Making contact in the Transit Zone

Anyone who has followed the goings-on in the SETI community over the past 10 or 20 years will know that there’s been a big turf war raging between those interested in beaming messages into space to try and contact extraterrestrial life, and those who worry that by revealing our presence to the cosmos we could be placing ourselves in jeopardy. At times, the arguments have reached fever pitch and neither side has really shown a willingness to budge on their position.

Yet, when calmer heads prevail, there’s some interesting debate to be had, new ideas to be voiced, and incrementally, advances in the way we think about SETI and the potential consequences of contact.

Which brings me to a new paper from Eamonn Kerins of the Jodrell Bank Centre for Astrophysics at my alma mater, the University of Manchester. Kerins picks up on an idea that has been floating around the SETI community for a while now – that we may be detectable to any technological extraterrestrial life on a planet orbiting a star in what we call the Earth Transit Zone.

Our transit signature

A planetary transit occurs when a planet moves in front of its star, temporarily blocking some of the star’s light light. We see Mercury and Venus – planets with inferior orbits to Earth’s – occasionally transit the Sun. We can also detect the dip in light when exoplanets transit their stars, but the alignment between those systems and ourselves has to be more or less edge-on for us to see the transit.

The dip in a star’s light as a planet moves across – transits – it is enough to reveal the presence of that planet.
Image: NASA Ames.

Similarly, it would be possible to see Earth transiting the Sun from a star system aligned with the plane of our own. In practice, this means on a planet that orbits a star that lies in the ecliptic plane on the sky. This is the Earth Transit Zone, from where transits of our planet are visible.

A transit of the Sun by Earth wouldn’t just reveal the existence of our planet. Transmission spectroscopy – the Sun’s light shining through our atmosphere and being absorbed by molecules  – could reveal the gases in our atmosphere that make Earth habitable, or even betray the presence of pollution from our factories. This could be enough for any technological aliens to take a closer look at our world, perhaps imaging it from afar, detecting our city lights or vegetation, or even listening for our radio chatter.

The SETI Paradox

Those already motivated to beam messages into space argue that this is all the justification they require. If ET can tell that we are here, what’s the point in trying to stay hidden?

More supposed justification for METI (the practice of Messaging ExtraTerrestrial Intelligence) has come from radio astronomer Alexander Zaitsev of the Russian Academy of Sciences, who has been one of the leading proponents for METI since the late 1990s. He has transmitted several deliberate signals into space using the RT-70 radio telescope and planetary radar at Yevpatoria in the Crimea, and has argued that it is imperative that we transmit to break the apparent ‘great silence’, because if everyone in the Universe is too scared to beam out messages, then SETI will never succeed. He called this the SETI Paradox, and to be fair, he has a point: why should we expect ET to do something that, right or wrongly, we are not willing to do ourselves?

In his new paper, Kerins has brought the concept of the Earth Transit Zone together with the SETI Paradox to show that there needn’t be a paradox at all, and to potentially show a way forward for both SETI and METI.

Kerins’ basic proposition is that the Earth Transit Zone allows for the possibility of mutual detectability. Technological aliens on a planet orbiting a star in the Zone will be able to see Earth transiting, and if their system is aligned favourably to us, we could see their planet transiting too. This creates the possibility for both parties to determine that the other party exists. Kerins posits that this plays up to a game-theoretic approach – the more that two civilisations know about each other’s environment and existence, the more likely they are to attempt communication with each other, which would have ramifications for Zaitsev’s SETI Paradox.

This model of mutual detectability is broken down by Kerins into four ‘laws’, which each form fascinating discussion points in their own right. So, let’s discuss them.

Law 1: Mutuality

This could be summarised as saying that contact between civilisations will be more likely should the odds be even that they can detect one another in the same fashion, in this case, through planetary transits. It takes two sides to partake in a conversation, and if only one side knows of the other’s existence, any communication isn’t going to get very far.

Of course, that is why we do SETI, to try and find evidence of that communication, but for much of the 60 years we’ve been doing SETI it has been very much like looking for a needle in a haystack. However, if two technological civilisations are mutually detectable, then that detection becomes more likely because they make for tempting targets on the sky for us to investigate.

Law 2: Symmetry

This builds upon the first law, wherein evidence of other’s existence doesn’t have to be only mutual, but also symmetrical. What does this mean?

SETI, and astrobiology in general, favours planets as the most likely places to find life, and this seems reasonable enough – after all, we live on a planet. Furthermore, SETI and astrobiology assume, in general, that life will live on a planet somewhat like Earth. If this is all true then we should be looking for planets like Earth, and aliens searching for us should be doing the same thing. [At this point let’s acknowledge there could be exceptions to this, for example space-faring civilisations not on a planet or even in a planetary system, or extremely technologically-advanced civilisations hanging out around black holes – see my earlier post on the subject.)

An artist’s impression of a potentially habitable exoplanet. Such worlds seen transiting in the Earth Transit Zone should be targeted by SETI searches. Image: ESO/M Kornmesser/Nick Risinger (

The Earth Transit Zone allows for detection to be both mutual and symmetrical, in that we should each be able to see the same thing: the transits of a planet. In this way, neither side will have an advantage over the other, and as Kerins writes, they’ll be able to evaluate the existence of each other using the same relative information.

Asymmetrical detection will not encourage mutual communication. The first known transiting exoplanet was not discovered until 1999 (for anyone interested, that planet was HD 209458b, which is a hot jupiter orbiting a star 159 light years away, and subsequently one of the best-characterised exoplanets), but astronomers had been conducting SETI for 39 years prior to that discovery. Before 1999 we wouldn’t have known where to look for a transiting planet, whereas a civilisation in the Earth Transit Zone could have seen our planet and known there was life here long, long before. Their attempts to contact us would have fallen on deaf ears brought about by our ignorance of their existence.

Law 3: Opportunity

The information that both parties could gain from their mutual and symmetrical evidence for each other is described by Kerins as ‘Common Denominator Information’, or CDI for short. For mutually symmetrical evidence to work, it has to fall under the umbrella of CDI – information that is as independent of technology as possible, making it equally accessible to both parties so that both have the opportunity to discover that evidence, irrespective of their relative technological levels. So, to pinch a science-fictional idea, suppose technologically-advanced aliens have wormholes that they can open up close to Earth through which to observe us; since we would not be able to detect those wormholes, or create our own, then we would not have the same opportunity to detect them as they could detect us.

In the strictest sense, planetary transits don’t necessarily fall into the category of CDI either – as we mentioned above, prior to 1999 we did not have the ability to detect transiting planets, so that information was dependent upon our technology of the time. Still, one must place limits somewhere. 

Naturally, technology will come into it. The James Webb Space Telescope (JWST), when it finally launches, will be able to characterise the atmospheres of nearby rocky exoplanets, and perhaps be able to detect biosignatures such as water vapour and oxygen, but not even the JWST could directly image and Earth-sized planet in the habitable zone of its star, and there are currently no telescopes on the drawing board that will be able to in the foreseeable future. That technology may come closer to being realised by the middle of this century, but without it we are clearly at a disadvantage compared to a technological extraterrestrial civilisation that has better telescopes than we have, so they might already know we are here. Still, if they are patient and willing to wait for us to catch up, we might have access to the same CDI as they do by the second half of this century. 

Regarding exoplanetary transits, the strength of the signal, and therefore the strength of the CDI, depends on the size and luminosity of the star, the size of the exoplanet, and the brightness of the star. The smaller the size difference between the star and the planet, the greater the fraction of light a planet will block. So, for example, an Earth-sized planet will block more light from a small M-dwarf (red dwarf) star than it will for a star the size of our Sun. This is one of the reasons why exoplanets orbiting M-dwarfs can be somewhat easier to detect – the CDI would be greater – but only out to a certain distance, beyond which the M-dwarf’s light will be too faint to notice any appreciable dip. Faintness is also an issue for transmission spectroscopy – it is easier to identify the absorption lines of molecules in a planetary atmosphere when the light of the star shining through that atmosphere is brighter, since the spectral lines in the light of a faint star will be difficult to pick out.

I’ve fully concurred with everything Kerins has proposed so far, and am happy that he places such emphasis on the idea that we should gather evidence, through observation, of another civilisation’s existence before making contact. This is the very same conclusion that I arrived at in my book, The Contact Paradox, to describe a way out of the SETI Paradox without recklessly shouting into the jungle before we know what’s out there. Let’s identify the potentially habitable planets, let’s characterise them carefully, let’s listen intently and search for any signs of technosignatures (including city lights). As our telescopes improve and grow ever larger, our range in to the Galaxy will increase. If and when we find someone, let’s take the time to learn all we can about them from afar, eavesdropping on them if you will. The more information we can gather, the better the chance that we can make an objective decision about whether it is safe to initiate contact with them.

However, Kerins’ final law of mutual detectability is where we’re going to disagree a bit, because I think the situation is far more complicated than he depicts. I’ll explain why in a moment, but first, what is his fourth law?

Detection of technosignatures, such as city lights, could form part of the Common Denominator Information. Image: David A Aguilar (CfA).

Law 4: Superiority

The imbalance mentioned above in the discussion about opportunity, resulting from relative technological levels, could mean that one civilisation has greater – or superior – CDI than the other civilisation. Therefore, writes Kerins, the onus falls on the superior party to initiate communication.

(Kerins goes to lengths in his paper to emphasise that the use of the word ‘superior’ relates only to the amount of information a civilisation has gathered about another civilisation, and should not be taken as commentary on the socio-cultural merits of any given society).

Kerins’ approaches the problem from the perspective of game theory – using mathematical models to analyse and determine logical courses of action for problems in which decisions by involved parties are interdependent. I don’t think there’s anything wrong with approaching it from this perspective, but I don’t think the decision about whether to make contact should rely solely on the amount or quality of the information that we have, but also on our value judgement of what we should make of that information.

To take an extreme example, suppose we find evidence that the extraterrestrial civilisation that we’ve discovered is violent. Maybe we’ve detected radioactive materials in their planet’s atmosphere that can be directly traced to nuclear weapons, or if we can image and resolve details on the surface of their planet, maybe we’ve even seen the flash of a nuclear detonation. We may then decide we want to steer well clear of that particular civilisation regardless of the superiority of our CDI. (This continues to be a narrative today in regards to one proposed explanation for the Fermi Paradox, which is that ET has seen us using our nukes and has decided we’re too dangerous to bother with – either that or they’ll send Klaatu.)

Furthermore, transmitting across interstellar distances is expensive. Even if we can find a planet harbouring an extraterrestrial civilisation, it doesn’t mean they’ve definitely detected us – they might not even be looking, or haven’t been looking long enough for evidence of our technological existence to have travelled across the stars at the speed of light to them. Vice versa, any civilisation beaming radio signals as us prior to the beginning of modern SETI in 1960 (or frankly, after that, so sporadic has SETI been over the years) would have gone unnoticed. The upshot of this is that we may have to prepare to send signals over a long timeframe – five minutes here or there just wouldn’t do. We’d have to continuously send a signal to maximise the likelihood that those on the receiving end will detect it.

When it comes to discussing the cost of transmitting, I often quote a paper by John Billingham (the former head of NASA’s SETI project) and Jim Benford, in which they find that calling ET isn’t cheap. If we wanted to communicate directly with an extraterrestrial civilisation a thousand light years away, Billingham and Benford point out that the cost of building and powering a continuous radio beacon would be $6.8 billion (in 2010 money), and $1.2 billion for a pulsed beacon, and that’s assuming the signal will be detected straight away. If we have to keep sending for years, decades, or even centuries, the cost becomes astronomical. 

Of course, aliens won’t have the same economy we have, but the point remains that interstellar transmissions cost resources, be they in terms of time, energy or bank notes. A society unwilling to devote those resources into initiating contact will therefore decide not to do so, irrespective of how superior their CDI is.

And it’s quite possible that the civilisation with the inferior CDI might initiate contact first anyway, perhaps because they are feeling bold, or foolish, depending on your viewpoint!

The RT-70 planetary radar in the Crimea has been used to transmit signals into space, but should we wait until we’ve found a mutually detectable civilisation? Image: S Korotkiy CC BY-SA 3.0.

Having a superior CDI would put a civilisation in the best position to decide whether they should initiate contact, but it shouldn’t alone dictate whether they will make contact. Just because we know where ET is would not be sufficient justification on its own to make contact. There’s also a case for saying the more technologically advanced civilisation, or at least the civilisation with the greatest resources at its disposal, should do the heavy lifting. Since we’re likely to be the new kids on the block, the onus then falls on the other guys.

The search begins now

Regardless, finding extraterrestrial life via remote observation would be a great start to confronting these issues, and the Earth Transit Zone offers the ideal opportunity for mutual detection to provide the foundations for contact should we wish to go down that road. It seems others in the SETI community agree. As mentioned earlier, the idea has been floating around for some time. I first came across it through Richard Conn Henry of Johns Hopkins University, who proposed a search of the ecliptic plane at a 2008 meeting of the American Astronomical Society (you can read the abstract of Conn’s poster presented at the meeting here, and a write-up on the subject by the always excellent Paul Gilster at Centauri Dreams here).

More recently, Sofia Sheikh of Penn State University is making good on Conn’s proposition by conducting a radio SETI search of the Earth Transit Zone in conjunction with the multi-million dollar Breakthrough Listen project. There are plenty of stars to choose from: a new study by Lisa Kaltenegger of the Carl Sagan Institute at Cornell University and Joshua Pepper of Leiden University has identified in the Earth Transit Zone 1,004 Sun-like stars that are within 300 light years of the Earth. Only a fraction of those 1,004 stars will have exoplanets that can be seen to transit from our point of view, but for those few systems, mutual detectability is a possibility.

For 60 years SETI has stumbled along, mostly performing sporadic or random searches, limited not by the imagination of its practitioners, but by funding and telescope time. Now, thanks to Breakthrough Listen, those limitations are being removed, and through insights such as those presented in Kerins’ paper, and via Sheikh’s Earth Transit Zone search, we have the beginnings of a new foundation for SETI – and METI – to build on that could ultimately see a more targeted hunt for ET pay huge dividends.


  1. Thanks for a great post – a really nice write-up of the ideas I discuss in the paper.

    I think I’d just like to add that the paper does not advocate that we only use game theory calculations to decide whether or not to transmit signals. I agree with you that other factors have to be considered and prioritised. In the event that we were to intercept an unambiguous signal then, in my view, it must be for society as a whole to decide how/whether to respond. The paper only looks at whether the incentive to send a signal is equal or not, where incentive is measured by a technology-independent measure of the strength of the signal that each observer receives about the other’s possible existence. It avoids the discussion of whether or not it is a good idea to actually send a signal, though I do cite two recent “for” and “against” discussions for those interested in the arguments.

    Regarding that controversial 4th law (“superiority” – or who should transmit), the reason for going with the civilisation with better CDI, rather than the civilisation with superior technology is that neither side knows who has the better technology but both can determine who has better CDI, so long as the CDI is framed as simply as possible based on the detection method used. The game theory behind it assumes that both sides are interested in contacting any civilisation that has mastered the ability to detect them using the relevant method (eg in this case the transit method), regardless of how advanced they are. I argue that the chosen method itself also needs to be simple in order to have as large an audience as possible – the more advanced the method used the fewer civilisations who will be able to use it. So even if there are civilisations who use more sophisticated methods that we are not yet able to use, I argue that their SETI search strategies should still be based on simple methods if they want to maximise the chances of making contact with someone (eg us). If, on the other hand, they are only interested in contacting civilisations as smart or smarter than they are, then they should use their more sophisticated methods to construct a more refined and optimal target list for their purposes.

    For a given strategy the chance of success (ie making contact) is maximised provided both sides stick to simple rules that both can understand. Neither side knows how advanced is the other, apart from the minimal requirement that they must be able to master the mutual detection technique being used. So the best strategy is to use CDI based on the simplest intrinsic (ie technology independent) signal that the technique yields. Both can then work out who sees the stronger signal and so has the best evidence to decide if it’s worthwhile to send a message. Of course, better still if both sides send messages to each other. But what we want to avoid is that standoff where neither sends a message because they’re both just listening to the other. If only one is to send a message then both sides would reasonably conclude that it should come from the civilisation that has access to better CDI.

    In any case, for potentially habitable transiting planets that lie within the Earth Transit Zone, I show in the paper that the game theory incentive almost always lies with others to message us. So for these planets at least, there is no incentive for us to send any message.

    Liked by 1 person

    • Hi Eamonn, thanks for your reply!

      I must admit, after years of reading and writing about this topic, I’ve kind of conditioned myself to look for complex answers to the messaging question. There really are so many factors involved, but I think where we both agree is that gaining more information would improve our decision-making process, which is why I really identified with your paper.

      I feel like I only scratched the surface of your paper in my write-up; there were lots of things in there worthy of discussion. For example, you showed that for civilisations around M-dwarfs, the onus would almost always be on them to transmit, and since there are so many M-dwarfs… but do we need to balance that with the question of whether M-dwarfs can even provide stable habitats given their tendency to flare, and would that mean we’ll find more civilisations orbiting more Sun-like stars, where the incentive to transmit may be more balanced between us and them?

      I also think it would be a useful exercise to draw up a list of all the different types of evidence that could form mutually detectable evidence, and then figure out what we would need to do to be able to detect all those different types of evidence, particularly with future telescopes. I find it interesting that we’ve only been able to detect the most basic of this kind of evidence – planetary transits – for the past 20 years, and only now are we reaching the stage where we can detect transits of Earth-sized planets orbiting Sun-like stars. But I think we all expect rapid advances in exoplanet detection and characterisation this century.

      I have my own thoughts about Zaitsev’s SETI Paradox, but I think I’ll save them for another time!


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