Presentation #233.02 in the session Technology, Policy, and the Search for Life.
It has been hypothesized that advanced technological civilizations will construct giant space colonies and supporting infrastructures to orbit about their home stars; these have been dubbed “Dyson spheres”. With data from recent satellites that operate at infrared and optical wavelengths (Spitzer, WISE, TESS, Kepler), in company with a few modest assumptions, it is now possible to begin to constrain observationally the frequency of such space-based civilizations in our Milky Way Galaxy. A Dyson sphere could be detected via the total infrared emission produced by the ensemble of structures that orbit a white dwarf. Detection could also be achieved by observing a dip in the light from a white dwarf as one of the structures in the Dyson sphere passes between the star and our telescopes. For detection at the current state of the art, the total infrared luminosity of the aggregate of all orbiting structures must be at least 0.1% of the bolometric luminosity of the white dwarf but does not depend on the size of the largest orbiting structure. For transit detection there must be at least one structure of diameter ~1000 km or larger (i.e., comparable to the size of Ceres), but the total ensemble luminosity can be much smaller than is required for a detection of excess infrared emission.
At present there is no unambiguous detection of a Dyson sphere by either technique. But, by incorporating Kepler survey results on planets in the habitable zone of stars, one can now ask: what fraction of potentially habitable planets that orbit solar-type stars spawn living organisms that eventually evolve to a technology that then constructs a Dyson sphere with fractional infrared luminosity at least as large as 0.1%? From existing white dwarf studies we estimate that this fraction is at most 3%. Similarly, with a few modest assumptions, it is also possible to place an upper limit on “N”, the total number of technological civilizations in the Milky Way galaxy; this limit for N lies between 100 million and 1000 million depending on what types of stars spawn technological life.