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SETI: A New Kind of ‘Water Hole’

Some of you may recall an episode of Star Trek: The Next Generation in which the inhabitants of a planet called Aldea use a planetary defense system that includes a cloaking device. The episode, “When the Bough Breaks,” at one point shows the view from the Enterprise’s screens as the entire planet swims into view. My vague recollection of that show was triggered by the paper we looked at yesterday, in which David Kipping and Alex Teachey discuss transit light curves and the ability of a civilization to alter them.

After all, if an extraterrestrial culture would prefer not to be seen, a natural thought would be to conceal its transits from worlds that should be able to detect them along the plane of the ecliptic. Light curves could be manipulated by lasers, and as we saw yesterday, the method could serve either to enhance a transit, thus creating a form of METI signaling, or to conceal one. In the latter case, the civilization would want to create a change in brightness that would essentially cancel out the transit light curve. It’s not exactly a ‘cloaking device,’ but it ought to work.


Image: The Next-Generation Transit Survey (NGTS) telescopes operating at ESO Paranal, Chile. Transit observations have SETI implications we are only beginning to explore. Credit: ESO/ G. Lambert.

A Galaxy of Xenophobes?

As I said yesterday, I’m not here to reignite the METI debate as much as to acknowledge that what an alien culture might do is unknown. Rather than asking whether any civilization should try to conceal itself, let’s simply ask what it could do if it made the attempt.

The idea has a brief history, with Eric Korpela (UC-Berkeley) and Shauna Sallmen (University of Wisconsin-La Crosse) suggesting in 2015 that ETI could effectively hide a planetary signature through the use of orbiting mirrors. This would, like the geometric masks envisioned by Luc Arnold, require engineering on a huge scale, and would also demand elaborate tuning for each target. Kipping and Teachey argue for a more affordable alternative using a directed laser beam:

In our scheme… the advanced civilization emits a laser directed towards the other planetary system at precisely the instant when the other system would be able to observe a transit. The power profile of the laser would need to be the inverse of the expected transit profile, leading to a nullified flat line eliminating the transit signature.

Screenshot from 2016-04-05 09:24:41

Image: Top: The unaltered light curve of the Earth transiting the Sun, as viewed by different broadband optical photometers (offset by 5 ppm). Middle: The power profile of a 600 nm laser array designed to cloak the Earth. An array of lasers producing a peak power of ∼ 30 MW over 13 hours nullifies the transit. Bottom: Residual light curve, as seen by the different photometers. Credit: David Kipping/Alex Teachey.

The Kepler mission has produced the vast majority of recent exoplanet discoveries, and we have upcoming transit surveys in the works including TESS (Transiting Exoplanet Survey Satellite), PLATO (PLAnetary Transits and Oscillations of stars) and NGTS (Next-Generation Transit Survey). If a civilization wanted to shield itself from this kind of broadband optical survey, a monochromatic optical laser should do the trick. The paper estimates that the Earth could be ‘cloaked’ — hidden from view from a particular star system by having its transit nullified — with a 600 nm laser array emitting a peak power of ~30 MW over 13 hours.

The power requirements are interesting because they are relatively low for a specific target, but the paper adds the obvious point that if we are trying to cloak a planet from a large number of targets, we would require larger power production. Nonetheless, we routinely use much larger numbers when talking about laser lightsails in the configurations that could enable interstellar flight. Kipping and Teachey point out that for a culture that develops those kinds of technologies, cloaking could become a secondary function of the laser arrays used primarily for propulsion.

Chromatic cloaking (across all wavelengths) could be achieved by using a large number of beams (although with an order of magnitude higher energy cost), while tunable (‘supercontinuum’) lasers may emerge that can simulate any spectrum. But even with these capabilities, is cloaking an entire planet the most efficient choice for a civilization trying to hide itself? Perhaps a better course from the standpoint of economics and efficiency is to cloak the biosignatures that announce life’s presence. Let me quote from the paper on this:

It is straightforward to use a chromatic laser array to cancel out the absorption features in the planet’s transmission spectrum, assuming laser emission can be produced at any desired wavelength. Indeed, the presence of an atmosphere could be cloaked altogether if the effective height changes of the planet as a function of wavelength are canceled out by lasers. The planet might then resemble a dead world totally devoid of any atmosphere and appear almost certainly hostile to life. Not only would this approach require a significantly smaller power output, it would also have the benefit of producing self-consistent observations insomuch as the presence of the planet might still be inferred by other means (i.e. through radial velocity analysis).

What SETI Can Learn

Kipping and Teachey refer to these methods as a ‘biocloak,’ and suggest that cloaking can be selective indeed, perhaps focusing on the absorption features of molecular hydrogen and ozone. In this case we are dealing with peak laser power of just ∼160 kW per transit. But the authors are clear about the limitations of these methods. Radial velocity methods can find a planet otherwise hidden by a chromatic transit cloak, and given technologies not so far advanced over what we have today, direct imaging can reveal atmospheric features of a planet even when a ‘biocloak’ is in place. “For these reasons” write the authors, “perhaps the most effective use of laser enabled transit distortion would be for broadcasting rather than cloaking.”

And it was on that note that I began yesterday’s look at these possibilities. If we have based fifty years-plus of SETI on the notion that another civilization may choose to contact us, we have to acknowledge what Kipping and Teachey make clear: There are ways to alter transit signatures that make it obvious we are dealing with an advanced technology. And you can make the argument, as the authors do, that transits offer a different kind of ‘water hole’ for SETI, comparable in its own way to the ‘water hole’ frequencies we monitor in radio SETI.

Thus while the cloaking aspects of this paper have received the most attention, I think the SETI implications are its strongest takeaway. It is a very short step from existing optical SETI to archival searches of transit signatures already in our files. Knowing what these signatures would look like is a step forward as we continue to probe for civilizations around nearby stars.

Addendum: This email from Dr. Kipping, excerpted below, further explains the authors’ thinking about cloaking possibilities:

…we never intended to solve cloaking from all detection methods in one paper (that would be a tall order to demand of any research paper). Rather, we started with the simplest and most successful technique, transits, and showed that it is energetically and technologically quite feasible for even our current level of technology to build an effective cloak. Whilst we acknowledge that there are ways to defeat the proposed cloak (e.g. polarization of laser beams, direct imaging), we see these as problems which are likely to be solved by more advanced civilizations than ourselves, or indeed in future work (by humans!). What we are trying to do on the cloaking side is stimulate a conversation- that it is surprisingly easy to hide planets. Given that many notable scientists are opposed to METI, it is not unreasonable that other civilizations may choose to do this. The scenario could be that they would have long ago observed the Earth as an inhabited planet, and then turned on a cloak as a insurance policy, buying them time to reveal their presence when they choose to, rather than our increasingly penetrating telescopes finding them before they wish.

The paper is Kipping and Teachey, “A Cloaking Device for Transiting Planets,” accepted at Monthly Notices of the Royal Astronomical Society (preprint), The Korpela and Sallmen paper is “Modeling Indications of Technology in Planetary Transit Light Curves – Dark Side Illumination,” Astrophysical Journal Vol. 809, No. 2 (abstract).


Comments on this entry are closed.

  • Alex Tolley April 5, 2016, 10:36

    Even assuming the cloaking works for transits, how will this prevent direct observations of planets by reflected light? An observer in the orbital plane can detect the planet in its orbit as it emerges behind its star all the way through nearly half an orbit until it transits and then repeats for the rest of the orbit. Observers out of the plane can observe the planet for the complete orbit.

    We have already observed planets directly, and in a short space of cosmic time will no doubt be able to image small worlds in detail using large telescopes with and without gravitational lensing.

    Without full cloaking, these “stealth” methods are too narrowly focused to work as serious methods to avoid detection. It’s like a child hiding under a blanket and thinking that he cannot be seen.

    As for signaling, transit signal modification seems like a solution in search of a real problem. The range of suggested radio and optical signaling methods are far more reasonable and cost effective, and cover a better range of target stars whose locations are not dependent on the transit alignment. For example, if we received monochromatic laser pulses in prime number sequences, that would be an unambiguous technology generated signal IMO.

    The proposal to re-examine transit signals for evidence of cloaking seems like a make-work proposal looking for funding.

    If a civilization really wants to hide, it will have to “go dark” by emitting only low power em signals and keeping its energy consumption in the noise level. If it also wants to hide all biosignatures, it would have to hide its biosphere under a shield that transmitted light inwards, but manage the outward signal to resemble a dead world. If that is possible, that might fool a distant observer, but probably not a probe sent to investigate.

    Our own technological development would require ever more sophisticated requirements to cloak a world.

    I’m reminded of a Stephen Baxter short story where astronomers using ever more powerful instruments suddenly reveal the true universe that was hidden from us by aliens behind projections. Earth’s telescopes eventually broke the capability of the projection screens to simulate the signals needed for the observations.

  • Mike Lorrey April 5, 2016, 11:05

    Wouldn’t coherent light be distinguishable from full spectrum emissions of a star?

  • sudheendra April 5, 2016, 12:47

    Best bet would be to couple ‘laser fencing’ with a global photon scattering mechanism that can smear out photon info exiting Earth. This would lead to suppression of temperature fluctuations as well as radiation flux. More importantly the spread in time of last photon smearing, might just cloak the region from where the last photon got smeared…..giving ample time for a getaway. This would amount to cheating redshift too. If you think this is downright preposterous you have got company. Me.

  • George King April 5, 2016, 15:17

    I acknowledge the stated objective in the author’s e-mail of starting a conversation.

    But, with that intriguing conversation started, it would seem more likely than not that:

    (a) any other world civilization having the only rudimentary (comparable, frankly, to our own current state of the art, which is growing by leaps and bounds, but . . . ) exoplanet discovery methods that could be defeated by laser transit or other cloaking would be unable to reach out across the vast distances and do anything to us even if they wanted to; and

    (b) any other world civilization that instead was able to reach out across the vast distances and potentially do something to us would be able to easily recognize and overcome our own comparatively rudimentary attempts to cloak against also comparatively rudimentary exoplanet discovery methods.

    So I question whether the game is worth the (cloaking of the) candle.

    And even once we are able to build the infrastructure required for laser propulsion of light sails (likely not before the 22nd Century from the propulsion end), I’m not sanguine that the laser placement required for propulsion necessarily will line up with that required for cloaking.

    All that said, the possibility that a civilization might seek to cloak or alter their transit or other signature still is something to think about in going back over prior apparently negative results from transit or other exoplanet surveys. It might be worthwhile for researchers to consider whether we might be seeing an artificially induced false negative. Although, at the other end at the other world, I would think that they would have gone through the same cost-benefit analysis at the outset and opted to not do this.

    Maybe if you actually knew that the Borg were actively looking for you and that they had a specific blind spot in their exoplanet search methodology, you might try cloaking to buy a little more time to build up your defenses . . . lol.

    But, as I say, if such an active threat already has FTL tech, their ability to detect target exoplanets with life signatures likely will be similarly advanced.

    The better utility of this technology arguably would be to send rather than attempting to conceal – if the policy decision were made to send. Perhaps an interrupted pulse laser signal could send clearly artificial intervals of prime numbers, sort of an interstellar Morse Code.

  • RobFlores April 5, 2016, 15:55

    George King :
    Now that is some common sense.

    The Cloak is not a long term solution, and really not practical.
    But I imagine a xenophobic race would build one, blocking the
    lesser tech civs from detecting them. They would still be detectable and be prey to a civ even only 500 years more advanced than they.

  • david lewis April 5, 2016, 16:49

    If we want to hide then why not just move the planet. If we made a wanderer out of the earth then aliens looking for habitable planets near stars would be looking in the wrong place. Alternatively we could just dismantle the planet to make space habitats, and hide those between the stars, or even, given enough time, between the galaxies.

    The simplest way though would be to crash Ceres into the earth. That way even if alien astronomers spotted the earth it wouldn’t be inhabitable, and they would have no reason to investigate further.

    Alternative we could just keep burning fossil fuels until the planet heats up. That way might be a bit difficult though; it would take an awful lot of coal to render the planet completely uninhabitable. We ourselves could move underground, or it could be done as part of a doomsday plot to keep the villain of the day from taking over the world. Better dead than red, as they used to say, though I myself don’t understand that line of reasoning.

  • Harry R Ray April 5, 2016, 17:32

    In a comment submitted to the PREVIOUS POST, I asked the question “Can a transit signal be completely cancelled?” I now want to focus on the “…COMPLETELY…” part of that question. It appears that it CAN(from the image,above), but what about ALL OF THE OTHER WAVELENGTHS? An infrared signal can, if you use infrared lasers. An ultraviolet signal can if you use ultraviolet lasers. AND SO ON! What I am getting at here is what is the TOTAL POWER OUTPUT NEEDED TO COVER THE ENTIRE ELECTROMAGNETIC SPECTROM? It must be EXPONENTIALLY higher than the total output derived in Kipping and Teachey’s paper. In the case of KIC211152484, if SOME of the transits WERE cloaked in the visible light spectrum, would they be cloaked IN THE EXACT SAME WAY in the infrared, ultraviolet, etc. The best way to find out would be to look at it SIMULTANEOUSLY in SEVERAL DIFFERENT SPECTRA using ALL AVAILABLE ground telescopes and spacecraft. I am almost CERTAIN that this is NOT an attempt by ET’s to contact us, but we could use this as a PRACTICE RUN for when a BETTER signal shows up. If Kipping and Teachey were made aware of my idea, I wonder if they could be able to(or would even WANT to) do this “practice run” in the near future?

    • Mephane April 7, 2016, 4:56

      And even then, if you had dozens of lasers in various wavelengths to conceil it across the entire spectrum – it is still detectable as spikes at distinct wavelengths.

  • Harry R Ray April 5, 2016, 19:15

    Sorry, I meant EPIC211152824, NOT KIC211152824.

    • Harry R Ray April 5, 2016, 19:21

      Sorry again, it’s EPIC211152484.

  • H. Floyd April 5, 2016, 20:43

    So granting the cloaking project for argument’s sake (and non-argument’s), it might be unfair to apply an Invisibility Or Bust standard. After all, things that exist are bound to be observable at some physical level or another.

    But suppose we were tasked with coming up with an interstellar “camouflage” strategy. The authors’ approach offers potential. Evolutionary camouflaging (both protective and predatory, spooky thought) increases the likelihood that noteworthy information will be confused with less noteworthy, ubiquitous information. To do that artificially you can’t avoid making probabilistic assumptions about an observer, nor can you anticipate every possible countermeasure. The project would work along a continuum of effort with greater sophistication as resources improve.

    As a side benefit, whatever cloaking technology a planetary society developed could be readily adapted for broadcast and communication purposes if policies changed. Plenty of jobs for SETI and METI minds alike. Hint, hint.

    • Alex Tolley April 6, 2016, 10:43

      An issue with the “evolutionary” cloaking is that in a civ’s case, the selection is happening at the population level, so there is no chance that improvements can be passed on to the next generation as it can with selection of the individual. A cloak failure and that is the end of that civilization (assuming the predator civ’s intent is to do that). There is no chance to improve the technology.

      As Dani Elder points out down thread, we are already on the brink of being able to develop high resolution telescopes with gravitational lensing, which completely obsoletes transit cloaking. As Dani also points out, observers will detect when a cloak is turned on, a high probability signal of a technological civ.

      In a c speed constrained universe, distance is the best protection, but it may only give that protection for tens of millennia at best from predators in the same galaxy. If they are out there, the clock has started to tick.

  • Horatio Trobinson April 5, 2016, 21:59

    I’m interested in the technique of cloaking, but from the exact opposite perspective.
    If we can selectively cloak Earth, we can send ones and zeroes.

    We can make Earth one huge modem broadcasting HERE WE ARE.

    One single laser could be used for semicloacking Earth in the direction of several star systems.
    A semicloaking transmission system would send a sequence of alternating eight bit bytes to establish a base frame of reference.

    First you have to show what’s the smallest information unit, sending out a number of bytes of alternating ones and zeroes.
    10101010 10101010 10101010 10101010
    10101010 10101010 10101010 10101010

    Next we have to let them know what’s our byte size.
    11111111 00000000 11111111 00000000
    11111111 00000000 11111111 00000000

    Then we can start communicating information that signals we have basic mathematical knowledge, much in the same way Carl Sagan described.
    Like the sequence of prime numbers Ellie Arroway decodes in “Contact”.

    00000001 00000010 00000011 00000101


    Followed perhaps by trigonometry data and a bitmap image of a circle.

    We can do it. Do we want to?

  • Dani Eder April 6, 2016, 8:52

    Laser cloaking of transits neglects that using the Sun as a gravitational lens telescope has a theoretical resolution of 2.8 meters per light year in visible light. That’s determined by simple optics assuming a lens diameter of 2 million km. The Sun is smaller than that, but you want to use photons that miss the surface of the Sun by a comfortable distance. That’s to avoid interference from the Sun’s own light output, and small scale distortions from turbulence. The miss distance is achieved by moving farther from the Sun, where the more weakly bent photons come to a focus.

    We don’t know how close an actual gravity lens instrument can come to the theoretical resolution, but if it is anywhere close, we could see city-sized objects from 1000 light years away. That means you could see cities at night, and other obvious indicators of civilization. Conversely, another civilization could do the same for us, regardless if they are located in our transit zone.

    The cloaking concept also seems to neglect speed of light time delays. A civilization 1000 light years away would see us as we were during the Medieval period. Biosignatures indicating life have existed far longer, but indications of intelligent life would not be obvious yet to distant observers. Those indications would continue to travel through space up to the point you start to cloak. The sudden stop when the cloak is activated is itself a signal of intelligent life (or that we destroyed ourselves).

  • djlactin April 6, 2016, 13:16

    I don’t know how they got “with a 600 nm laser array emitting a peak power of ~30 MW”.

    I calculate the amount of solar energy (at 1361 W/m^2 = 1361 x 10^6 W/km^2) blocked by the earth (radius 6371 km; x-sectional area 1.28 x 10^8 km^2) and get 1.74 x 10^17 W.
    This is 1.74 x 10^11 MW; i.e their estimate is low by about 10 orders of magnitude.
    To clarify, every 13-h night the civilization would have to beam 1.74 x 10^17 J/s x =46800 s = 8.12 x 10^21 J.
    A gigaton of TNT is 4.184×10^18 J, to this total is nearly 2000 gigatons of TNT. (EVERY NIGHT)

    If anybody can show me where I’ve gone wrong, I will thank you.

    • Alex Tolley April 6, 2016, 14:03

      I think the answer is that the coherent laser power diminishes much less than the incoherent stellar output. While the star’s output diminishes by the radius^2 of distance, the laser power is much more focused and therefore becomes relatively more powerful than the star’s output when observed from many light years away.

      If this wasn’t the case, we wouldn’t be worrying about signaling our presence by radio or laser emissions as their power is insignificant compared to our sun’s over the earth.

      I think equations 1 – 5 in the Kipping and Teaching paper are the relevant to study.

      • djlactin April 6, 2016, 15:57

        A signal from a coherent laser would be exactly the opposite of hiding.

    • Ron S April 6, 2016, 14:21

      The problem is you’re comparing apples and oranges. Apples are insolation per square meter on the planet and oranges are the flux differential between an obstructed and obstructed stellar disk seen at infinity. We want only oranges.

      Take the fraction of the area of the stellar surface reduced by the transiting planet. Calculate the flux at some selected astronomical distance. Replace the lost flux. Walk it back to the planet to determine the power and beam width to equal that flux.

      • Alex Tolley April 7, 2016, 12:52

        A much clearer explanation than mine.

  • Michael T April 6, 2016, 20:45

    This cloaking system assumes you know where the potential observers are. But what if there are several or many potential observers in different directions? This could enormously increase the effort and energy required, requiring many laser systems. The very concept of cloaking, taken to its logical conclusion, implies that you couldn’t trust ANY system with any chance of life on it. So you’d have to conceal yourself from hundreds, even thousands of systems, depending on your degree of paranoia!

    • Michael T April 9, 2016, 22:21

      To clarify, “ANY system” refers to any close enough to the plane of the Ecliptic to observe us transiting, which could still be many systems.

  • djlactin April 7, 2016, 1:16

    ” Apples are insolation per square meter on the planet and oranges are the flux differential between an obstructed and obstructed stellar disk seen at infinity.”

    Please explain how these could be different. A transiting planet (e.g., Mercury as seen from Earth) is seen (oranges) as a black spot, because it has blocked (apples) all light that would have passed through the area that it currently occupies. For Mercurians to hide from us they would have to replace that flux.

    (Also, to replicate the Sun’s black-body spectrum when doing so.)

    • Ron S April 7, 2016, 15:14

      Draw yourself a diagram to visualize the geometry. It doesn’t have to be to scale, only be sure you draw the planet smaller than its star. Draw a line from each side of the stellar disk that graze the planetary disk, and keep going to infinity. You will see that the radiation the planet blocks would diverge to a large solid angle if the planet were not there. Only a tiny fraction of that will impinge on the observer’s planet (or telescope). It is only that fraction you need to replace.

      Of course as mentioned many times in the comments here and the previous article that “replacing” this light is a poor solution to the general problem since it only addresses one observer.

      • djlactin April 7, 2016, 23:41

        “It is only that fraction you need to replace.”

        I wrote a rebuttal but then realized that you present it yourself in the last sentence.

        I think we can agree that the strategy is not the very realistic.

  • Jim Strom April 7, 2016, 1:35

    Upfront admission: didn’t read the paper. That said. If transit sensors of putative alien civilizations are highly sensitive, wouldn’t the hypothetical cloaking laser need to vary it’s intensity for all possible remote observation angles and distances? Seems ridiculously difficult, considering the variables and also the need to “broadcast” non-stop.

  • Mephane April 7, 2016, 5:00

    I think more likely than achieving the goal of cloaking the planet, the laser would serve as an indicator that this planet is indeed inhabited. One would just have to perform a spectral analysis of the neutralized light curve to see the anomaly that light is blocked in all but one wavelengths, and by then ignoring this single wavelength, the original light curve is revealed.

    Not only would the observer now still know about the planet, they would also assume that there is indeed a civilization there to create the anomaly that only in a single wavelength the light curve is neutralized.

  • Harry R Ray April 8, 2016, 9:30

    To top everything off, a NEW preprint is up on the exoplanet.eu website. “The Search For Directed Intellegence” by Philip Lubin. What it all boils down to is this: We should not only be looking for PULSED signals, but we also should be looking for CONTINUOUS OUTPUT from phased laser arrays. CASE IN POINT: The Bouquet OSETI observatory found NO pulsed signals coming from KIC8462852, but is it even EQUIPPED to look for continuous laser leakage? Can anybody help me out here?

  • Ron S April 11, 2016, 20:42

    “The scenario could be that they would have long ago observed the Earth as an inhabited planet, and then turned on a cloak as a insurance policy, buying them time to reveal their presence when they choose to, rather than our increasingly penetrating telescopes finding them before they wish.”

    With all respect to Dr. Kipping this is poor reasoning. Any advanced civilization that truly poses a threat would see right through this ploy due to the imperfection of any such cloak. Indeed the situation would be made worse. Now they would not only see the planet but know that an intelligent species is present. Better to remain silent. At least then there would only be an ambiguous bio-signature that alone would not spur into rapid action.