  Most of the first-discovered extrasolar multi-planet systems were found to
lie close to dynamically unstable configurations. However a few observed
multi-planet systems (e.g. HD 74156) did not show this trait. Those systems
could share this property if they contain an additional planet in between those
that are known. Previous investigations identified the properties of
hypothetical planets that would place these systems near instability. The
hypothetical planet in HD 74156 was expected to have a mass about equal to that
of Saturn, a semi-major axis between 0.9 and 1.4 AU, and an eccentricity less
than 0.2. HD 74156 d, a planet with a mass of 1.3 Saturn masses at 1.04 AU with
an eccentricity of 0.25, was recently announced. We have reanalyzed all
published data on this system in order to place tighter constraints on the
properties of the new planet. We find two possible orbits for this planet, one
close to that already identified and another (with a slightly better fit to the
data) at ~0.89 AU. We also review the current status of other planet
predictions, discuss the observed single planet systems, and suggest other
systems which may contain planets in between those that are already known. The
confirmation of the existence of HD 74156 d suggests that planet formation is
an efficient process, and planetary systems should typically contain many
planets.
