Hunting for Alien Cities: The Search for Technosignatures

In 2015, astronomers noticed a star called KIC 8462852 -- nicknamed 'Tabby's Star' -- dimming in strange, irregular patterns unlike any known planet or dust cloud. For a brief moment, some scientists asked seriously: could this be evidence of an alien megastructure built to harvest starlight? It turned out to be an unusual dust cloud, but the search that followed defined an entire new field: technosignature science.

- What a technosignature is and how it differs from a biosignature - Three real technosignature candidates astronomers actually search for - Why Dyson spheres are a serious (if unproven) hypothesis - How the Drake Equation estimates the odds of finding a signal

A biosignature is evidence of life in general, like oxygen or methane in a planet's atmosphere that could be produced by living organisms. A technosignature is more specific: evidence of technology built by an intelligent civilization. Radio signals, laser pulses, and industrial pollution in a planet's atmosphere would all count as technosignatures, because only a technological civilization produces them -- not just any life.

In 1960, physicist Freeman Dyson proposed that any sufficiently advanced civilization would eventually need more energy than its home star naturally provides to them on the planet's surface. His solution: build a massive structure -- a swarm of solar collectors, or a full shell -- around the star itself to capture nearly all of its energy output. If such a Dyson sphere existed, it would block or reshape the star's light in ways telescopes could detect, which is exactly the pattern astronomers checked for with Tabby's Star.

Some scientists search for chlorofluorocarbons (CFCs) in exoplanet atmospheres -- chemicals that, on Earth, are manufactured only by industrial processes like refrigeration, never produced naturally. Detecting CFCs on a distant planet would strongly suggest an industrial civilization exists there, since no known natural process creates them in large quantities.

In 1961, astronomer Frank Drake wrote an equation multiplying several factors together: the rate of star formation, the fraction of stars with planets, the fraction of those planets that could support life, the fraction where life actually develops intelligence, the fraction that develop detectable technology, and how long such a civilization broadcasts signals before going silent. The equation does not give one certain answer -- some values are pure guesses -- but it organizes exactly what we would need to know to estimate how many communicating civilizations might exist in our galaxy right now.