Planetary dynamos in evolving cold gas giants

Context. The discovery of thousands of exoplanets has started a new era of planetary science that expands our ability to characterize diverse planetary features. However, magnetic fields remain one of the least understood aspects of exoplanetary systems. A deeper understanding of planetary dynamos and the evolution of surface magnetic properties throughout the lifetime of a planet is a key scientific purpose. It has implications for planetary evolution, habitability, and atmospheric dynamics. Aims. We modeled the evolution of magnetic fields generated by dynamo action in cold gaseous giant planets. We explored the change in the morphology and strength of the magnetic field at different evolutionary stages, providing a comprehensive view of the planetary life-cycle. Methods. We solved the resistive magnetohydrodynamic (MHD) equations under the anelastic approximation with the 3D pseudospectral spherical shell MHD code MagIC. We employed 1D thermodynamical hydrostatic profiles taken from gas giant evolutionary models (using MESA) as the background states of our MHD models. The numerical integration led to saturated dynamo solutions. These calculations were performed with radial profiles corresponding to different planetary ages, so that we were able to interpret them as different snapshots of the planetary dynamo during the long-term planetary evolution. Results. We characterized the magnetic field at different stages in the evolution of a cold gaseous planet. A transition from a multipolar to a dipolar-dominated dynamo regime occurs throughout the life of a Jovian planet. During the planetary evolution and the cooling-down phase, we observe a decrease in the average magnetic field strength near the dynamo surface as ≈ t−0.2−t−0.3. This trend is compatible with previously proposed scaling laws. We also find that some dimensionless parameters evolve differently for the multipolar to the dipolar branch, possibly reflecting a force balance change. Conclusions. Our method captures the long-term evolution of the internal dynamo phases of magnetic fields by considering snapshots at different ages. We find a slow decay and a transition in the dynamo behavior. This approach can be extended to the study of hot gaseous planets, and it is a versatile method for predicting the magnetic properties of giant planets and for identifying promising candidates for exoplanetary low-frequency radio emission.