A predictive framework for realistic star planet radio emission in compact systems

TL;DR

This work addresses the challenge of detecting exoplanetary star–planet interaction (SPI) radio emissions by developing a data-driven forward-modeling framework that combines Zeeman-Doppler Imaging (ZDI) maps, 3D MHD stellar wind simulations, and the ExPRES radio-emission simulator to predict SPI-induced radio frequencies. It uses the Radio–Magnetic Scaling Law to estimate radio power from wind-derived quantities and evaluates detectability with current and upcoming facilities. Applying the framework to Tau Boo, HD 179949, and HD 189733 demonstrates how emission frequencies and beaming patterns depend on magnetic geometry, wind conditions, and observer viewpoint, highlighting HD 189733 as the strongest candidate across instruments like SKA1-Low, LOFAR, and NenuFAR. The study provides a practical pathway to prioritize targets and optimize telescope scheduling, while noting that improvements in magnetic-field reconstructions and wind modeling will sharpen predictions and enable systematic SPI radio surveys across many exoplanetary systems.

Abstract

Radio emission from star planet interactions (SPI) beyond our solar system has yet to be firmly detected, primarily due to challenges such as weak signals, directional beaming effects, and low frequency emissions that are blocked by the ionosphere of Earth. Addressing these obstacles calls for strategic target selection. This proof of concept study aims to improve SPI target prioritization by simulating SPI induced radio emission frequencies and estimating associated radio power to identify systems most likely to produce detectable signals. We combine Zeeman Doppler Imaging (ZDI) maps with 3D magnetohydrodynamic (MHD) stellar wind simulations and use the ExPRES code to model SPI driven radio emissions. We also estimate the intensity of these emissions using the Radio Magnetic Scaling Law, based on the magnetic field and plasma density parameters from the 3D wind models. This approach is applied to systems such as Tau Boo, HD 179949, and HD 189733 to assess their detectability with current and future radio telescopes. This framework, tested on benchmark systems, is applicable to any star planet system with available ZDI maps and wind models. As magnetic field reconstructions and wind simulations improve, the method will become more robust. It provides a data driven approach to prioritize targets and optimize telescope scheduling. This shall enable systematic exploration of magnetic SPI radio emissions across a wide range of exoplanetary systems.

Figures (16)

• Figure 1: Artist's rendering illustrating star–planet interaction-induced radio emission. This illustration summarizes the physical processes driving radio emission, from the star’s activity to the direct energy flux generated by Alfvén waves (green color) electron acceleration at relativistic velocities (red color circles), resulting in cyclotron radio emission (red cone). The image also highlights the use of a 3D MHD stellar wind simulation, specifically the Alfvén Wave Solar Model (AWSoM) and WindPredict_AW. These wind models are coupled with the ExPRES code to simulate and predict the SPI-induced radio emission.
• Figure 2: Conceptual illustration of radio wave propagation from the star toward the planet. The star is shown with a simplified dipolar magnetic field, and its rotation axis is indicated by the upward arrow. Two emission cones are depicted: the cone closer to the star lies in regions of stronger magnetic field (strong $B$), while the outer cone is located in weaker fields (weak $B$). The background color represents the magnetic field strength (orange–red: strong; green–blue: weak). As radio waves propagate into regions of stronger field (inner cone), the refractive index decreases, leading to reflection at the iso-$f_{\rm ce}$ surface and a flattening of the cone on that side. The illustration is not to scale.
• Figure 3: Simulated stellar wind environments for two F-type main-sequence stars (Tau Boo and HD 179949) and one K-type star (HD 189733). Each panel shows the magnetic connectivity between the star and the planetary orbit: Tau Boo (top), HD 179949 (middle), and HD 189733 (bottom). For Tau Boo, the system is shown at its inclination relative to the observer, while for HD 179949 and HD 189733 a slightly tilted viewpoint is used to highlight the magnetic field topology. The stellar surface displays the radial magnetic field (in Gauss), reconstructed from ZDI maps, using a yellow-to-blue color scale. The translucent gray surface marks the Alfvén surface. For clarity, only a subset of magnetic field lines is shown, colored according to total wind pressure ($P_\mathrm{tot}$) from orange to purple, with darker tones indicating higher pressure. Both forward-connected (star to planet) and backward-connected (planet to star) field lines are included. The 3D axes (x, y, z) indicate the orientation of the stellar rotation axis, aligned with the z-axis.
• Figure 4: Time–frequency spectrograms showing the simulated radio emission from star–planet magnetic interactions in the Tau Boo system for selected months; October 2010 (top), January 2011 (middle), and April 2011 (bottom). The y-axis denotes the radio frequency, while the x-axis represents time, with each tick corresponding to a single day within a given month (formatted as "day:month:year"). Frequencies are color–coded by the degree of circular polarization: blue indicates emission from the magnetic southern hemisphere, corresponding to field lines directed from the star to the planet, while red indicates emission from the magnetic northern hemisphere. In both cases, the color refers to the polarity of the field at the stellar surface (or carried by the wind).
• Figure 5: A simple illustration of how the observed radio frequency pattern changes as the star–planet system rotates, assuming a dipolar, axisymmetric magnetic field. We show the frequency pattern as seen from an observer located at mid/high magnetic latitudes from the star system. The planet is shown moving from positions 1 to 3. It appears larger when it is closer to the observer (position 2), and the same perspective effect applies to the radio beam cones. Colored cones indicate the radio emission directed toward the observer: blue/yellow (top/bottom), green/red (right/left), and gray indicates directions with no emission toward the observer (represented by the eye symbol). As the planet orbits, different portions of the emission cone become visible to the observer, leading to changes in the observed frequency pattern. The magnetic field line is shown in magenta color. The illustration is not to scale.