  Approximately half of Sun-like stars harbor exoplanets packed within a radius
of ~0.3 AU, but the formation of these planets and why they form in only half
of known systems are still not well understood. We employ a one-dimensional
steady state model to gain physical insight into the origin of these close-in
exoplanets. We use Shakura & Sunyaev alpha values extracted from recent
numerical simulations of protoplanetary disk accretion processes in which the
magnitude of alpha, and thus the steady-state gas surface density, depends on
the orientation of large scale magnetic fields with respect to the disk's
rotation axis. Solving for the metallicity as a function of radius, we find
that for fields anti-aligned with the rotation axis, the inner regions of our
model disk often falls within a region of parameter space not suitable for
planetesimal formation, whereas in the aligned case, the inner disk regions are
likely to produce planetesimals through some combination of streaming
instability and gravitational collapse, though the degree to which this is true
depends on the assumed parameters of our model. More robustly, the aligned
field case always produces higher concentrations of solids at small radii
compared to the anti-aligned case. In the in situ formation model, this bimodal
distribution of solid enhancement leads directly to the observed dichotomy in
exoplanet orbital distances.
