  Terrestrial planets form in a series of dynamical steps from the solid
component of circumstellar disks. First, km-sized planetesimals form likely via
a combination of sticky collisions, turbulent concentration of solids, and
gravitational collapse from micron-sized dust grains in the thin disk midplane.
Second, planetesimals coalesce to form Moon- to Mars-sized protoplanets, also
called "planetary embryos". Finally, full-sized terrestrial planets accrete
from protoplanets and planetesimals. This final stage of accretion lasts about
10-100 Myr and is strongly affected by gravitational perturbations from any gas
giant planets, which are constrained to form more quickly, during the 1-10 Myr
lifetime of the gaseous component of the disk. It is during this final stage
that the bulk compositions and volatile (e.g., water) contents of terrestrial
planets are set, depending on their feeding zones and the amount of radial
mixing that occurs. The main factors that influence terrestrial planet
formation are the mass and surface density profile of the disk, and the
perturbations from giant planets and binary companions if they exist. Simple
accretion models predicts that low-mass stars should form small, dry planets in
their habitable zones. The migration of a giant planet through a disk of rocky
bodies does not completely impede terrestrial planet growth. Rather, "hot
Jupiter" systems are likely to also contain exterior, very water-rich
Earth-like planets, and also "hot Earths", very close-in rocky planets. Roughly
one third of the known systems of extra-solar (giant) planets could allow a
terrestrial planet to form in the habitable zone.
