High energy planetary astrophysics: exomoons and their geological signatures in the magnetospheres of giant planets and dwarf stars

Volcanic satellites — active moons orbiting giant planets, or close-in active rocky planets orbiting dwarf stars — are among our best windows into geological activity beyond Earth. Even within our own solar system, the 2014 claim of cryovolcanism at Jupiter’s satellite Europa has now been retracted (Roth et al. 2026), owing to the difficulty of inferring transient water vapor from low signal-to-noise aurorae. By contrast, NaCl sourced from an evaporating or volcanically active ocean world such as Io or Enceladus provides one of the brightest available signatures, namely in atomic sodium and potassium. Io ignites a sodium exosphere extending ~1000 Jupiter radii. Similar to the first radial-velocity detections of exoplanets, we report the first Doppler shift of an exomoon system: an unambiguous redshift that, in transmission geometry, opposes the natural blueshift vector of radiation pressure (Oza et al. 2024, ApJL). At the same system, new KECK observations confirm a repeating ~40-minute transient sodium signature, coincident with its Doppler shift, first observed with HARPS/3.6-m (Unni et al. 2025, MNRASL).  We characterize the orbit to be roughly 8 hours, consistent with recent independent N-body simulations (Sucerquia & Cuello 2025, A&A Letters). Without high-resolution spectroscopy , JWST low-res spectroscopy cannot easily infer the periodicity of a satellite without a Doppler shift; nevertheless, recent variability of SO₂ — a known volcanic gas — at WASP-39b, seen by MIRI and NIRCam, appears to require an exomoon and its associated plasma torus (8-15h orbit) rather than H₂S photochemistry of a planetary atmosphere alone (Oza et al. 2026, MNRAS). Beyond optical/IR, roughly a decade before the sodium cloud was detected at Io, decametric radio emission at Jupiter notably provided the first evidence of Io’s volcanic escape (Bigg 1964). In this light, we also report evidence of a volcanic satellite in a dwarf-star system based on its radio emission, originally reported as a radiation belt (Kao et al. 2023).  Our modeling suggests a putative satellite is fueling the detected electron cyclotron maser instability (ECMI) observed as short bursts. Together, these initial optical, infrared, and radio detections of active satellites opens a window onto planet–satellite formation and evolution, informing the broader population of exomoons in the galaxy.