If there really are advanced alien civilizations out there, you’d think they’d be easy to find. A truly powerful alien race would stride like gods among the cosmos, creating star-sized or galaxy-sized feats of engineering. So rather than analyzing exoplanet spectra or listening for faint radio messages, why not look for the remnants of celestial builds, something too large and unusual to occur naturally?
The most common idea is that aliens might build something akin to a Dyson sphere. In their need for more powerful energy sources, an advanced civilization might harness the entire output of a star. They wrap a star within a sphere to capture every last photon of stellar energy. Such an object would have a strange infrared or radio spectrum. An alien glow that is faint and unique. So astronomers have searched for Dyson spheres in the Milky Way, and have found some interesting candidates.
One major search was known as Project Hephaistos, which used data from Gaia, 2MASS, and WISE to look at five million candidate objects. From this they found seven unusual objects. They appear to be M-type red dwarfs at first glance, but have spectra that don’t resemble simple stars. This kind of star-like infrared object is exactly what you’d expect from a Dyson sphere. But of course extraordinary claims require extraordinary evidence, and that’s where things get fuzzy.
Almost immediately after the paper was published, other astronomers noted that the seven objects could also be hot Dust-Obscured Galaxies, or hotDOGs. These are quasars, so they appear star-like, but are obscured by such a tremendous amount of dust that they mostly emit in the infrared. And their spectra can be quite different from a M-type star. So the challenge is to distinguish between a hotDOG and a Dyson sphere. Which is where a new paper on the arXiv comes in.
Rather than trying to specifically distinguish between the two, the authors instead look at the distribution of known hotDOGS. They found that statistically about 1 in 3,000 quasars are of the hotDOG type, so that a broad search for Dyson spheres would likely include some dusty quasars. The authors go on to note that any civilization powerful enough to build star-scale structures would also have the ability to obscure their infrared signal. We can’t simply assume that aliens would build a Dyson sphere in such an obvious way. Overall, the authors argue, the seven candidate superstructures can be accounted for by hotDOGs and other phenomena, thus there is currently no clear evidence for alien superstructures.
Reference: Suazo, Matías, et al. “Project Hephaistos–II. Dyson sphere candidates from Gaia DR3, 2MASS, and WISE.” Monthly Notices of the Royal Astronomical Society 531.1 (2024): 695-707.
Reference: Blain, Andrew W. “Did WISE detect Dyson Spheres/Structures around Gaia-2MASS-selected stars?.” arXiv preprint arXiv:2409.11447 (2024).