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Planetary Days as a SETI Factor

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).


Comments on this entry are closed.

  • Thomas W. Hair June 19, 2020, 13:32

    Imagine a rotating space habitat orbiting the moon of a planet that itself orbits its home star and also orbits the galaxy. Of course it will be intermittent if not aimed intentionally at us. Additionally, the Benford brothers have shown, using straightforward math, that an omni-directional beacon is pretty much a non-starter years ago as stated in this article.

    Using simple Bayesian mathematics I showed, years ago, that any unintentionally detected signal would clearly be intermittent without resorting to the imponderables (that’s code word for making it up as you go along). Math is and always will be the king of science because it does not involve speculation (aka imponderables).


    In consideration of this it is important to realize that the finest mathematical sieve will always gather up far more straw from the haystack than it does those very special needles.

    If they exist at all, and that is a very big if, then they are ancient, they can directly image the Earth and know this is a special place, and then for whatever reason leave us completely alone. If we are not alone however, then the only plausible answer to the Fermi Paradox is my Modified Zoo Hypothesis published by Cambridge in 2011.


    We have to dismiss our Star Wars and Star Trek fantasies. They are just Cowboys and Indians in space.

    • Harold Shaw June 20, 2020, 10:10

      I agree that the interval between emerging civilizations is crucial to understanding the Fermi paradox. The first civilization to survive into deep time could shape the behavior of successive civilizations but I think a policy of non-interference would need objective benefits to take hold among all civilizations.

      Imo, the economics of navigating deep time encourages non-interference policies. The more unique neighbors are, the more likely they are to have opportunities to trade using comparative advantage rather than competitive advantage. Interfering in the early development of a civilization could drastically decrease the likelihood that the people will develop unique resources. Interference would be analogous to humans burning down a forest and planting crops when the market is demanding wildlife. As a taboo, it would be analogous to crippling an infant Mozart’s hands.

      • Wojciech J June 22, 2020, 13:35

        An untouched civilisation can potentially propose new maths solutions, science and of course will produce it’s own art and philosophy. If technological life is rare, uniqueness can be very valuable resource.

        • Thomas W. Hair June 23, 2020, 15:06

          Not sure where you are going with this thread Woj (if I may call you that, it sounds kinda cool). The mathematics of reality are surprisingly well understood. Any ancient intelligence has much to offer our nascent ten thousand year old civilization, but we will not appear to them like ants. We think, ants don’t, and I can contemplate a five billion year old intelligence and an ant can’t.

  • Geoffrey Hillend June 19, 2020, 15:58

    One hour or a couple of hours every day would be more than enough if we consider that any ET signal has to have some information in it that we can decode, a simple digital message in universal language. Language has universal building blocks and mathematics is even more universal. If there clearly is no code, or no message then it must be a false positive. Why would ET’s send a signal with no information? My point is to illustrate that we have not received any ET messages, so the WOW signal and gamma ray bursts etc. are most likely not any form of messages from an intelligent ET origin.

    They may be a lot of reasons why we have not received a signal yet like they don’t know we are here, they are not advanced enough, or like us they are advanced enough but are still early in the search and the practicality of funding is not recognized: It makes sense to send of message to a planet we know has biosignatures rather than any exoplanet. We don’t have any biosignatures yet, but a planet with the highest potential for intelligent life like an Earth sized planet around a G class star might have the best potential.

  • Alex Tolley June 19, 2020, 16:14

    Robert Gray calculates the power requirement for a broadcast radio signal to be receivable within 1000 ly with a 100m antenna with an S/N of 10, to be 10^15 W, 100x that of our current energy production.

    The problem before we even start is that for there to be just 2 intelligent species in that bubble to communicate suggests that there must be 15,000 such species in the galaxy. This is a rather high number and suggests that this bubble around us is empty of ETI.

    Getting past that obstacle, and reducing power requirements by narrow beams, for 2 species to find each other using those narrow, intermittent transmitting beams and narrow aperture telescopes, the probability of transmitting and receiving coincidently is very low, even over years of observing time. Worse, the receiving species must have the technological sophistication to create the necessary technology as well as the impetus to do searches (which we humans have largely not done). Our capability to receive radio signals at all is barely 100 years, although we have had optical telescopes for hundreds of years.

    To increase teh receiving odds, even for an intermittent signal, the ability to widely scan teh sky needs to be overcome. Ideally, the technology needed is simple, allow for a longer time for any species to have had that technology available. The most obvious solution is optical signaling with an intensity visible to the naked eye. Every intelligent being of the species then becomes a potential receiver, and the assumption of technological readiness disappears. Our ancestors for tens of thousands of years might notice a blinking point of light that is fixed in relation to the other, unblinking (merely twinkling) points of light.

    A laser transmitting in pulses that can readily be discerned as pulses by the eye is the best solution. The pulses might generate a pattern that lasts just a few minutes every year or decade, but it would eventually be noticed by any pre-industrial, even pre-civilized, species, and attention could be focussed on predicting the next expected occurrence of the pulses.

    The pulses themselves could contain far higher frequency modulation to provide much more information that could be extracted once the receiving species gains the technology required to move beyond just recognizing the signaling aspect. Eventually, if desired, this could translate to a METI program.

    However, we cannot just ignore the density of species in our galaxy issue. If the likelihood of another species being within the same 1000 ly radius bubble is extremely low, and most likely zero, then any reasoned strategy is an exercise in futility. Both the signaling and receiving must be far more powerful so that it can cover a much larger fraction of the galaxy. The distances involved then preclude any conversation. The solution is to make the transmitter local to the receiving species. The best solution is to launch von Neumann probes that will occupy each star and maintain itself by any means over many millions of years. Its signaling will still be optical as described. Its proximity will allow communication when discovered and crude signaling is attempted by the receiving species. The communication mode can be selected to match that of the receiving species.

    If there is at least one other ETI in our galaxy, then the “lurker” model makes the most sense. If there are many more species of ETI, even competitive species, then the scenario depicted in David Brin’s Existence makes most sense to me.

  • Henry Cordova June 19, 2020, 16:31

    The last two entries in Centauri Dreams have only reinforced a conclusion this long-time SETI enthusiast has reluctantly come to accept over the last few years. Even with the most optimistic numbers plugged into the Drake Equation. technical civilizations in the galaxy are simply too far apart, in both space and time, for us to be likely to be close enough to to one to pick up evidence of it. Furthermore, these civilizations have probably figured that out too and haven’t bothered to try. An omnidirectional signal is extremely expensive in energy terms, and even civilizations that are capable of doing so may feel its just not worth the effort. There are better things to spend your energy on.

    Tight beam transmissions aimed at suitable candidate stars are much more appealing, and technically much more achievable, but it would still require maintaining a continuous transmission program aimed at large numbers of targets for very long periods of time . A suitable candidate doesn’t necessarily mean anybody is home, and if they’re home, it doesn’t mean they’re paying attention. Maybe earth was bathed with signals during the Triassic, but ET finally gave up.

    I suspect single cell life is common throughout the universe, there is all sorts of circumstantial evidence to support that conclusion. But it took billions of years for multi-cellular organisms to evolve here, and half a billion years for metazoans to evolve the ability to construct radio telescopes. I know its only one data point, but if those numbers are representative of the entire Galactic biosphere, then there may be only one, or at best, a handful of advanced technical civilizations currently operating. Even if I’m totally wrong, and there are a million SETI programs out there trying to contact us, that still works out to only one transmission for every 10,000 stars. There are 10K stars within a hundred light years of us. And of course, there’s always that last pesky Drake Equation term, the one about “average lifetime of a technical civilization”. If we know this, they know it too. That explains the Fermi Paradox.

    Oh, and one more thing. If we assume the most optimistic parameters for SETI, that suggests most SETI-capable civilizations have already contacted all the correspondents they want. Maybe the enthusiasm for
    talking to the neighboring alien wears off quickly once they have several
    on the local party line. Why bother spending energy and time looking for others, I doubt we will.

  • Thomas W. Hair June 19, 2020, 23:40

    Henry, what you posted is exactly how I view it. If they exist at all, (and abiogensis is one tough nut) then the temporal and spacial dispersion of intelligence based upon star formation rate makes the point that they can see us at a distance (us being our planet thousands? of years ago).

  • tesh June 20, 2020, 4:15

    Jared Diamond wrote an interesting book called Guns, Germs and Steel. Between that and Dawkins work, they pretty much explain everything about how life began and stumbled into civilisation and all amazing stupidity (world events) that is now accommodated by it. One conclusion of the book, is that what is now Iraq/Syria was pretty much the only place in the world (at the time) where resources and geography were accommodating enough to allow for a significant departure from the hunter gatherer mode. Arable land, crop types and domesticate-able live stock were sufficiently amenable/available to allow for civilisation to take hold. As tech developed, other civs took hold relatively quickly.

    In a similar vein, could it be that though intelligence may emerge frequently, and may even overlap in some instances, until the right set of resources/geography are/is available they are doomed to peter out… Are we a dead end hominid species or humans who have journeyed to a dead end, such as the Australians or Europeans – before civilisaton took off? Are we going be the ones that build Sumeria/Mesopatania or will we have to wait for the Sumerian to come to us? Do we have the right geography?

    Looking for other civilisations, should we be first asking what sort of resources need to be present to allow civs to become multi-planet then multi-solar civs, and then look for them there. I guess this is a bit tricky as there is no data and there may be more than one way to skin a cat – however, there are definitely places where the chances are highly unlikely and (through our eyes) more likely.

    p.s. on the whole i agree space is too big and time too long (and short) for civs to corss swords or paths.
    p.p.s. trodden paths we may find…

  • Harold Shaw June 20, 2020, 9:48

    A civilization could use intermittent signaling because it makes room for the consent of the receiving civilization. The longer the interval, the longer someone must watch the sky. Looking is proportional to the consent to see. A civilization that transmits a continuous and easy to see signal is an aggressive civilization. They are refusing to be ignored and are willing to influence the development of other civilizations. They are willing to become a religion.

  • Law Wong June 20, 2020, 11:42

    If the aliens are targeting Sol specifically for broadcasts (presumably because they’re going on a biosignature detection or habitable zone planet or something), it seems that a space-based emitter is not implausible, especially if interplanetary economies exist.

  • Gary Wilson June 20, 2020, 13:03

    I think we will always have to content ourselves with knowing very little about a very small segment of our own galaxy. We won’t communicate across galaxies obviously and light is relatively slow as a medium to communicate within our own galaxy. We will have a finite lifespan as a species and we will look for answers as long as we can. I like Alex’s idea of Von Neumann probes sent out to “inhabit” various star systems and attempt to communicate within those systems. Even with advanced methods to detect possible technological civilizations it’s still a long shot that we ever find anybody. That’s just the way it is in a really, really big universe like ours.

  • Mike Serfas June 20, 2020, 16:09

    Is there an innate physical way to define a usable time interval that would stand out? The Planck time is a bit too short … but maybe multiply it by 2^n? Or do the same with the frequency you broadcast at?

  • Christopher Phoenix June 20, 2020, 16:18

    Fascinating speculations indeed, Paul. I will have to read the original paper for sure! The challenge with SETI is that while a confirmed hit would prove the presence of ETI, not finding anything doesn’t conclusively rule out aliens. They may not be transmitting, or we may not have looked long enough. We always have to make assumptions when picking the parameters of our search.

    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.

    It is interesting to note that those numbers aren’t so far off from what is required for physical interstellar travel. A civilization using 1015 watts for METI would be consuming sufficient energy to propel a Space Shuttle to 10% C every two weeks just for that one project. That suggests direct missions would be feasible for them (imagine using that energy for a beamed energy propulsion system, for instance).

    Of course directly targeting likely stars is a vastly more efficient way to undergo METI, albeit a strategy that will limit the likelihood that the signal will be observed… especially if the signals are transient as Robert Gray’s paper suggests.

  • Robin Datta June 22, 2020, 12:47

    A number of issues may be considered.

    Visual perception can be highly variable. Most birds of prey have extremely sensitive visual acuity: a hawk sitting atop the Empire State Building would be able to see a dime on the sidewalk below. Conversely, echolocators such as chiroptera and cetacea tend towards poor visual acuity. Some species perceive well in the ultraviolet (insecta) and others well in the infrared (ophidia). Elsewhere, xenobiologic intelligence may have developed such characheristics.

    If they do not rapidly drown out in the cosmic microwave background radiation, one might find in it modulated signals in specific directions. Even neutrinos might have been harnessed to carry information.

    Possible technosignatures could be scrutinized in radio frequencies.

  • Wojciech J June 22, 2020, 13:36

    Considering the amount of energy, time and resources needed involved in such a project, wouldn’t it make more sense to send probes instead? Especially since it would be likely that at this stage of development you can pin point planets with biospheres and potential future civilisations.

  • Ljk June 23, 2020, 0:15

    Frank Drake’s daughter writes about her pioneering father and how his SETI efforts changed astronomy….


  • AlexTru June 23, 2020, 3:05

    I do not met in this calculations very important point, when we are talking about distant objects (cosmic distances), we are looking at target trough some type of time machine, so every scientific data we can learn about object is related to object’s past, we can hardly suppose what are actual parameters of this object and what will be parameters in the future.
    So if someone planing to calculate narrow beam communication, it have to use data actual to the future of the target. Higher distance , higher uncertainty in place and time.
    And we do not have any possibility to get feedback and correct beam direction or schedule “on the fly”.
    As sequence, before we try to communicate we must collect lot of astronomical data about target, i.e. long time direct observation, before communication. Only astronomical observations can give us this information and it is not fact that it will be enough for correct communication window scheduling.
    So step by step we can understand that, narrow beam it is not good solution and decreasing power by increasing antenna gain, does not solve main SETI concept problems, still gigantic power resources required that are comparable with EM power level radiated by stars… Dead end.

  • Michael T June 23, 2020, 3:40

    A blank signal would STILL communicate the existence of ETI, if its other characteristics were unmistakably artificial.

    Also, are we SURE the WOW signal (or dare I say it, FRBs) have no data in them? Was the WOW signal recorded with sufficient bandwidth to KNOW that it wasn’t modulated?

    • AlexTru June 23, 2020, 9:37

      Michael T, all available data about WOW signal, you can find even in wikipedia, famous picture that is published in previous article contains almost every information we got with this signal, if you can decipher from this signal hidden ETI message, you are welcome :-)
      I suppose no any usable information can be extracted from 6 (or 8) non repetitive samples …
      Even super SETI optimists do not dare to see the message :-)

      • Michael T June 26, 2020, 1:03

        Yes I agree the Wow Signal was not recorded in great detail. That was my point. I did not express myself very well, sorry for the confusion. Haha re SETI optimists! The Truth May Be In There, but we cannot tell :-/ .

    • ljk June 23, 2020, 14:39

      If you want to learn the details about the Wow! Signal, especially to understand why there was no discernible data in the transmission, start here at the source:


      This very recent Astronomy Picture of the Day (APOD) page has information and more links on the subject:


      I also recommend this book, The Elusive Wow:


      • Michael T June 26, 2020, 1:09

        @ljk: Thanks for the links!

        I have done some reading on it but this article inspired me to do more.

        As always, you’re a wonderful source of references.