  Models of planet formation have shown that giant planets have a large impact
on the number, masses and orbits of terrestrial planets that form. In addition,
they play an important role in delivering volatiles from material that formed
exterior to the snow-line (the region in the disk beyond which water ice can
condense) to the inner region of the disk where terrestrial planets can
maintain liquid water on their surfaces. We present simulations of the late
stages of terrestrial planet formation from a disk of protoplanets around a
solar-type star, and we include a massive planet (from 1 Earth mass to 1
Jupiter mass) in Jupiter's orbit at ~5.2 AU in all but one set of simulations.
Two initial disk models are examined with the same mass distribution and total
initial water content, but with different distributions of water content. We
compare the accretion rates and final water mass fraction of the planets that
form. Remarkably, all of the planets that formed in our simulations without
giant planets were water-rich, showing that giant planet companions are not
required to deliver volatiles to terrestrial planets in the habitable zone. In
contrast, an outer planet at least several times the mass of Earth may be
needed to clear distant regions from debris truncating the epoch of frequent
large impacts. Observations of exoplanets from radial velocity surveys suggest
that outer Jupiter-like planets may be scarce, therefore the results presented
here suggest the number of habitable planets that reside in our galaxy may be
more than previously thought.
