  The presence of giant planets influences potentially habitable worlds in
numerous ways. Massive celestial neighbors can facilitate the formation of
planetary cores and modify the influx of asteroids and comets towards
Earth-analogs later on. Furthermore, giant planets can indirectly change the
climate of terrestrial worlds by gravitationally altering their orbits.
Investigating 147 well characterized exoplanetary systems known to date that
host a main sequence star and a giant planet we show that the presence of
'giant neighbors' can reduce a terrestrial planet's chances to remain
habitable, even if both planets have stable orbits. In a small fraction of
systems, however, giant planets slightly increase the extent of habitable zones
provided the terrestrial world has a high climate inertia. In providing
constraints on where giant planets cease to affect the habitable zone size in a
detrimental fashion, we identify prime targets in the search for habitable
worlds.
