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What types of signals do SETI projects search for?

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Planet TOI 700e — extraterrestrial life An illustration of the Earth-size planet TOI 700e orbiting within the habitable zone of its star. The habitable zone is the area around a star where liquid water, and thus possibly life, can survive on a planet's surface. Its Earth-size sibling, TOI 700d, can be seen in the distance.

SETI (Search for Extraterrestrial Intelligence) projects primarily search for electromagnetic signals that may be sent deliberately as beacons or leaked inadvertently from other worlds. These searches generally focus on two types of signals:1

Radio Signals

Most radio SETI searches utilize receivers tuned to the microwave band, specifically near 1,420 megahertz. This frequency corresponds to the natural emission of hydrogen, a universal reference point known to any technically competent civilization. Researchers hunt for narrowband signals—typically 1 hertz wide or less—which would be distinct from the broadband radio emissions naturally produced by cosmic objects like pulsars.23

Optical Signals

Optical SETI projects search for brief, intense flashes of light lasting a nanosecond or less. These signals could be produced by extraterrestrial societies using high-powered pulsed lasers to momentarily outshine the natural light of their host star.43

By scanning for these specific, non-natural patterns, SETI aims to identify evidence of technical civilizations capable of interstellar communication.23