The recent high spatial/spectral resolution observations have enabled constraining formation mechanisms of giant planets, especially at the final stages. The current interpretation of such observations is that these planets undergo magnetospheric accretion, suggesting the importance of planetary magnetic fields. We explore the properties of accreting, magnetized giant planets surrounded by their circumplanetary disks, using the physical parameters inferred for PDS 70 b/c. We compute the magnetic field strength and the resulting spin rate of giant planets, and find that these planets may possess magnetic dipole fields of either a few 10 G or a few 100 G; the former is the natural outcome of planetary growth and radius evolution, while the resulting spin rate cannot reproduce the observations. For the latter, a consistent picture can be drawn, where strong magnetic fields induced by hot planetary interiors, lead both to magnetospheric accretion and to spin-down due to disk locking. We also compute the properties of circumplanetary disks in the vicinity of these planets, taking into account planetary magnetic fields. The resulting surface density lies between the predictions of two empirically derived models of circumplanetary disks:the minimum mass subnebula model and the gas-starved model. Our model predicts a positive gradient of the surface density, which invokes the traps for both satellite migration and radially drifting dust particles. This work thus concludes that the final formation stages of giant planets are similar to those of low-mass stars such as brown dwarfs, as suggested by recent studies.
After protoplanets have acquired sufficient mass to open partial gaps in their natal protostellar disks, residual gas continues to diffuse onto some horseshoe streamlines under effect of viscous dissipation, and meander in and out of the planets’ Hill sphere. Inside the Hill sphere, the horseshoe streamlines intercept gas flow in circumplanetary disks. The host stars’ tidal perturbation induce a barrier across the converging streamlines’ interface. Viscous transfer of angular momentum across this tidal barrier determines the rate of mass diffusion from the horseshoe streamlines onto the circumplanetary disks, and eventually the accretion rate onto the protoplanets. We carry out a series of numerical simulations to test the influence of this tidal barrier on super-thermal planets. In weakly viscous disks, protoplanets’ accretion rate steeply decreases with their masses above the thermal limit. As their growth time scale exceeds the gas depletion time scale, their masses reach asymptotic values comparable to that of Jupiter. In relatively thick and strongly viscous disks, protoplanets’ asymptotic masses exceed several times that of Jupiter. Such massive protoplanets strongly excite the eccentricity of nearby horseshoe streamlines, destabilize orderly flow, substantially enhance the diffusion rate across the tidal barrier, and elevate their growth rate until their natal disk is severely depleted. Based on the upper fall-off in the observe mass distribution of known exoplanets, we suggest their natal disks had relatively low viscosity (α ∼ 10−3), modest thickness (H/R ∼ 0.03 − 0.05), and limited masses (comparable to that of minimum mass solar nebula model).