Terrestrial planet formation in extra-solarplanetary systems

摘要

Terrestrial planets form in a series of dynamical steps from the solid component of cir-cumstellar 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 thethin disk midplane. Second, planetesimals coalesce to form Moon- to Mars-sized protoplanets,also called "planetary embryos". Finally, full-sized terrestrial planets accrete from protoplanetsand planetesimals. This final stage of accretion lasts about 10-100 Myr and is strongly affectedby gravitational perturbations from any gas giant planets, which are constrained to form morequickly, during the 1-10 Myr lifetime of the gaseous component of the disk. It is during this finalstage 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 factorsthat 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 accretionmodels 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 impedeterrestrial planet growth. Rather, "hot Jupiter" systems are likely to also contain exterior, verywater-rich Earth-like planets, and also "hot Earths", very close-in rocky planets. Roughly onethird of the known systems of extra-solar (giant) planets could allow a terrestrial planet to formin the habitable zone.