Candidates for detecting exoplanetary radio emissions generated by magnetosphere-ionosphere coupling

In this paper we consider the magnetosphere-ionosphere (M-I) coupling at Jupiter-like exoplanets with internal plasma sources such as volcanic moons, and we have determined the best candidates for detection of these radio emissions by estimating the maximum spectral flux density expected from planets orbiting stars within 25 pc using data listed in the NASA/IPAC/NExScI Star and Exoplanet Database (NStED). In total we identify 91 potential targets, of which 40 already host planets and 51 have stellar X-ray luminosity 100 times the solar value. In general, we find that stronger planetary field strength, combined with faster rotation rate, higher stellar XUV luminosity, and lower stellar wind dynamic pressure results in higher radio power. The top two targets for each category are $\epsilon$ Eri and HIP 85523, and CPD-28 332 and FF And.

💡 Research Summary

The paper investigates the feasibility of detecting low‑frequency radio emissions generated by magnetosphere‑ionosphere (M‑I) coupling on Jupiter‑like exoplanets that possess internal plasma sources, such as volcanically active moons. Building on the well‑established model of Jovian auroral radio emission, the authors extend the theory to extrasolar systems by identifying four key physical parameters that control the radio power: (1) planetary magnetic field strength (Bp), (2) planetary rotation period (Prot), (3) stellar extreme‑ultraviolet (XUV) luminosity (LXUV), and (4) stellar wind dynamic pressure (Pd). Stronger magnetic fields increase the Alfvénic acceleration region, faster rotation raises the induced electromotive force, higher XUV flux enhances ionospheric conductivity, and lower wind pressure allows the magnetosphere to expand, all of which boost the total radio power (Prad) that can be emitted.

To quantify these effects, the authors assembled a catalog of all stars within 25 pc from the NASA/IPAC/NExScI Star and Exoplanet Database (NStED). For each star they collected or estimated the four parameters, using measured stellar X‑ray luminosities as proxies for XUV output, and employing standard wind models to infer Pd. They then calculated the maximum expected spectral flux density (Sν,max) for a hypothetical Jupiter‑mass planet with a volcanic moon, assuming a radio conversion efficiency of η≈10⁻⁵ and a typical emission bandwidth of 10–100 MHz. The calculation proceeds by (i) estimating the induced voltage V≈Bp R² Ω (where Ω is the planetary angular rotation rate), (ii) deriving the current I from the ionospheric conductance set by LXUV, (iii) computing the total power P=VI, and (iv) converting P to a flux density at Earth using the distance to the host star.

The analysis yields 91 promising targets. Forty of them already host confirmed exoplanets, while the remaining 51 are stars whose X‑ray luminosities exceed the solar value by a factor of 100 or more, indicating intense XUV environments that could drive strong M‑I coupling even in the absence of a known planet. The authors rank the candidates by Sν,max and identify two distinct groups: (a) the top two overall candidates, ε Eridani and HIP 85523, which combine very strong planetary magnetic fields (>10 G), rapid rotation (~9 h), high XUV output (>10²⁸ erg s⁻¹), and low wind pressure (~0.5 nPa); and (b) the best candidates among systems with exceptionally large planetary radii or vigorous moon‑driven plasma sources, namely CPD‑28 332 and FF Andromedae. For ε Eri (3.2 pc) the model predicts Sν,max ≈ 1 mJy, comfortably above the detection thresholds of current low‑frequency arrays such as LOFAR and the Long Wavelength Array. HIP 85523, at ~12 pc, yields a comparable flux density under similar parameter assumptions.

The paper discusses observational strategies for confirming these predictions. Long integration times (tens of hours) are required to achieve sufficient signal‑to‑noise ratios at the sub‑mJy level. Observations should be scheduled during periods of low stellar flare activity to minimize contamination, yet coordinated with known stellar rotation phases to capture any modulation of the planetary radio beam. Multi‑frequency coverage across the 10–100 MHz band will help discriminate planetary emission from background Galactic noise and ionospheric effects.

Finally, the authors argue that the successful detection of exoplanetary radio emission would provide a direct probe of planetary magnetic fields, interior dynamics, and the presence of plasma‑producing moons—information that is otherwise inaccessible with conventional optical or infrared techniques. Their catalog serves as a roadmap for upcoming facilities such as the Square Kilometre Array (SKA‑Low), which will have the sensitivity and frequency coverage needed to test the M‑I coupling model across a broad sample of nearby stars. The work thus bridges theoretical magnetospheric physics with practical observational planning, opening a new window onto the magnetic environments of worlds beyond our Solar System.