How Scientists Search for Intelligent Life Beyond Earth: SETI Methods Explained

Researchers looking for life beyond Earth split the effort into two distinct searches. One listens for artificial signals that an advanced civilization might send or leak into space, a field called SETI (Search for Extraterrestrial Intelligence). The other examines the chemistry of distant planets for traces of biology or industry, an approach astrobiologists call biosignature and technosignature science, according to NASA's Science Mission Directorate.

Listening for artificial signals: radio and optical SETI

The SETI Institute runs its search mainly through radio astronomy. Its Allen Telescope Array in Northern California, which the institute owns and operates, scans the sky for narrowband radio signals using machine learning and real-time computing to sift through large volumes of data. A second project, COSMIC, taps into the Karl G. Jansky Very Large Array in New Mexico, copying data from each dish to analyze in real time for unusual signals, per the SETI Institute's description of its own research.

On the optical side, the SETI Institute's LaserSETI project looks for ultrashort flashes of light lasting only a few nanoseconds that have no known natural origin, aiming eventually to build a global network of cameras monitoring the entire sky continuously, the institute states.

The largest privately funded effort, Breakthrough Listen, combines radio and optical searches with a $100 million budget over a 10-year span, according to the Breakthrough Initiatives. The program surveys roughly one million of the closest stars, the galactic center and plane, and the 100 nearest galaxies beyond the Milky Way, covering, by its own account, ten times more sky and five times more of the radio spectrum than earlier programs — sensitive enough, it says, to pick up a 100-watt laser from 25 trillion miles away.

Reading starlight for chemical clues

Technosignature research also looks for chemistry rather than deliberate signals. NASA lists possible markers such as nitrogen dioxide pollution, chlorofluorocarbons (CFCs) once used as refrigerants on Earth, carbon-based chemicals tied to fossil-fuel use, artificial night-side city lights on rocky planets, and the infrared "waste heat" or light dimming that a Dyson-sphere-style megastructure around a star might produce.

The same atmospheric techniques feed into the search for basic biology. In April 2025, a team led by Professor Nikku Madhusudhan of the University of Cambridge reported that the James Webb Space Telescope had detected chemical fingerprints consistent with dimethyl sulfide and/or dimethyl disulfide in the atmosphere of the exoplanet K2-18b — compounds that, on Earth, come from marine microbes — as published in The Astrophysical Journal Letters, according to Sci.News. The reported concentrations were far higher than on Earth, which the researchers said could also point to an unknown non-biological chemical process.

Signals that were never confirmed

The most famous SETI detection remains unresolved. On August 15, 1977, the Ohio State Radio Telescope recorded a strong, narrowband signal lasting just over a minute. Astronomer Jerry Ehman circled the printout and wrote "Wow!" next to it, giving the event its name, according to the SETI Institute. Despite follow-up observations, the signal has never repeated, and the SETI Institute's own account leaves its origin — natural source, human interference, or something else — officially open.

A near-real-time version of the same process played out in 2026. After the interstellar object 3I/ATLAS was identified in July 2025 as the third confirmed interstellar visitor to the Solar System, SETI Institute researchers pointed the Allen Telescope Array at it within 24 hours, scanning frequencies from 1 to 9 gigahertz, according to a ScienceDaily report on the study. The team detected nearly 74 million narrowband signals; most were quickly identified as human-made interference, and after narrowing the search to about 200 candidates matching the object's motion, further analysis traced every remaining signal to technology on Earth or in satellites, the report said. The study set limits suggesting any transmitter on the object could be no stronger than roughly 10 to 110 watts — consistent with 3I/ATLAS being a natural comet-like body rather than artificial technology.

Why nobody can say how many civilizations exist

Estimating how many detectable civilizations might exist relies on a 1961 framework, not a measurement. Astronomer Frank Drake devised the Drake Equation to organize discussion at an informal meeting at the Green Bank Observatory, attended by scientists including Carl Sagan, Joshua Lederberg and Melvin Calvin, according to the SETI Institute. The equation multiplies seven factors — the rate of star formation, the fraction of stars with planets, the number of potentially habitable planets per system, and the fractions where life, intelligence and detectable technology emerge, combined with how long a civilization broadcasts signals — to estimate a number, N, of communicative civilizations in the galaxy.

Because several of those factors remain unknown, the resulting estimates vary enormously. The SETI Institute notes that estimates for N have ranged from 1 — meaning Earth would be the only transmitting civilization in the galaxy — to several million. Drake himself has suggested a figure of about 10,000, based on an assumption that one new transmitting civilization emerges per year and that such civilizations broadcast for an average of 10,000 years — illustrating why the equation is used as a research prompt rather than a precise prediction.