one light travel time after it did then occur.

[16]

In a JBIS article, Siebrand [17] suggested that any “astrophysical photon source flare” could act as a SETI time marker, even extragalactic radio signals such as quasar activity. Oliver continued in his interview by explaining why he is skeptical of such an approach:

The only thing wrong with that is, if you use the kind of high gain antennas we’re talking about, which you have to do to get across the miles, you have to point them in a million directions to cover the sky. So you’d only be sampling one-millionth of the sky if you point it in the directions he suggests. Therefore, if there are N races in the galaxy, you’d have something like 2 million over N events that would have to transpire before you got 63 percent of the directions covered. And those events aren’t that often. I’m not willing to wait 2 million centuries for this to happen.

[15]

Oliver correctly described a problem that SETI already struggles with: how to sample the entire sky. He noted that, “If we had some way of knowing preferred directions as well as times, it would change the whole picture,” [15].

Oliver may have been correct that such events are unpredictable and rare, but it would only be a few years later before the observation of a supernova in the Large Magellanic Cloud in 1987 (known as SN1987A) renewed interest in the SETI Ellipsoid. Hilton and Almár [18] noted that SN1987A generated a new search space as an ellipsoid expanding at the speed of light from its origin point at the progenitor star. Several years later, Lemarchand [19] developed a search strategy for examining several hundred systems of potential interest within the SN1987A SETI Ellipsoid (further discussed by Lemarchand and Tarter [20]), noting that the geometry of the search space is different for listening versus transmitting:

SN1987A is certainly the most important cosmic event, in several hundred years, that clearly determines two possible SETI strategies for Earth: (a) Passive (searching for signals inside the SETI Ellipsoid in the direction of SN1987A) and (b) Active (transmitting terrestrial messages toward the SETI Hyperboloid in antipode direction of SN1987A to call the attention of other possible galactic civilizations).

[19]

A more recent study by Davenport et al. [21] examined the Gaia Catalog of Nearby Stars to find that the majority of nearby stars remain viable candidates for searching as the SN1987A SETI Ellipsoid expands. Nilipour et al. [22] noted that only a fraction of these nearby stars have crossing times that would be observable in the near future, but such systems would still be ideal candidates to search for anomalies in stellar observations. Similarly, a study by Cabrales et al. [23] analyzed data from the Transiting Exoplanet Survey Satellite (TESS) mission for possible anomalous light curves from stars during the Ellipsoid crossing event, although no anomalous signatures were found.

The idea of using transient astrophysical events as time markers remains alive in SETI today, with studies noting that phenomena such as gamma-ray bursts [24] and pulsars [25] could serve as functional time markers in addition to supernovae. Seto [26,27] proposed an interstellar signaling scheme with an alternative geometry to the traditional SETI Ellipsoid, either by coordinating the timing prior to the occurrence of a “conspicuous astronomical event such as a coalescence of a double neutron star binary” [26] or by signaling after a reference transient event using a strategy based on optimal geometric and game-theoretic considerations [27]. Radio technosignature monitoring of the SN2023ixf SETI Ellipsoid is already underway with the Allen Telescope Array and the Green Bank Telescope [28]. Analysis pipelines for SETI have also explored the use of real-time alert brokers from all-sky surveys [29], which could enhance the ability to search for anomalies in the SETI Ellipsoid, the Seto [27] scheme, or any other location in space in relation to an astrophysical transient event.

3. Extrasolar Planets as SETI Time Markers

The discovery of extrasolar planets provides a new method for prioritizing SETI targets, which has led to some suggestions for how they might also be used as SETI time markers. Corbet [24] speculated that any extraterrestrial civilizations interested in Earth would listen for signals when Earth is at the point of opposition from the Sun—which would require precise prior observations of the motion of the Earth–Sun system.

Another idea could take advantage of the timing of transiting events, where Earth appears to transit the Sun from the vantage point of extraterrestrial observers [30]. Heller and Pudritz [31] suggested that SETI searches could be optimized within Earth’s “transit zone,” giving priority to star systems that would be more likely to detect Earth, or other solar system planets [32], through transit photometry. A Breakthrough Listen campaign using the Green Bank Telescope observed 20 stars within the Earth transit zone, finding no evidence for radio technosignatures during the 28.8 min of observing time per target [33].

A third possibility is the possibility of detecting evidence of radio communication in a multi-planet system with planets and stars that are co-planar with Earth [34]. Broadcasts in such a system would necessarily be beamed if the signals are primarily for planet-to-planet communication. A single planet could be restricted to localized weak transmission, such as as cell-to-cell radio, fiber optics, or even keeping everything is underwater, which may provide no detectable radio signals. But planet-to-planet communication absolutely demands strong and beamed signals, and they would be in the plane of the system’s planets. Such a signal would be modulated and peak strongly at the times when both planets are in simultaneous conjunction (both near to being in transit as seen from Earth). The signal would be strong any time the two planets align along our line of sight but would remain very weak the rest of the time. This possibility was examined in a study by Tusay et al. [35], which used observations of the TRAPPIST-1 system with the Allen Telescope Array to search for technosignatures during planet–planet occultations. Another analysis by Barrett et al. [36] examined 27 eclipsing exoplanets from TESS data to search for radio anomalies during occultation events; however, any events of interest in these studies were eventually identified as radio frequency interference.

Extending this approach for the sake of argument, suppose there was a system with two known transiting planets, and we periodically detected sharply peaked levels of radio signals at the times of alignment. But we also find, on rare occasions, other radio spikes when there is no alignment. Such an observation would provide evidence of another yet-undetected farther planet that is not transiting. The width of the spike would provide clues as to the undetected planet’s location (i.e., semi-major axis). Such a (wildly improbable) technique would be capable of simultaneously detecting a planet and technological civilization through a single observation.

4. Circumbinary and Circumprimary Planets as SETI Time Markers

We now return full circle to the original Pace and Walker [7] suggestion, but with observations of planets in binary star systems. Four years of observations of eclipsing binaries by the Kepler mission led to the discovery of a dozen circumbinary planets (Welsh and Orosz [37] and references therein) as well as numerous circumprimary planets. Some of these giant planets orbit within the habitable zone of their host stars; although the planets themselves may not be habitable in an Earth-like sense, terrestrial moons orbiting such planets could conceivably be habitable. Observations with TESS have found a handful of additional circumbinary planets, e.g., [38,39,40], and ground-based observations have suggested tentative evidence for a polar-inclined orbiting circumbinary exoplanet [41]. The James Webb Space Telescope and the next generation of planet-finding missions—such as the Habitable Worlds Observatory mission concept—will provide further opportunities to constrain the abundance of both giant and terrestrial planets in circumbinary and circumprimary systems. Modeling studies suggest that terrestrial circumbinary planets do not face any unique challenges to habitability compared with single-star counterparts, e.g., [42], so any such systems that are discovered may be ideal candidates to search for signs of life.

Prior studies have shown that eclipses in a binary system provide a possible time marker for SETI, but this paper suggests that the combination of a binary eclipse with a planetary transit could be an even more optimal configuration. An observer would notice a planetary transit coincident with a binary eclipse, which could provide a time marker for listening (by the observer) and transmitting (from the circumbinary planet). If the planet is also orbiting in the habitable zone of the circumbinary system, then this may also provide greater justification for SETI; however, a planet orbiting outside the habitable zone might also be useful as a time marker for SETI (for example, giant planets that are easier to detect). As long as the possibility of habitable planets in a circumbinary system cannot be precluded, any transit timing events—especially that coincide with an eclipse—could be a useful time marker of the best time to search for directed transmissions.

Circumprimary systems that are co-planar to Earth provide time markers that could be used for eavesdropping on planet-to-planet communication. A transiting system with planets orbiting both the primary and secondary stars could yield opportunities where both planets are aligned to the observer. Although detecting radio leakage would be difficult for such systems, any communication between these two planets would require strong beamed signals that could be detectable during such an alignment. The figure shows the planets aligned with the stars, but any alignment of the planets with the observer’s line of sight would suffice as a time marker of the best time to eavesdrop. If the binary stars are widely separated, then the timing of the alignment will occur approximately at the same time as the eclipse.

Only a handful of exoplanets in binary systems have been detected so far, and none of these may represent ideal candidates in the search for life in general or for SETI timing considerations in particular. The detection of such planets in transiting data remains challenging, but ongoing searches may eventually find binary systems that contain terrestrial planets within the system’s liquid water habitable zone, which would be ideal candidates for biosignature searches as well as timed SETI observations. If and when such planets are discovered, then these observations could be used to calculate optimal timings based on the known planets within the habitable zone of the system. Systems with giant planets in the habitable zone may also be viable targets for timing considerations, as giant planets could host terrestrial exomoons that could conceivably be habitable. For other known circumbinary or circumprimary systems, a dynamical code could be used to add fictitious planets into the system, within the habitable zone, and then optimal SETI timings could be derived for any dynamically stable cases. Another extension of this idea could be to use dynamical models to calculate timings for an ideal circumbinary or circumprimary planetary system that would represent a strong candidate for hosting a habitable Earth-like planet. Other ideas for examining possible SETI timing considerations should continue to be explored as new planet candidates are discovered in binary systems.

5. Next Steps

The search for circumbinary and circumprimary planets is in its infancy, but such systems might be some of the best targets for SETI. The next generation of space telescopes will enhance the ability to discover such planets as well as provide methods for characterizing them through spectroscopy. Planets in binary systems that are also within the habitable zone will inevitably be desirable targets for SETI. As new binary targets are discovered, we recommend that the timing of events such as eclipses and planetary alignments be considered as optimal times to listen (and, perhaps, transmit). Such an approach would complement existing SETI surveys by conducting focused, single-system observations that coincide with extremely rare transit events.

It is difficult to predict the frequency of line-of-sight alignments between Earth and yet-to-be-discovered planets in circumbinary and circumprimary systems, but further theoretical and observational studies will help to provide better constraints. Most single-star systems are expected to host at least one planet, but it remains an area of investigation as to whether the occurrence rate of planets in binary systems is similar. This problem itself can be addressed by current and future missions seeking to detect transiting planets orbiting nearby stars. But if we hypothesize that planets are just as common in binary stars as single stars, and if we likewise assume that habitable planets are equally probable, then this would suggest that potential circumbinary targets for SETI might be about as numerous as transiting single-star systems.