Magnetic Fields and Accreting Giant Planets around PDS 70

TL;DR

This study investigates the magnetic fields and accretion processes of giant planets around PDS 70, highlighting the role of magnetic fields in planetary growth, spin regulation, and satellite formation during final formation stages.

Contribution

It models the magnetic field strength, spin rate, and circumplanetary disk properties of accreting giant planets, revealing the significance of strong magnetic fields and their effects on planet and satellite formation.

Findings

Giant planets may have magnetic fields of 10-100 G.

Strong magnetic fields can lead to magnetospheric accretion and spin-down.

Circumplanetary disk surface density is low with a positive gradient.

Abstract

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 dipole magnetic 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 becomes very low, compared with the canonical models, implying the importance of radial movement of satellite-forming materials. 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.