Princeton University Extrasolar Planet Discussion Group

Planetary-scale magnetic fields provide a unique window into a planet's deep interior. From the magnetic field of Earth to that of Jupiter, the existence of such fields is tied to the presence of an electrically conductive convecting fluid (dynamo source region) in the interior. Thus, detections of planetary-scale magnetic field signals offer constraints on the planets’ thermal state, interior structure, and dynamics. In this thesis, we present thermal evolution calculations for rocky exoplanets. We aim to determine whether super-Earths can host dynamo-generated magnetic fields and explore how dynamo lifetimes scale with planet properties (such as planet mass, M_pl, and core mass fraction, CMF). To achieve this, we couple a 1D thermal evolution model with a Henyey solver to calculate their thermal evolution. The code solves the energy balance equation in the Fe-dominated core and the silicate mantle. We use a modified mixing length formulation to model convection in the silicate mantle with low and high Reynolds numbers. In addition, by including the Henyey solver, the model self-consistently accounts for adjustments in the interior structure as the planet evolves in time. We explore the possibility of the planet-hosting a dynamo source in its Fe-dominated core and/or magma ocean. We find that the heat loss rate of the core scales with M_pl. This results in a greater dynamo lifetime in the core of a more massive planet, as long as convection shuts off before the core fully solidifies. However, the dynamo lifetime in the core decreases with increasing M_pl if the core fully solidifies before convection shuts off, owing to the short lifetime of the liquid core associated with the high core heat loss rate. Depending on our choices of the rheology of post-perovskite, we predict 6M_Earth planet with CMF=0.1 and 3M_Earth planet with CMF=0.1 to have the longest dynamo lifetimes in their cores. In addition, a magma ocean could only host a dynamo if its melt fraction is high enough to have liquid-like convection. The dynamo in the magma ocean in an Earth-like planet (M_pl=1M_Earth and CMF=0.33) could only last ~0.25Myr. However, a magma ocean may sustain a long-term dynamo on a lava planet or a sub-Neptune, whose silicate mantle could stay molten or partially molten on a billion-year timescale. Future studies of these planets may shed light on the role of a magma ocean sustaining a planetary-scale magnetic field.