Detecting Distant Dark Planets via Magnetospheric Radio Emissions: A Low-Frequency Interferometric Strategy for Planet Nine

The hypothesized Planet Nine remains one of the most intriguing unresolved problems in modern planetary astronomy. Traditional search strategies rely on reflected sunlight or thermal infrared emission, both of which scale unfavorably with heliocentric distance. In contrast, magnetospheric radio emissions scale more favorably with distance and may provide an alternative detection pathway. This paper explores the feasibility of detecting Planet Nine via cyclotron–maser radio emissions produced by interactions between a planetary magnetosphere and the solar wind. Using standard magnetopause pressure-balance models and empirical radiometric scaling relations derived from solar system planets, we estimate magnetosphere sizes, expected radio power, and observable flux densities for distant planetary dynamos. We show that a Neptune-class or super-Neptune planetary magnetic field at distances of ∼ 600 AU could produce detectable low-frequency radio bursts, particularly if enhanced by moon-driven electrodynamic interactions analogous to the Jupiter–Io system. The expected observational signature is a strongly circularly polarized, low-frequency transient source exhibiting periodic modulation and slow proper motion across the sky. A mission concept based on a dual-spacecraft lunar-orbit interferometer is proposed as a practical architecture for detecting such signals. Unlike traditional imaging telescopes, the system would function as a transient locator, mapping burst coordinates and tracking motion over multi-year baselines. This approach reframes the search for distant planets as a search for magnetospheres rather than reflected light sources, potentially opening a new observational window for detecting otherwise invisible objects in the outer solar system.