The Influence of Magnetic Field Geometry on the Formation of Close-in Exoplanets

Abstract

Approximately half of Sun-like stars harbor exoplanets packed within a radius of ~0.3 AU, but the formation of these planets and why they form in only half of known systems are still not well understood. We employ a one-dimensional steady state model to gain physical insight into the origin of these close-in exoplanets. We use Shakura & Sunyaev alpha values extracted from recent numerical simulations of protoplanetary disk accretion processes in which the magnitude of alpha, and thus the steady-state gas surface density, depends on the orientation of large scale magnetic fields with respect to the disk's rotation axis. Solving for the metallicity as a function of radius, we find that for fields anti-aligned with the rotation axis, the inner regions of our model disk often falls within a region of parameter space not suitable for planetesimal formation, whereas in the aligned case, the inner disk regions are likely to produce planetesimals through some combination of streaming instability and gravitational collapse, though the degree to which this is true depends on the assumed parameters of our model. More robustly, the aligned field case always produces higher concentrations of solids at small radii compared to the anti-aligned case. In the in situ formation model, this bimodal distribution of solid enhancement leads directly to the observed dichotomy in exoplanet orbital distances.