The best evidence for extraterrestrials may be their massive engineering projects

The classic approach to the scientific search for extraterrestrial intelligence (SETI) is to scan the skies for radio transmissions from intelligent civilizations. While we definitely won't hear anything if we don't listen, SETI Institute senior astronomer Seth Shostak urges us to also keep our eyes (and sensors) peeled for another kind of alien technosignature: alien megastructures, "massive engineering works that an advanced society has constructed somewhere in space." We haven't found one yet but the possibility made headlines several years ago when a team of astronomers led by Tabetha Boyajian described a star that periodically dims in a very odd way. Shostak writes:

One explanation was that the star was surrounded by a Dyson sphere. The idea, proposed years ago by physicist Freeman Dyson, is that really advanced aliens would construct a gargantuan, spherical swarm of solar panels in orbit beyond their own planet sort of the way you might cup your hands around a candle to collect the heat. The swarm would gather enough starlight to energize the aliens souped-up lifestyles, and could sometimes get in the way of light from the star, causing it to intermittently dim as seen from afar.

That explanation for Tabbys star seems less likely today. Astronomical measurements show that it gets redder when it dims, suggesting that its surrounded by naturally produced dust, not a gargantuan group of light collectors.

But its reasonable to believe that Dyson spheres exist somewhere. In the past, astronomers looked for clues to such massive engineering projects by trawling star catalogs for systems that show an excess of infrared light produced by the warm backside of the panels.