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Using the Transit Method for SETI Detections

Every study using transit methods to detect objects around other stars is looking for planets. But a paper by Luc Arnold (Observatoire de Haute-Provence, France), soon to be published in The Astrophysical Journal, suggests that the same methods could be employed to find artificial planet-sized objects in orbit around stars. Arnold sees this as a possible SETI ploy, for transits of multiple objects could be used to emit signals that might be detected by other civilizations.

What would such objects be? Giant solar sails, perhaps, or huge low-density structures of other configuration built purposely as a means of interstellar communication. Arnold’s work inevitably recalls Freeman Dyson’s 1960 Science article “Search for Artificial Stellar Sources of Infrared Radiation,” which developed the idea that would later be known as a Dyson Sphere, an artificial cluster of rotating objects the size of a planetary orbit that would collect almost all the solar energy available and create a vast habitat for life.

But Arnold falls back on Jill Tarter’s suggestion that advanced technologies might try to send signals that would be discovered by other civilizations in the course of their normal astronomical observations. So he is developing a new spin on ‘optical SETI,’ while noting that the light-curves of objects from spheres to triangles and even more exotic shapes will have their own distinctive signature, even as multiple objects could send a ‘message’ whose timing and number would announce the willingness of their makers to communicate.

Upcoming missions like Kepler and the European Corot may be able to detect such objects as they look for planetary transits. “Transit of artificial objects also could be a mean for interstellar communication from Earth in the future,” Arnold concludes. “We therefore suggest to future human generations to have in mind, at the proper time, the potential of Earth-size artificial multiple structures in orbit around our star to produce distinguishable and intelligent transits.”

A preprint of Arnold’s paper “Transit Lightcurve Signatures of Artificial Objects” can be accessed at the ArXiv site.

The study of Dyson Spheres remains intriguing. None has as yet been observed (the paper to consult is Bradbury, R.J. “Dyson Shells: A Retrospective,” which appeared in The Search for Extraterrestrial Intelligence (SETI) in the Optical Spectrum III, 2001 Proc. SPIE Vol. 4273, pp. 56-62). But it is also true that some stars do display an excess of infrared that has not been explained. The most likely cause is a hitherto unknown natural process, but the data also fit a possible Dyson signature.

Dyson sphere visualized

Image: A Dyson Sphere would consist provide a vast amount of habitable space, while taking advantage of almost all the solar energy available. Credit: Steve Bowers.

That work was done at the University of California at Berkeley. Charles Conroy (working with SETI@Home chief scientist Dan Werthimer) determined that a Dyson Sphere would radiate with an excess temperature of about 300 degrees Kelvin, which would translate to surplus radiation at the 12 micron wavelength. Using a list of candidate stars, each one billion years of age or older (those whose protoplanetary disks would have dissipated, thus eliminating a possible source of excess infrared), Conroy found 33 stars whose infrared radiation seemed excessive in the 12 micron range.

Followup studies using the SETI resources at Berkeley yielded no unusual radio emissions or light signals, leaving the mystery of the excess infrared unsolved. You can read more about Conroy’s work with Dyson sphere candidates in this article by Amir Alexander.

The original paper on these objects is Dyson, F.J. “Search for artificial stellar sources of infrared radiation,” Science 131, pp. 1667-1668 (3 June 1960). The Bradbury paper mentioned above, “Dyson Shells: A Retrospective,” offers refinements to the Dyson concept, an analysis of earlier work, and extrapolations on new signatures for optical SETI study. Bradbury is particularly valuable in discussing the distinction between an all-encompassing Dyson ‘sphere’ and a Dyson ‘shell.’

Maybe the term ‘swarm’ is even better: In a later letter to Science, Dyson noted that his concept of a ‘sphere’ had been misunderstood: “The form of ‘biosphere’ which I envisaged consists of a loose collection or swarm of objects traveling on independent orbits around the star. The size and shape of the individual objects would be chosen to suit the inhabitants. I did not indulge in speculations concerning the constructional details of the biosphere, since the expected emission of infrared radiation is independent of such details.”

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  • ljk June 4, 2007, 17:49


    Art for E.T.’S Sake

    By Stuart Atkinson

    National Space Society

    posted: 31 May 2007
    06:18 am ET

    For a long time there was an assumption in SETI circles that when Contact came it would be through the detection of an unambiguous radio signal, a clarion call from the depths of space that would be unmistakably alien. SETI scientists and enthusiasts alike maintained that an ET signal will be both simple and easy to identify as being extraterrestrial in nature. Now we realize it might not be that straightforward. ETs might be attempting communication via lasers, or holograms, or some other way we can’t even imagine. We’re looking for a needle in a haystack whilst wearing a blindfold and boxing gloves.

    While the best case scenario would be for our first ET signal to be a Sputnik-like beeping from the depths of space, telling us how far we have to turn the galactic radio dial before we hear the clear tones of “Voice Of the Milky Way”, it’s more likely that the transmission will contain a LOT of information which will be hard to decode, especially if that information wasn’t created and transmitted with the purpose of making contact, but ‘leaked’. Earth leaks signals out into space all the time, and the radio ripples spreading away from us carry not only entertainment programs but documentary-style factual programs too. Wildlife documentaries, natural history features, gardening shows, cookery shows, sports events, all of them carrying a wealth of accurate information for any ETIs who stumble upon them.

    Might alien civilizations be “leaking” too? If they are, then we should cross our fingers that there are ET equivalents of “National Geographic” specials heading towards us, packed with useful information, instead of their versions of “Big Brother”. What a disaster that would be…

    But what about programs concerning art? Art contains lots of confusing and conflicting signals. Art is subjective and very personal. And although many forms of art are self explanatory, realistic and easily interpreted, so-called modern art, with its geometrical patterns, chaotic curves, random patterns and psychedelic swirls and whorls of color, take some figuring out.

    The odds against us stumbling across an ET episode of “Art Today” are ridiculously high, but not impossible. Perhaps a civilization might, after millennia spent refining its sciences, value art more highly, so highly they felt a desire to share their artistic achievements with the rest of the Galaxy, (and preserve them in the process too, of course). Such a civilization might broadcast the contents of their galleries, scattering them among the stars like confetti, distributing them like a cosmic form of “shareware”…

    But one civilization’s art could be mistaken by another for scientific images, graphical representations of scientific concepts, or illustrations from some advanced physics textbook, and vice versa. Would we recognize – and/or aesthetically appreciate – any incoming examples, whole or fragmented, of ET art? And it works both ways: what would an ETI make of Picasso’s works, when only a very few of us here on Earth have a clue what they’re meant to represent? And are we really sure that ETs will correctly interpret those pulsar maps on the sides of our Voyagers and Pioneers, or will they just think “Hmmm, interesting composition, but too abstract for me..”?

    The nature of ET art will be dictated by their range of senses, their environment, their evolutionary path, psychology and physiology. Of course, they could create familiar-looking compositions and provide us with stunning landscapes and portraits of the living things they share their corner of the Galaxy with, but it’s more likely that the art created by ETs might be so different to our own that it would be unrecognizable as art. What sort of art might a mechanical ET produce? Would they rejoice in perfect design, see beauty in purely functional forms and shapes, have only disdain for soft lines and subtle, soft colors?

    Perhaps some ETs will be so advanced they might create art on a literally astronomical scale, manipulating astronomical objects or entire regions of space…

    It seems to me that ETs centuries or even millennia ahead of us would have so much power and so much energy at their disposal that the lines between art and engineering would eventually become blurred. As their artists strove to produce bigger and better works, they would need increasingly larger “canvases” for their works. Think of the evolution of our own art. Once we painted on cave walls, then discovered canvases, and how to sculpt stone. We then moved on to illuminating the sides of buildings with lasers and carving faces in mountainsides. Where next? Laser sculptures in the sky? Sculptures in Earth orbit? Images projected onto the Moon?

    Now put yourself in the shoes of an artist a thousand, ten thousand years ahead. What canvas is big enough for your ambitions and imagination…? How about deliberately crashing asteroids or comets into gas giants to create exotic and wonderful patterns in their clouds, to be enjoyed by millions watching the show from across your solar system..?

    Perhaps alien artists are painting with the very light of the stars themselves. Look at those breathtaking pictures of planetary nebulae taken by the Hubble Space Telescope, with all their multi-colored, intricately-structured shells, veils and curtains of starfire swirling around them. By interfering with the stars lurking inside such a nebula, maybe by dumping material on their surfaces, alien artists might be able to change the density and “gusts” of the solar wind shaping it, and in so doing manipulate the shape of the nebula into patterns and forms of their choosing. Would advanced civilizations be able to create epic-scale light-and-gas sculptures in this way?

    Think about it. How many times have you looked at a Hubble image and thought “That’s a work of art…”?

    Maybe you were right.

  • ljk February 20, 2008, 10:48

    Rearranging Stars to Communicate with Aliens

    A proposal to create special constellations that nature
    would never produce

    by Jaron Lanier

    This month I seriously propose that we begin the process
    of repositioning the sun and other nearby stars in order to
    send signals to aliens, and that we begin the search for
    signs that aliens might have done the same for our benefit.

    Full article here: