candidate radio-emitting systems, and confirming orbital modulation of the radio signal, which has not been definitively accomplished for any radio SPI candidate to date. Testing for orbital periodicity can require hundreds of hours of radio monitoring. In identifying targets for long-term monitoring, studies may choose to focus on either young magnetically-active stars or older inactive stars. Young strongly-magnetized stars offer the benefit of a high predicted SPI luminosity, enabling sensitivity to weak planetary fields, which makes the recent radio non-detection of two rapidly-rotating M dwarf planet hosts particularly surprising (Ortiz Ceballos et al. 2025).

SPI Radio Search 3 However, even in the case of a radio detection, strong stellar activity also causes a foreground of non-SPI-driven stellar radio bursts. So far, long programs targeting a young planet-hosting M dwarf (Bloot et al. 2024) and G dwarf (Ilin et al. 2025a) have found no evidence of orbitally-phased radio emission, instead finding that the radio emission is stochastic or phased with the star’s rotation, as expected for non-planet-driven stellar activity. To mitigate the challenge of non-SPI stellar activity, another approach focuses on identifying candidate SPI systems as radio emitters with relatively weak stellar activity. One method to find low-activity SPI candidates is to use radio surveys to identify stars with anomalously strong radio emission compared to other activity indicators, since the radio emission could indicate an undiscovered SPI-inducing planet (e.g., Vedantham et al. 2020; Pope et al. 2021; Frail et al. 2025). This blind-survey method has exciting potential as a novel exoplanet detection method, although it faces the challenge that initial radio detections may be exceptional events that do not recur in follow-up (Narang et al. 2024). Another method to find low-activity SPI candidates is targeted radio observations of known close-in exoplanet systems with older stars, yielding polarized radio detections on M dwarfs Prox Cen (P´erez-Torres et al. 2021), YZ Cet (Pineda & Villadsen 2023), GJ 3323 (Ortiz Ceballos et al. 2024), and K+M dwarf HD 189733 (Zhang et al. 2025), and upper limits for M dwarf planet hosts GJ 486 (Pe˜na-Mo˜nino et al. 2025) and K2-18 (Wandia et al. 2026). In a promising conjunction of the blind-survey and targeted-exosystem approaches, a recent broad stellar survey with LOFAR found a radio burst with Jupiter-Io-like morphology, which originated from an M dwarf with a known planet (Tasse et al. 2026). In this work, we expand the search for low-activity, sub-Alfv´enic radio SPI candidates, through longduration, GHz observations of 5 recently-discovered close-in exoplanet systems around slowly-rotating M dwarfs. In Section 2, we present the 5 targets and a comparison sample from the literature. In Sections 3-5, we describe the observations, data analysis, and results, including quiescent detections of 2 systems but no detections of SPI-like polarized bursts. In Section 6, we prepare to interpret the non-detections by setting up a wind model for our targets based on rotation-magnetism and rotationmass loss correlations. In Section 7, we consider 4 possible causes of the SPI non-detections, one of which (low luminosity) we expand on in Section 8 by modeling our radio upper limits as constraints on exoplanet magnetic field, with the conclusions summarized in Section 9.
2. TARGETS
We have targeted five M dwarfs known to host ultra-short-period terrestrial exoplanets (Table 1). None of these targets have prior targeted radio observations in the literature. For multi-planet systems, the table lists only the innermost planet, since the strong dependence of SPI power on orbital distance (PSPI ∝a−4 to a−5; Section 8.5) favors detecting close-in planets. We also compare to 4 slowly-rotating M dwarf terrestrial planet hosts with literature radio observations, and a hypothetical reference system with easily scalable basic parameters. Our 5 target systems were all discovered via the transit method and thus have radii available, whereas for RV-discovered comparison systems YZ Cet b, Prox Cen b, and GJ 3323 b we assume Earth density and edge-on orientation: Rp = (Mp sin i)1/3 in Earth units. The nearness to Earth and short orbits of our targets make them interesting follow-up targets even though they are too hot to be habitable, for instance LHS 3844 b has potential for JWST