A Hybrid Scenario for Planet Formation

The standard scenario for the formation of planets has two critical problems: (1) Turbulence in the disk stirs up dust particles and prevents the dust sedimentation onto the disk mid‐plane that is required for the fragmentation of the dusty layer prior to planetesimal formation. (2) The gravitational interaction of (proto)planets and the disk tends to result in their migration onto the central star within a short timescale. These problems have stimulated extensive theoretical work but still remain to be solved. In contrast, the recent increase of our understanding of the star formation process enables us to depict the long‐term evolution of protoplanetary disks: the resultant gravitational fragmentation and the formation of gaseous planetary‐mass objects in the disks. We critically review recent progress in our understanding of these processes and propose a possible hybrid scenario for the formation of planetary‐mass objects in variety.