Magnetic games between a planet and its host star: the key role of topology

TL;DR

This paper models magnetic interactions between close-in planets and their host stars using 3D MHD simulations, revealing how magnetic topology influences energy transfer, emissions, and planetary migration.

Contribution

It provides the first comprehensive 3D simulation study of star-planet magnetic interactions considering different magnetic configurations and topologies.

Findings

Poynting fluxes can reach ~10^{19} W, powering enhanced emissions.

Magnetic topology significantly affects the shape of magnetic obstacles and the magnitude of torques.

Magnetic torques can cause planetary migration in strong stellar wind magnetic fields.

Abstract

Magnetic interactions between a star and a close-in planet are postulated to be a source of enhanced emissions and to play a role in the secular evolution of the orbital system. Close-in planets generally orbit in the sub-alfv\'enic region of the stellar wind, which leads to efficient transfers of energy and angular momentum between the star and the planet. We model the magnetic interactions occurring in close-in star-planet systems with three-dimensional, global, compressible magneto-hydrodynamic numerical simulations of a planet orbiting in a self-consistent stellar wind. We focus on the cases of magnetized planets and explore three representative magnetic configurations. The Poynting flux originating from the magnetic interactions is an energy source for enhanced emissions in star-planet systems. Our results suggest a simple geometrical explanation for ubiquitous on/off enhanced emissions associated with close-in planets, and confirm that the Poynting fluxes can reach powers of the order of $10^{19}$ W. Close-in planets are also showed to migrate due to magnetic torques for sufficiently strong stellar wind magnetic fields. The topology of the interaction significantly modifies the shape of the magnetic obstacle that leads to magnetic torques. As a consequence, the torques can vary by at least an order of magnitude as the magnetic topology of the interaction varies.