The search for life: what SETI is actually doing
Looking for signals nature does not make — and the one candidate that never repeated.
4 min read · Published 28 August 2026
The short answer
SETI searches primarily for narrowband radio signals — transmissions concentrated in a very narrow frequency range, which natural astrophysical processes do not produce. Nothing has ever been confirmed. The most famous candidate, the 1977 Wow! signal, matched expectations for an artificial transmission but was never detected again despite repeated searches, which means it cannot be verified and cannot count as evidence.
What it looks for
The core target is narrowband signals. Natural radio sources — pulsars, hydrogen clouds, galactic emission — spread their energy across broad frequency ranges. Concentrating power into a very narrow band is something transmitters do and physics generally does not.
So the search is not for a message. It is for a carrier: a signal whose structure indicates technology, regardless of content.
Certain frequencies receive extra attention, particularly the water hole — the quiet region between the hydrogen line at 1420 MHz and hydroxyl emission. The reasoning is that this band is relatively free of natural noise and that the components spell out water, which any water-based chemistry might notice. It is an assumption about alien reasoning, and its weakness is acknowledged.
The Wow! signal
On 15 August 1977 a radio telescope at Ohio State recorded a narrowband signal lasting 72 seconds — exactly the time the target would remain in the beam as Earth rotated, which is what a genuine sky source would produce.
It was strong, narrowband, near the hydrogen line, and appeared to come from a fixed point in Sagittarius. The astronomer reviewing the printout wrote 'Wow!' in the margin, which is how it got its name.
It has never been detected again despite many searches. Explanations proposed include a comet's hydrogen cloud, a reflected terrestrial transmission, and an interstellar cloud of hydrogen stimulated by a passing source; none is established, and objections exist to each.
The honest position: it remains unexplained, and unexplained is not evidence of anything. A signal that occurs once and never repeats cannot be verified, which is why it is a famous anomaly rather than a discovery.
Why nothing has been found
Several possibilities, and no way yet to distinguish them
- We have barely looked. The searched volume of parameter space — sky positions, frequencies, times, sensitivities — is a vanishingly small fraction of the total. Comparisons to sampling a swimming pool with a glass are commonly made and are broadly fair.
- Radio may be a brief phase. Our own broadcasts are becoming lower-power and more directional as technology improves. A civilisation might be radio-loud for only a century or two.
- They may not be transmitting deliberately. Detecting incidental leakage is far harder than detecting a beacon aimed at us.
- They may not be there. The simplest explanation, and one that cannot currently be distinguished from the others.
The two searches
SETI looks for technosignatures — evidence of technology. The other approach looks for biosignatures — evidence of biology, through atmospheric composition, as attempted with exoplanet spectroscopy and in the subsurface oceans of icy moons.
Biosignature searches are more likely to succeed sooner, because microbial life is presumably far more common than technological civilisation. They are also far harder to interpret: any candidate molecule has abiotic production routes, which is why the K2-18b claim remains contested.
Both fields have adopted deliberately high standards, precisely because the claim is so consequential and the history of premature announcements is so poor.
Why the discipline is admirable
SETI operates under conditions that would tempt anyone toward overclaiming: an enormous prize, a public appetite for the answer, and decades of null results.
Its response has been the opposite. Candidate signals are published as candidates, follow-up is demanded before announcement, and the field has repeatedly identified its own apparent detections as interference — including signals that turned out to originate from a microwave oven at the observatory.
That is the standard worth holding any extraordinary claim to, and it is the direct contrast with the astrology content this journal spends most of its time examining. The difference is not the subject matter. It is that one field publishes what would falsify it.
Common questions
- What does SETI actually search for?
- Primarily narrowband radio signals — transmissions concentrated into a very narrow frequency range, which natural astrophysical sources do not produce. It searches for evidence of a transmitter rather than for a decodable message.
- What was the Wow! signal?
- A narrowband radio signal detected in 1977 lasting 72 seconds, exactly matching what a fixed sky source would produce as Earth rotated. It has never been detected again despite repeated searches, so it remains unexplained but cannot be verified.
- Why has SETI found nothing?
- Possibly because the searched fraction of sky positions, frequencies and times remains vanishingly small; because radio may be a brief technological phase; because incidental leakage is far harder to detect than a deliberate beacon; or because nothing is there. These cannot currently be distinguished.
- What is the difference between a technosignature and a biosignature?
- A technosignature is evidence of technology, such as an artificial radio signal. A biosignature is evidence of biology, such as an atmospheric composition difficult to produce without life. Biosignature searches are more likely to succeed sooner but far harder to interpret.