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A radio signal is a possible technosignature when it has features compatible with technology—such as a very narrow frequency range or a changing frequency that fits the motion of a transmitter—and remains unexplained after researchers check for natural sources and human-made interference. Those features make it a candidate, not proof of extraterrestrial technology. Repeat observations and independent verification are essential.
What “possible technosignature” means
A technosignature is observable evidence of technology built or used by extraterrestrial beings. Radio is one possible channel; others include optical emissions, infrared excess, anomalies in astronomical measurements, or an artifact. The International Academy of Astronautics’ June 2026 updated declaration defines the term and sets out principles for verifying putative evidence.
In practice, “possible” means an observation is worth testing against technological, natural, and local explanations. An unusual signal—or one that cannot yet be explained—is not thereby confirmed as extraterrestrial.
What signal features can make it interesting?
Narrow bandwidth
A narrowband signal concentrates its energy in a small part of the radio spectrum. Many familiar natural cosmic radio sources, including pulsars, quasars, and interstellar gas, emit across broader bands, so a particularly narrow signal can be a useful search clue. The SETI Institute describes examples only a few hertz wide or less. Its FAQ also says that signals “less than about 300 Hz wide” would, as far as we know, be artificially produced. That is an explanatory statement from the FAQ, not a universal instrument-independent cutoff or a standalone test: Earth-based radar and telecommunications satellites can also produce narrowband signals. (SETI Institute FAQ)
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A carrier that drifts in frequency
A continuous or slowly pulsed carrier may be detectable as a signal whose frequency changes over time. Relative motion between transmitter and receiver changes the observed frequency through the Doppler effect. The orbit and rotation of a planet, as well as Earth’s own motion, can contribute. Researchers describe the rate of frequency change as the drift rate and search across possible drift rates rather than assuming a signal must remain at one observed frequency. (SETI Institute FAQ)
The SETI Institute’s COSMIC system at the Very Large Array, operated with UC Berkeley and Breakthrough Listen, is one example of an observatory system that searches for slowly drifting radio signals. Its approach illustrates current search practice; it does not establish a universal drift-rate cutoff for technosignatures. (COSMIC)
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Why researchers first rule out Earth-based interference
Human technology produces radio signals that can look artificial because they are artificial: radar and telecommunications satellites, among other sources, can produce narrowband emissions. A signal may therefore be unusual and still originate on or near Earth. (SETI Institute FAQ)
Evaluation involves checking whether the signal’s apparent location is consistent with the astronomical target, comparing it with known sources of radio-frequency interference, and seeking follow-up observations. Repeat detection matters because a one-off event is difficult to distinguish from interference or an unrepeatable natural event. The SETI Institute notes that unexplained signals, including the Wow! signal, were never detected again; their lack of repetition has not established a SETI discovery. (SETI Institute FAQ)
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The IAA declaration describes verification and reporting principles. If researchers reach a credible consensus that evidence is extraterrestrial, it recommends prompt, full, and open reporting to the public, the scientific community, and the UN Secretary-General. (IAA declaration, updated June 2026)
Why narrowband searches can miss some signals
Narrowness is a useful search clue, but it is not a complete description of every signal that might be transmitted or received. A 2026 SETI Institute report discusses how plasma near a transmitting planet—including fluctuations in stellar-wind density or eruptive events such as coronal mass ejections—may broaden an initially narrow signal and reduce its peak strength. A broadened signal could fall below the detection thresholds of searches optimized for very narrow signals. This is a possible propagation effect with implications for search design, not evidence that a candidate has been hidden or missed. (SETI Institute, 2026)
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What a modeled detection range does—and does not—show
A SETI Institute report on a 2025 “Earth Detecting Earth” modeling study says Earth’s planetary radar emissions could potentially be detectable up to 12,000 light-years under the model. This is a model-based estimate of potential detectability for those emissions, not an observed signal from another civilization and not a general range for every kind of radio transmitter. (SETI Institute, February 3, 2025)
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How to read a technosignature claim
- Interesting feature: A narrow signal or a plausible frequency drift can justify further analysis.
- Candidate: The observation remains worth investigating after initial checks, but explanations such as terrestrial interference and natural sources still need to be tested.
- Confirmed technosignature: This requires credible verification and independent agreement, not merely an unexplained or one-time observation.
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