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 …