  In the search for life beyond our Solar system, attention should be focused
on those planets that have the potential to maintain habitable conditions over
the prolonged periods of time needed for the emergence and expansion of life as
we know it. The observable planetary architecture is one of the determinants
for long-term habitability as it controls the orbital evolution and ultimately
the stellar fluxes received by the planet. With an ensemble of n-body
simulations and obliquity models of hypothetical planetary systems, we
demonstrate that the amplitude and period of eccentricity, obliquity, and
precession cycles of an Earth-like planet are sensitive to the orbital
characteristics of a giant companion planet. A series of transient,
ocean-coupled climate simulations show how these characteristics of
astronomical cycles are decisive for the evolving surface conditions and
long-term fractional habitability relative to the modern Earth. The
habitability of Earth-like planets increases with the eccentricity of a
Jupiter-like companion, provided that the mean obliquity is sufficiently low to
maintain temperate temperatures over large parts of its surface throughout the
orbital year. A giant companion closer in results in shorter eccentricity
cycles of an Earth-like planet but longer, high-amplitude, obliquity cycles.
The period and amplitude of obliquity cycles can be estimated to first order
from the orbital pathways calculated by the n-body simulations. In the majority
of simulations, obliquity amplitude relates directly to the orbital inclination
whereas the period of obliquity cycles is a function of nodal precession and
the proximity of a giant companion.
