  Habitability is usually defined as the requirement for a terrestrial planet's
atmosphere to sustain liquid water. This definition can be complemented by the
dynamical requirement that other planets in the system do not gravitationally
perturb terrestrial planets outside of their habitable zone, the orbital region
allowing the existence of liquid water. We quantify the dynamical habitability
of 85 known extrasolar planetary systems via simulations of their orbital
dynamics in the presence of potentially habitable terrestrial planets. When
requiring that habitable planets remain strictly within their habitable zone at
all time, the perturbing influence of giant planets extends beyond the
traditional Hill sphere for close encounters: terrestrial planet excursions
outside of the habitable zone are also caused by secular eccentricity
variations and, in some cases, strong mean-motion resonances. Our results
indicate that more than half the known extrasolar planetary systems (mostly
those with distant, eccentric giant planets) are unlikely to harbor habitable
terrestrial planets. About 1/4 of the systems (mostly those with close-in giant
planets), including 1/3 of the potential targets for the Terrestrial Planet
Finder, appear as dynamically habitable as our own Solar System. The influence
of yet undetected giant planets in these systems could compromise their
dynamical habitability. Some habitable terrestrial planets in our simulations
have substantial eccentricities (e > 0.1) which may lead to large seasonal
climate variations and thus affect their habitability.
