Formation of intermediate-mass planets via magnetically controlled disk fragmentation

Intermediate-mass planets, from super-Earth to Neptune-sized bodies, are the most common types of planet in the Galaxy¹. The prevailing theory of planet formation—core accretion²—predicts the existence of substantially fewer intermediate-mass giant planets than have been observed^3,4. The competing mechanism for planet formation—disk instability—can produce massive gas giant planets on wide orbits, such as HR 8799⁵, by direct fragmentation of the protoplanetary disk⁶. Previously, fragmentation in magnetized protoplanetary disks has been considered only when the magneto-rotational instability is the driving mechanism for magnetic field growth⁷. However, this instability is naturally superseded by the spiral-driven dynamo when more realistic, non-ideal magneto-hydrodynamic conditions are considered^8,9. Here, we report on magneto-hydrodynamic 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^10,11. These light clumps survive shear and do not grow further owing to the shielding effect of the magnetic field, whereby magnetic pressure stifles the 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.