  We investigate whether any multi-planet systems among Kepler candidates (2011
February release) can harbor additional terrestrial-mass planets or smaller
bodies. We apply the "packed planetary systems" hypothesis that suggests all
planetary systems are filled to capacity, and use a Hill stability criterion to
identify eight 2-planet systems with significant gaps between the innermost and
outermost planets. For each of these systems, we perform long-term numerical
integrations of 10^7 years to investigate the stability of 4000-8000 test
particles injected into the gaps. We map out stability regions in orbital
parameter space, and therefore quantify the ranges of semi-major axes and
eccentricities of stable particles. Strong mean-motion resonances can add
additional regions of stability in otherwise unstable parameter space. We
derive simple expressions for the extent of the stability regions, which is
related to quantities such as the dynamical spacing Delta, the separation
between two planets in units of their mutual Hill radii. Our results suggest
that planets with separation Delta < 10 are unlikely to host extensive
stability regions, and that about 95 out of a total of 115 two-planet systems
in the Kepler sample may have sizeable stability regions. We predict that
Kepler candidate systems including KOI 433, KOI 72/Kepler-10, KOI 555, KOI
1596, KOI 904, KOI 223, KOI 1590, and KOI 139 can harbor additional planets or
low-mass bodies between the inner and outer detected planets. These predicted
planets may be detected by future observations.
