Protoplanetary disks and planet formation around brown dwarfs and very low-mass stars

摘要

Brown dwarfs and very low-mass stars are very common in the Galaxy, yet we know little about the planetary systems they may host. Here we review observational evidence emerging from comparative studies of disks around brown dwarfs and sun-like stars. These studies show that very young brown dwarfs and very low mass stars have disks as frequently as sun-like stars do, arguing for the same formation processes. There are indications, but no conclusive evidence yet, for a longer disk lifetime around the lowest-mass stars and brown dwarfs. At the same time, evidence for faster dust processing and more strongly reduced disk scale heights is found, demonstrating that the first steps of planet formation also take place around brown dwarfs. With increasingly sensitive infrared instruments a new window is opening on gas-phase chemistry in these disks and the first surveys indicate a different gas-phase chemistry and, perhaps, a suppressed nitrogen chemistry. Sub-millimeter surveys reveal disk masses of a few Jupiter mass, which core accretion models show is enough to form few Earth-mass and smaller planets, but not gas giant planets.