SETI Institute warns stellar plasma can smear alien radio signals

Radio SETI searches are built to catch needles: ultra-narrow spikes that stand out against the hiss of the sky. In a paper in The Astrophysical Journal, SETI Institute researchers Vishal Gajjar and Grayce C Brown argue that the last metres of a signal’s journey out of its home star system may be turning those needles into straw, spreading the power across a wider range of frequencies and pushing it below standard detection thresholds.

According to The Guardian, the team focuses on plasma turbulence and eruptive stellar events such as coronal mass ejections. The mechanism is familiar to radio engineers on Earth: when radio waves pass through a medium with fluctuating electron density, the wave’s phase and frequency can be perturbed. For SETI, the practical consequence is that a transmission that is intrinsically narrowband at the source may arrive at Earth as a broadened feature that no longer triggers pipelines tuned for sharp spectral lines.

The work matters because “no signal detected” is not a single fact but the output of a chain of assumptions: which frequencies are monitored, what bandwidth counts as “artificial”, how long the telescope dwells on a target, and what signal-to-noise ratio is required before software flags a candidate. Narrowband searches are attractive because most known astrophysical emitters are broadband; narrowing the net reduces false alarms. But the same choice also creates a blind spot if propagation effects smear a signal enough to make it look natural or simply too weak.

The researchers calibrate these distortions using spacecraft transmissions within the solar system and then extrapolate to other stellar environments, The Guardian reports. That approach tries to separate two sources of confusion that SETI routinely fights: Earth-side interference (telecoms, satellites, instrument artefacts) and space-side propagation (ionised gas between transmitter and receiver). The new claim is that the latter can be strong close to the transmitting star, before the signal even enters interstellar space.

One implication is methodological rather than philosophical: if the receiver assumes the transmitter’s signal stays narrow, it may be optimising for the wrong target. Brown suggests that future surveys should include higher-frequency observations, where some plasma effects are reduced, and that search software should be adapted to look for broadened technosignatures instead of only razor-thin lines.

SETI’s “radio silence” has always been compatible with multiple stories: there may be no transmitters; transmitters may be rare or short-lived; they may choose other bands or modulation schemes; or they may be loud but hard to recognise. Gajjar and Brown add another mundane filter: a signal can be present, but a star’s own weather can make it fail the receiver’s definition of a signal.

The paper’s argument ultimately returns to the same operational detail SETI lives and dies on: a detection threshold is a human decision embedded in software, and plasma does not care what the software expects.