Formation of intermediate-mass planets via magnetically-controlled disk fragmentation

TL;DR

This study uses MHD simulations to show that magnetic fields in protoplanetary disks can lead to the formation of intermediate-mass planets through fragmentation, aligning with observed exoplanet distributions.

Contribution

It introduces a new model of disk fragmentation driven by spiral dynamo magnetic fields, producing lighter protoplanets than traditional models.

Findings

Formation of long-lived, intermediate-mass protoplanets.

Magnetic pressure prevents further growth of these protoplanets.

Results align with observed exoplanet mass distribution.

Abstract

Intermediate mass planets, from Super-Earth to Neptune-sized bodies, are the most common type of planets in the galaxy. The prevailing theory of planet formation, core-accretion, predicts significantly fewer intermediate-mass giant planets than observed. The competing mechanism for planet formation, disk instability, can produce massive gas giant planets on wide-orbits, such as HR8799, by direct fragmentation of the protoplanetary disk. Previously, fragmentation in magnetized protoplanetary disks has only been considered when the magneto-rotational instability is the driving mechanism for magnetic field growth. Yet, this instability is naturally superseded by the spiral-driven dynamo when more realistic, non-ideal MHD conditions are considered. Here we report on MHD simulations of disk fragmentation in the presence of a spiral-driven dynamo. Fragmentation leads to the formation of long-lived bound protoplanets with masses that are at least one order of magnitude smaller than in conventional disk instability models. These light clumps survive shear and do not grow further due to the shielding effect of the magnetic field, whereby magnetic pressure stifles local inflow of matter. The outcome is a population of gaseous-rich planets with intermediate masses, while gas giants are found to be rarer, in qualitative agreement with the observed mass distribution of exoplanets.