Dead Zones and the Origin of Planetary Masses

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 magnetorotational instability. 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.