Internal structures and magnetic moments of rocky planets

Abstract

Context. For a planet to be considered habitable on its surface, it is an important advantage for it to have a magnetic field that protects its atmosphere from stellar winds as well as cosmic rays. The magnetic protection of potentially habitable planets plays a key role in determining the chances of detecting atmospheric biosignatures. In making an estimate of a planet’s magnetic field, its internal structure must first be known. Aims. This study proposes to use the Preliminary Reference Earth Model (PREM) internal structure as the base of a numerical model, as it is considered the best available option for estimating the internal structure of rocky exoplanets. With this model, we estimate the magnetic properties of dry and water-rich Earth-like and Super-Earth-like planets and we apply it to the first 176 planets of this kind confirmed by the Transiting Exoplanet Survey Satellite exoplanet mission (TESS). Methods. Using PREM as a reference, we estimated the internal structure of dry and water-rich rocky planets. This model provides an estimation of the average density and core size of the planet, which are the key ingredients for estimating its magnetic moment, depending on whether it is tidally locked or not. Our model estimates the thermodynamic variables as a function of pressure and including saltwater as a component of water-rich exoplanets. In addition, we did not use the perfect layer differentiation approximation. We validated our model based on the selection of planets and satellites in the Solar System with similar characteristics. Results. Using our model, we estimated the internal structure and magnetic moment of selected dry and water-rich rocky planets and satellites in the Solar system. The differences with the observed values in the internal structure characteristics, mass, average density, moment of inertia factor, and local Rossby number are remarkably low or even negligible. The estimated magnetic moments are also very similar to the observed ones. We applied the model to the first dry and water-rich rocky planets confirmed by TESS, 176 in total, and we find, from an astrobiological perspective, that TOI-700 d and TOI-2257 b are the most interesting examples, as they are located in the habitable zone. However, we note that their magnetic moments are only about 0.01 of the Earth’s magnetic moment.