  Protoplanets accrete material from their natal protostellar disks until they
are sufficiently massive to open a gap in the face of the disk's viscosity that
arises from the magneto-rotational instability (MRI). By computing the
ionization structure within observationally well-constrained disk models, we
demonstrate that poorly ionized, low viscosity "dead zones" stretch out to 12
AU within typical disks. We find that planets of terrestrial mass robustly form
within the dead zones while massive Jovian planets form beyond. Dead zones will
also halt the rapid migration of planets into their central stars. Finally, we
argue that the gravitational scattering of low mass planets formed in the dead
zone, to larger radii by a rapidly accreting Jupiter beyond, can explain the
distribution of planetary masses in our solar system.
