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A radio signal is a possible technosignature when it has measurable characteristics compatible with technology—such as very narrow bandwidth or a frequency that drifts over time—and remains unexplained after checks for natural sources and human-made interference. Those clues make a signal a candidate, not proof of extraterrestrial technology. Repeat observations and independent verification are essential.
What scientists mean by a possible technosignature
A technosignature is observable evidence of technology built or used by extraterrestrial beings. Radio is one possible channel; others include optical emissions, infrared excess, unusual astronomical measurements, or an artifact. The International Academy of Astronautics (IAA) uses this definition in its June 2026 updated declaration.
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A “candidate” or “possible technosignature” is therefore an observation worth testing against technical, natural, and local explanations. It is not a claim that its source has been identified. A confirmed technosignature would require credible verification and independent agreement.
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A narrowband signal concentrates its energy in a small portion of the radio spectrum. Many familiar natural radio sources, including pulsars, quasars, and interstellar gas, emit across broader frequency ranges, so a very narrow signal can be a useful clue in a search. The SETI Institute describes examples that may be only a few hertz wide or less.
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The SETI Institute 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, not a universal cutoff that applies identically to every instrument or a stand-alone test for extraterrestrial origin. Earth-based military radar and telecommunications satellites can also produce narrowband signals, as the Institute notes in its SETI FAQ.
Why a candidate may drift in frequency
Relative motion between a transmitter and receiver changes the frequency we observe through the Doppler effect. If a signal comes from a planet orbiting another star, motion in that system can shift its apparent frequency. Earth’s own orbit and rotation also affect the received signal. The rate at which observed frequency changes is called the drift rate.
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For that reason, radio searches do not always look only for a perfectly constant tone. They may process fine frequency channels and test for a signal that slopes or drifts over time. The SETI Institute explains this Doppler-related approach in its FAQ. As one current example, COSMIC—the VLA Search for Extraterrestrial Intelligence project at the Very Large Array—describes its search for slowly drifting radio signals. A particular observatory’s search settings illustrate one approach; they do not define a universal drift-rate limit for technosignatures.
How researchers distinguish a signal from interference
Earth is a major source of false positives. Radar, satellites, telecommunications, and other human technology can generate radio signals that are narrow, structured, or otherwise interesting to a search pipeline. A signal’s unusual shape alone cannot show that it came from space or from extraterrestrial technology.
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Researchers investigate whether the signal is consistent with the astronomical target, examine possible local radio-frequency interference, and seek follow-up observations. Repeat detection matters: the SETI Institute notes that unexplained signals, including the Wow! signal, were never detected again, so they did not establish a successful detection. Non-repetition does not prove a signal was natural; it means the evidence was not sufficient to confirm it.
The IAA’s June 2026 declaration sets out verification and reporting principles. If researchers reach a credible consensus that evidence is extraterrestrial, it recommends prompt, full, and open reporting to the public, scientific community, and UN Secretary-General.
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Why narrowband searches have limits
Narrowband searches are effective for a particular class of possible signal, but a signal may not remain narrow while travelling through space. A 2026 SETI Institute report describes how plasma near a transmitting planet—including stellar-wind density fluctuations and eruptions such as coronal mass ejections—could broaden an initially narrow signal and reduce its peak strength.
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If broadening spreads the signal’s energy across more frequencies, it may fall below detection thresholds tuned for very narrow signals. This is a possible propagation effect and a search-design consideration, not evidence that a particular candidate has been hidden or missed. It also means narrowness is a useful clue, not a complete description of every signal a civilization might transmit or an observatory might detect.
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What a modeled detection distance does—and does not—show
A SETI Institute summary of a 2025 “Earth Detecting Earth” modeling study says Earth’s planetary radar emissions could be detectable up to 12,000 light-years under the model. This is a modeled potential reach for those emissions, not an observed signal from another civilization and not a general range for every radio transmitter. The Institute describes the result in its February 3, 2025 report.
What makes the evidence stronger
No single trait settles the question. A candidate becomes more compelling when multiple checks support a technological explanation and competing explanations are addressed. Relevant questions include:
- Is its bandwidth and structure compatible with the kind of signal the search targets?
- Does its frequency behavior fit a plausible drift, rather than an assumption that every signal must stay fixed?
- Is it consistent with the astronomical target, and have terrestrial transmitters or other interference been considered?
- Can the signal be detected again or independently verified?
- Could propagation effects have broadened a signal beyond the search’s narrowband focus?
These are evaluation dimensions, not a universal checklist with a single pass/fail threshold. A one-off unexplained signal can justify follow-up, but without repeat detection or independent verification it remains a candidate rather than a confirmed technosignature.
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