  We have obtained a steady-state, 1-D model of the accretion disk of a
protostar taking into account the magneto-rotational instability (MRI). We find
that the disk is divided into an outer turbulent region (OTR), a MRI suppressed
region (MSR), and an inner turbulent region (ITR). The outer turbulent region
is fully turbulent because of MRI. However, in the range, r_{out} (= 8 - 60 AU)
from the central star, MRI is suppressed around the midplane of the gas disk
and a quiet area without turbulence appears, because the degree of ionization
of gas becomes low enough. The disk becomes fully turbulent again in the range
r in (= 0.2 - 1 AU), which is called the inner turbulent region, because the
midplane temperature become high enough (\gt 1000 K) due to gravitational
energy release.
  Planetesimals are formed through gravitational instability at the two
distinct sites, outer and inner MRI fronts (the boundaries between the MRI
suppressed region (MSR) and the outer and inner turbulent regions), because of
the radial concentration of the solid particles. At the outer MRI front, icy
particles grow through low-velocity collisions into porous aggregates with low
densities. They eventually undergo gravitational instability to form icy
planetesimals. On the other hand, rocky particles accumulate at the inner MRI
front, since their drift velocities turn outward due to the local maximum in
gas pressure. They undergo gravitational instability in a sub-disk of pebbles
to form rocky planetesimals at the inner MRI front.
  The tandem regime is consistent with the ABEL model, in which the Earth was
initially formed as a completely volatile-free planet. The water and other
volatile elements came later through the accretion of icy particles by the
occasional scatterings in the outer regions.

Earth and Planetary Astrophysics