Exo–IPM Scattering as a Hidden Gatekeeper of Narrowband Technosignatures

Abstract Narrowband radio technosignatures can be significantly modulated by the host star’s exoplanetary interplanetary medium (Exo-IPM), where turbulence in stellar winds and coronal mass ejections (CMEs) imprint spectral broadening. We present a novel framework that maps isotropic wind properties, turbulence strength, observing frequency, and geometry to the spectral broadening of narrowband technosignatures. Anchored to what is likely the largest compilation of empirical spectral-broadening measurements from solar-system spacecraft, we validate and derive a robust radial dependence of spectral broadening from the host star. For Sun-like stars, wind speeds and turbulence strengths are constrained directly from empirical measurements, while for M-dwarfs, these properties are scaled from solar values. Applied to a simulated 1 GHz survey of the nearest 10 6 stars across orbital properties, orientation, stellar population, and Exo-IPM conditions, the survival function indicates that ∼70% of systems produce >1 Hz and >30% produce >10 Hz of broadening, disproportionately affecting M-dwarf systems, which constitute ∼75% of the stellar population. At 100 MHz, the effects are even more pronounced, with >60% of systems exhibiting >100 Hz of spectral broadening. Although the probability of encountering a CME during a typical technosignature observation is low (<3%), nearly all such encounters induce additional broadening by several orders of magnitude (>10 3 Hz). This redistribution of power from the expected intrinsic δ -like line into Lorentzian wings suppresses the peak signal-to-noise ratio targeted by standard narrowband pipelines, biasing sensitivity limits and plausibly contributing to the persistent “Great Silence” in narrowband radio technosignature searches over the past several decades.