Magnetic properties of Proxima Centauri b analogues

Abstract: The discovery of a planet around the closest star to our Sun, Proxima Centauri, represents a quantum leap in the testability of exoplanetary models. Unlike any other discovered exoplanet, models of Proxima b could be contrasted against near future telescopic observations and far future in-situ measurements. In this paper we aim at predicting the planetary radius and the magnetic properties (dynamo lifetime and magnetic dipole moment) of Proxima b analogues (solid planets with masses of $\sim 1-3\,M_\oplus$, rotation periods of several days and habitable conditions). For this purpose we build a grid of planetary models with a wide range of compositions and masses. For each point in the grid we run the planetary evolution model developed in \citet{Zuluaga2013}. Our model assumes small orbital eccentricity, negligible tidal heating and earth-like radiogenic mantle elements abundances. We devise a statistical methodology to estimate the posterior distribution of the desired planetary properties assuming simple prior distributions for the orbital inclination and bulk composition. Our model predicts that Proxima b would have a mass $1.3\leq M_{\rm p}\leq 2.3\,M_{\oplus}$ and a radius $R_{\rm p}=1.4^{+0.3}{-0.2}\,R{\oplus}$. In our simulations, most Proxima b analogues develop intrinsic dynamos that last for $\geq$4 Gyr (the estimated age of the host star). If alive, the dynamo of Proxima b have a dipole moment ${\cal M}{\rm dip}>0.32^{\times 2.3}{\div 2.9}{\cal M}_{\rm dip,\oplus}$. These results are not restricted to Proxima b but they also apply to earth-like planets having similar observed properties.