  The final masses of Jovian planets are attained when the tidal torques that
they exert on their surrounding protostellar disks are sufficient to open gaps
in the face of disk viscosity, thereby shutting off any further accretion. In
sufficiently well-ionized disks, the predominant form of disk viscosity
originates from the Magneto-Rotational Instability (MRI) that drives
hydromagnetic disk turbulence. In the region of sufficiently low ionization
rate -- the so-called dead zone -- turbulence is damped and we show that lower
mass planets will be formed. We considered three ionization sources (X-rays,
cosmic rays, and radioactive elements) and determined the size of a dead zone
for the total ionization rate by using a radiative, hydrostatic equilibrium
disk model developed by Chiang et al. (2001). We studied a range of surface
mass density (Sigma_{0}=10^3 - 10^5 g cm^{-2}) and X-ray energy (kT_{x}=1 - 10
keV). We also compared the ionization rate of such a disk by X-rays with cosmic
rays and find that the latter dominate X-rays in ionizing protostellar disks
unless the X-ray energy is very high (5 - 10 keV). Among our major conclusions
are that for typical conditions, dead zones encompass a region extending out to
several AU -- the region in which terrestrial planets are found in our solar
system. Our results suggest that the division between low and high mass planets
in exosolar planetary systems is a consequence of the presence of a dead zone
in their natal protoplanetary disks. We also find that the extent of a dead
zone is mainly dependent on the disk's surface mass density. Our results
provide further support for the idea that Jovian planets in exosolar systems
must have migrated substantially inwards from their points of origin.
