Abstract
Most of the large terrestrial bodies in the solar system display
evidence of past and/or current magnetic activity, which is thought to
be driven by thermo-chemical convection in an electrically conducting
fluid layer. The discovery of a large number of extrasolar planets
motivates the search of magnetic fields beyond the solar system. While
current observations are limited to their radius and minimum mass,
studying the evolution of exoplanetary magnetic fields and their
interaction with the atmosphere can open new avenues for constraining
interior properties from future atmospheric observations.
Here, we investigate the evolution of massive planets
($0.8-2$~$M_{\rm Earth}$) with
different bulk and mantle iron contents. Starting from their temperature
profiles at the end of accretion, we determine the structure of the core
and model its subsequent thermal and magnetic evolution over
$5$~Gyr. We find that the planetary iron content
strongly affects core structure and evolution, as well as the lifetime
of a magnetic field. Iron-rich planets feature large solid inner cores
which can grow up to the liquid outer core radius, shutting down any
pre-existing magnetic activity. As a consequence, the longest magnetic
field lifetimes ($\sim 4.15$~Gyr) are
obtained for planets with intermediate iron inventories
($50-60$~wt.\%). The presence of a
small fraction of light impurities keeps the core liquid for longer and
extends the magnetic field lifetime to more than
$5$~Gyr. Even though the generated magnetic fields are
too weak to be detected by ground facilities, indirect observations can
help shedding light on exoplanetary magnetic activity.