Beyond Radio Signals: How Scientists Use Technosignatures to Hunt Alien Civilizations

In 1977, a radio telescope at Ohio State University picked up a 72-second signal so striking that the astronomer reviewing the data wrote "Wow!" in the margin. It was never heard again. For the better part of six decades, the search for extraterrestrial intelligence ran on one assumption: if another civilization exists, it will broadcast radio waves, and we will catch them. That assumption was not wrong, exactly. It was just narrow.
The field has a name for the broader approach now: technosignatures. A technosignature is any detectable marker of technology, not biology, not chemistry, but the specific fingerprint of a civilization that builds things. The shift matters because it opens the search to every instrument astronomers already use, from space telescopes measuring atmospheric chemistry to infrared observatories scanning for waste heat. The aliens do not have to be calling. They just have to exist.
Biosignatures are the chemical signs of life: oxygen, methane, ozone, dimethyl sulfide. NASA's James Webb Space Telescope, launched in December 2021, is already capable of reading the atmospheric chemistry of exoplanets through a technique called spectroscopy, splitting the light that passes through a planet's atmosphere into a spectrum that reveals which molecules are present. If a planet shows oxygen and methane together, that combination is chemically unstable without a biological source constantly replenishing it. That is a biosignature.
Technosignatures go further. Nitrogen dioxide, NO2, is produced in tiny quantities by lightning and wildfires. It is produced in large quantities by internal combustion engines and industrial furnaces. A 2022 study published in Nature Astronomy, led by researchers including Ravi Kopparapu at NASA's Goddard Space Flight Center, calculated that the James Webb Space Telescope could detect industrial-level nitrogen dioxide pollution on an Earth-like exoplanet within about 400 light-years, given sufficient observation time. The logic is clean: if the NO2 reading is too high for natural processes to explain, something unnatural is producing it. Spectroscopy becomes, in that reading, a pollution detector aimed at other worlds.

Every machine wastes energy as heat. A civilization that builds at scale, cities, factories, data centres, spacecraft, radiates that heat into space. Freeman Dyson, the physicist, proposed in 1960 that a sufficiently advanced civilization might build a structure around its star to capture all of its energy output. The waste heat from such a structure would glow in the mid-infrared, at wavelengths no natural stellar process produces in that pattern. Searches of infrared survey data from the WISE telescope, which mapped the entire sky in infrared, have looked for this signature. No confirmed Dyson sphere has been found, but the search identified several anomalous stars worth continued study, including the star KIC 8462852, known informally as Boyajian's Star, whose irregular dimming still lacks a fully satisfying natural explanation.
You do not need a Dyson sphere for the logic to hold. Any civilization producing significant industrial output radiates detectable infrared. The sensitivity of current and next-generation infrared observatories keeps improving. The Roman Space Telescope, planned for launch later this decade, will survey wide fields with a sensitivity that makes earlier infrared searches look preliminary.

Perhaps the most precise technosignature candidate is chlorofluorocarbons. CFCs have no known natural source. On Earth, they were synthesised in the 1920s as refrigerants and propellants, and their presence in the atmosphere is entirely industrial in origin. A planet showing CFC absorption in its spectral signature would be, by definition, a planet with chemistry that nature does not produce. Sara Seager at MIT and her collaborators have catalogued dozens of molecules in this category, compounds that biology does not make and geology does not make, but technology does.
The James Webb Space Telescope cannot detect CFCs at interstellar distances with current sensitivity. But the concept defines what the next generation of extremely large ground-based telescopes, the Thirty Meter Telescope and the Extremely Large Telescope, might be capable of within the coming decades. The detection method already exists. The question is aperture and time.

ISRO's ambitions have so far centred on the solar system: Mangalyaan reached Mars in 2014, Chandrayaan-3 landed near the lunar south pole in 2023, and Aditya-L1 is studying the Sun from the L1 Lagrange point. These missions look for biosignatures and geological data within reach. The technosignature search operates at interstellar distances and requires the kind of space-based spectroscopy infrastructure that currently sits with NASA, ESA, and a handful of large ground observatories.
That gap is not permanent. India's astronomical community participates in international collaborations, and the Giant Metrewave Radio Telescope near Pune, one of the world's largest radio telescope arrays, has contributed to SETI-adjacent research on fast radio bursts and anomalous signals. As the field of technosignature science formalises, the instrumentation question becomes as important as the theoretical one. Detection depends on who has the telescopes pointed in the right direction at the right sensitivity.
The deeper shift the technosignature approach represents is not just methodological. For most of the history of SETI, the search assumed civilizations would be communicating, that intelligence, if it existed, would be reaching outward. Technosignatures make no such assumption. A civilization that stopped broadcasting centuries ago, or never broadcast at all, still leaves chemical and thermal marks on its planet's atmosphere. The search stops depending on intent and starts depending only on existence. That change in premise is what makes a nitrogen dioxide reading on a planet 300 light-years away the same kind of evidence as a radio pulse, and in some ways, a more honest one.