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Into the ‘Transit Zone’

Given how powerful the transit method has proven for detecting exoplanets, we can assume great things are ahead. It won’t be that many years before we’re actually analyzing the atmospheric constituents of worlds much smaller than the gas giants for which we perform such studies now. That would make it possible for us to discover possible biosignatures. As I’ve speculated in these pages before, it may well be that we discover life on a planet around a distant star before we manage to discover it — if it exists — elsewhere in our Solar System.


We’re looking at worlds around other suns with something of the same spirit that Carl Sagan and the Voyager team looked back from the outer reaches and saw the Earth as a ‘pale blue dot.’ It’s a comparison that René Heller and Ralph E. Pudritz draw in their recent paper on SETI strategy. Except here we’re talking about extraterrestrial observers looking at our planet, the assumption being that if we can make these studies using our current technologies, so can another species, and certainly one with tools that may be much improved over our own.

Image: Astrophysicist René Heller. Credit: Nick Kozak, for Science et Vie (2015).

A Thin Strip of Sky

Thus we get what the researchers are calling the Earth’s Transit Zone, or ETZ. It’s that region from which another civilization would be able to detect the Earth as a transiting planet in front of the Sun. Heller (Max Planck Institute for Solar System Research, Göttingen) and Pudritz (McMaster University, Ontario) analyze this thin region, a strip around the ecliptic projected out onto the galaxy. The entire ETZ amounts to about two thousandths of the entire sky, which is precisely why the authors like it. Says Heller:

“The key point of this strategy is that it confines the search area to a very small part of the sky. As a consequence, it might take us less than a human life span to find out whether or not there are extraterrestrial astronomers who have found the Earth. They may have detected Earth’s biogenic atmosphere and started to contact whoever is home.”

What the researchers provide is an extension of ideas that go back at least into the 1980s, and were discussed in a 1990 paper by the Russian astronomer L. N. Filippova, who presented a list of nearby stars near the ecliptic that would be good targets for SETI. A 2008 poster at the American Astronomical Society produced by Richard Conn Henry, Steve Kilston and Seth Shostak addressed the matter as well, making this case in its abstract:

…the best hope for success in SETI is exploration of the possibility that there are a few extremely ancient but non-colonizing civilizations; civilizations that, aeons ago, detected the existence of Earth (oxygen, and hence life) and of its Moon (stabilizing Earth’s rotation) via observations of transits of the Sun (hence, ecliptic, which is stable over millions of years…, and have been beaming voluminous information in our direction ever since, in their faint hope (now realized) that a technological “receiving” species would appear. To maintain such a targeted broadcast would be extremely cheap for an advanced civilization. A search of a swath centered on our ecliptic plane should easily find such civilizations, if they exist.


Image: Narrow band: This image illustrates the transit zone, in which distant observers could see the Earth pass in front of the Sun. Credit and copyright: Axel Quetz (MPIA) / Axel Mellinger, Central Michigan University.

But Heller and Pudritz don’t limit themselves to intentional communications of this sort. Whether it’s leakage radiation or directed signals, their intent is to present a rigorous geometric description of the Earth Transit Zone for SETI use, one that contains almost 100,000 stars, with a small sub-group of 82 nearby stars that can serve as an early target list. The authors point out that the PLATO mission, to be launched by the European Space Agency in 2024, will use the transit method to find small planets around many bright stars like those on the list.

“PLATO might even detect the transits of exoplanets, whose possible inhabitants would be able to see the Earth transiting the Sun,” Heller adds. “Such a crazy setup would offer both them and us the possibility of studying each other’s planets with the transit method.”

As to the size of the ETZ, the paper notes that the galactic disk has a width of some 600 parsecs at the Sun’s location. Bear in mind that the Solar System is tilted against the galactic plane by about 63,° which gives us an Earth Transit Zone whose path through the disk is about 1 kiloparsec long (3261 light years). Heller and Pudritz do not consider M-dwarfs, but place their focus on K and G-class stars. The paper describes the selection of the 82 priority stars:

By rejecting all F, A, and B stellar types we make sure that we only take into account stars with lifetimes long enough to actually host habitable planets for billions of years. A more sophisticated approach would make use of the stellar ages (if known) for the remaining K and G stars, as done by Turnbull and Tarter (2003a), as some of these targets might still be very young with little time for the emergence of an intelligent species. Nevertheless, most of these stars should be of similar age as the Sun since they are in the solar neighborhood of the Milky Way. The rejection of giants and subgiants finally leaves us with 45 K and 37 G dwarfs.

What we see in the Earth Transit Zone is a way of confining our SETI search to a high-priority region that runs as a 0.528° ribbon along the ecliptic, defining that place where extraterrestrial astronomers would be able to see non-grazing transits of the Earth in front of the Sun. Heller and Pudritz estimate the total number of K and G dwarfs within 1 kiloparsec inside the ETZ is about 100,000, with estimates for habitable zone terrestrial planets from among this number reaching as high as 10,000. SETI thus gets a highly confined search area within which to focus its attention as we look for signs that planets we can discover may have discovered us.

The paper is Heller, “The Search for Extraterrestrial Intelligence in Earth’s Solar Transit Zone,” Astrobiology Vol. 16, No. 4 (2016). Preprint available. See also this news release from the Max Planck Institute for Solar System Research. If you’re interested in digging into the early history of the ecliptic and ETZ concept, check the Filippova paper mentioned above, which is “A List of Near Ecliptical Sun like Stars for the Zodiac SETI Program,“ Astronomicheskii
1544:37 (1990). See also Filippova’s 1998 paper with V. S. Strelnitskij, “Ecliptic as an Attractor for SETI,” Astronomicheskii Tsirkulyar 1531:31.


Comments on this entry are closed.

  • Michael March 9, 2016, 15:11

    It might be premature to exclude late F types, they still have fair lifespans past the 4-5 billion year mark due to lower masses.

  • Andrew_W March 9, 2016, 16:38

    While it’s true that the power required to continually send a signal towards Earth would be extremely cheap for an advanced civilization, such a civilization would also not be challenged much by direct imaging of extrastellar planets.

  • sglover March 9, 2016, 17:47

    Echoing Andrew_W…. We are already on the cusp of doing large-scale extrasolar planet searches based on direct imagery. The strategy described here assumes that civilizations much more sophisticated than ours will be limited to techniques that we ourselves might not be bothering with in a few decades. It’s incoherent.

  • Mark_S March 9, 2016, 17:57

    Alien astronomers observing Earth transits, being able to predict transits would therefore take pains to observe during the transit periods and up the probability of us having an audience for that specific time. Do with it what you will; hide, or send a signal…eg with large shade/sails located further out. Interstellar shadow-puppet show anyone? Hide and seek maybe?

  • JonW March 9, 2016, 19:11

    Andrew_W: sure, such a civilisation could have discovered our planet by direct imaging or any other means–how does the fact this is not much of a challenge for them affect the possibility they could be transmitting a signal to us? I don’t think the article is suggesting they would be transmitting for any purposes that would be rendered obsolete by superior imaging capabilities.

  • kamal ali March 10, 2016, 4:36

    This makes sense to me: civilizations that are at a slightly-superior technical level as us (up to a few hundred years ahead) will , through direct-imaging, learn of an ever-increasing inventory of planets with biospheres that have high probability of life. They may also be able to estimate our system’s age and maybe detect markers of artificial life.

    My only fear is of that pesky last factor in Drake’s “equation”: L. Unless civilizations routinely reach millions of years during which they are technically capable and choose to be active, the probability that we are cotemporaneous with them and both civilizations are within a few hundred LY of each other seems small.

  • Michael March 10, 2016, 6:55

    I think it would be a mistake to leave out late F-type stars as the UV output may aid complex life by producing more oxygen earlier on. Although they are fewer in number they have larger HZ’s able to have many more earth’s in the zone at once.


    • Ashley Baldwin March 14, 2016, 20:06

      Jim Kasting’s 2010 book ( a strongly recommended read ) about how to make a habitable planet shows rather counterintuitively that on a planet with an O2 dominated atmosphere ,orbiting ( presumably in the habitable zone , be it further out than 1 AU) , a late F class star , the additional UV it produces splits a greater number of atmospheric O2 molecules into atomic oxygen ,much of which then combines with remaining O2 to form MORE ozone than say a planet orbiting a G class star like the Sun. So much so that the UV reaching ground level is significantly LESS than that of the Earth ! So UV is not the problem one might reasonably assume . With main sequence lives significantly in excess of the current age of the Sun, certainly late F class stars represent good potential for orbiting habitable planets . Sadly they are rarer than G stars and not common in the solar neighbourhood , though the JWST should be able to spectroscopically characterise any transiting planet found by either TESS or K2 within a few hundred light years .

  • Andrew_W March 10, 2016, 10:20

    JonW, my point is that I don’t see any sound reason to preferentially search the Earth transit zone for ET transmissions.

  • sglover March 10, 2016, 12:26

    the probability that we are cotemporaneous with them and both civilizations are within a few hundred LY of each other seems small.

    As in lightning strike or lottery ticket small. I.e., nothing to base a strategy on at all.

  • Astronist March 11, 2016, 6:20

    “…the best hope for success in SETI is exploration of the possibility that there are a few extremely ancient but non-colonizing civilizations; civilizations that […] have been beaming voluminous information in our direction ever since, in their faint hope (now realized) that a technological “receiving” species would appear.” – In other words, civilizations which both do and do not maintain an outward-looking technological culture.

    The entire SETI project is premised upon either the existence of a state of society which, given current evidence, is fundamentally implausible, or upon a highly unlikely coincidence in development of the first two industrial civilizations to evolve in the Galaxy.

    We would do better trying to resolve the century-old question of whether there really is life on Mars!

    Oxford, UK

  • Wojciech J March 11, 2016, 7:49

    “…the best hope for success in SETI is exploration of the possibility that there are a few extremely ancient but non-colonizing civilizations; civilizations that, aeons ago, detected the existence of Earth (oxygen, and hence life) and of its Moon (stabilizing Earth’s rotation) via observations of transits of the Sun (hence, ecliptic, which is stable over millions of years…, and have been beaming voluminous information in our direction ever since”

    A civilization so long lived, continuously sending high energy signals in hope somebody receives them? This would imply a colossal project, requiring constant upkeep and attendance to machines through millions of years.
    The costs of such endeavour would seem higher than sending an interstellar probe, especially as such civilization would be able to directly image planets with potential for life(if not signs of life itself). I wouldn’t therefore consider it likely situation, although there is no harm in pursuing such search.
    As others above noted, the chance of civilization existing at the same time as ours with similar level(or slightly above) of development is almost impossible statistically.

    • Harold March 11, 2016, 12:07

      The cost of broadcasting a signal could fall to a near zero percentage of civilization’s GDP. That scenario only involves assuming more advanced technology and does not demand that a civilization would need to choose between probes and broadcasting. Imo, it would be good strategy to gather intelligence about a system with discreet probes within the system but initiate communication from several systems away.

      The Milky Way is more hospitable to life than it was 1,2,3 billion years ago and continues to become more hospitable as planet sterilizing events become less frequent. The probability of multiple technological civilizations coexisting should be increasing.

      • Eniac March 13, 2016, 17:54

        What planet-sterilizing events? There hasn’t been a single one around here in 4 billion years, how could they be getting less frequent?

  • Steve Muise March 11, 2016, 11:26

    The bottom line is that this type of SETI search is still dependent upon the senders being willing to send easily detectable signals. It just narrows the search area but makes the assumption that the sender both knows that Earth exists and is found to be interesting enough to beam signals at for an extended period of time.

    If I were to be stranded on a desert island and I knew that there was the possibility that cannibals could be lurking the absolute LAST thing I would do would be to light a signal fire, and the second to last thing I would do would be to respond to a potential signal fire.

    Given the assumption that potential signal-sending civilizations are long-lived that would imply that they would be vulnerable to potential hostile civilizations; being a long-lived civilization would mean that threats that would be restricted to traveling at some percentage of C would be a realistic fear.

  • Michael March 11, 2016, 14:47

    There is a passive way to communicate our presence to the whole galaxy through this transit method if we built large spheres enclosing habitats. We could also have them orbit in prime numbers so there will be little doubt to there artificial creation.

  • Michael Simmons March 14, 2016, 1:29

    I had this idea along time ago.
    I’m glad someone did a paper on this.
    Its a pretty obvious thing to do.
    If someone is going to be beaming signals at us, then they need to know we are here.
    With advanced technology they could already have a good idea their is life on earth.

    For SIMBAD the query to run is
    plx > 16.308 & region(ZONE ecl,0.0 +0.0,360.0d 27.98m)
    It returns 47 stars of which 16 are G type.

    I made several posts on cosmoquest http://cosmoquest.org/forum/archive/index.php/t-126869.html
    And on setiquest

    Someone should check out LHS 121 imo

    • Michael Simmons March 15, 2016, 3:41

      The other thing about this, is that the alien intelligence might only send a burst message timed to arrive when the earth is in the middle of a transit. This solves the when to listen problem that would otherwise require a extremely high powered dedicated transmitter just for the earth.