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.”