Exoplanetary magnetic fields as a tool for future interior characterization

Most planets located within the solar system display evidence of past and/or current magnetic activity. Magnetic fields of rocky bodies are thought to be driven by thermo-chemical convection taking place in an electrically conducting fluid in their deep interior (the liquid outer core for Earth), and are thus evidence of strong internal dynamics. Furthermore, magnetism is thought to play a crucial role for the development and the long-term stability of habitable surface conditions, as it shields the upper atmosphere from mass loss induced by stellar winds and extreme space weather events. 

The discovery of a large number of rocky exoplanets motivates the search and the study of magnetic fields beyond the solar system. While current observations are limited to providing the planetary radius and minimum mass, future missions aimed at the exploration of exoplanetary atmospheres will open up new avenues for the inversion of interior properties starting from atmospheric parameters. Such a goal requires knowledge of the planetary cores and the development of exoplanetary magnetic fields, as well as their influence on atmospheric evolution and its interaction with the surrounding stellar wind. 

The aim of the current study is to identify trends and parameter(s) controlling the core evolution and magnetic field sustainment in super-Earths. To do this we investigate the evolution of the cores of planets having different masses (0.8-2 Earth masses) and iron inventories (bulk iron content and mantle iron number). Starting out from the internal temperature profile after the complete solidification of a global magma ocean (Noack and Lasbleis, 2020), we determine the size and the structure of the core, and model its thermal and magnetic evolution during the subsequent 5 billion years. By taking into account the energy release resulting from the growth of a solid inner core, we compute the thermal and compositional buoyancy fluxes, as well as the generated magnetic field strengths and lifetimes. 

Our findings show that while the planetary mass is not a controlling parameter, both the bulk iron content and the mantle iron number strongly influence inner core growth and the lifetime of the magnetic field. Iron-rich planets having a high mantle iron number tend to start out and end up with solid inner cores that are substantially larger than iron-poor bodies, sometimes even reaching up to the radius of the outer core and thus shutting down magnetic activity. We therefore find that there is a “sweet spot” for longer-lasting magnetic fields, located at intermediate bulk iron contents and low mantle iron numbers.  

We also varied the content of light elements in the core and found that the addition of a small fraction of light elements helps keeping the magnetic field active for longer, even at high bulk iron contents. Field strengths can reach up to several times the one of Earth, even though such a signal might still be too weak to be detected by current radio telescopes. Nevertheless, the development of new observation techniques and the multiple future missions devoted to atmospheric exploration will provide useful insights on the presence and frequency of planetary magnetic fields.