  Recent work has suggested that many planetary systems lie near instability.
If all systems are near instability, an additional planet must exist in stable
regions of well-separated extra-solar planetary systems to push these systems
to the edge of stability. We examine the known systems by placing massless test
particles in between the planets and integrating for 1-10 million years. We
find that some systems, HD168443 and HD74156, eject nearly all test particles
within 2 million years. However we find that HD37124, HD38529, and 55Cnc have
large contiguous regions in which particles survive for 10 million years. These
three systems, therefore, seem the most likely candidates for additional
companions. Furthermore HD74156 and HD168443 must be complete and therefore
radial velocity surveys should only focus on detecting more distant companions.
We also find that several systems show stable regions that only exist at
nonzero eccentricities.
