  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 Yet, this instability is naturally superseded by the spiral-driven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.
