Introduction
As of now, thousands of extrasolar planets have been detected, indicating the ubiquity of planets around stars. However, how these planets—including those in our own Solar System—were formed remain in question. We generally focus on studying the so-called core accretion model, in which planets are formed by coagulating solid materials, especially in gaseous environments.
This process takes interstellar, micron-sized dust grains into planetary cores and hence covers about 13 orders of magnitude in size and 30 orders of magnitude in mass, which needs to be completed before the disk is evaporated. Given such a huge difference in length scales, we have to break the process down into different stages. We highlight in the following what we have been working on.
Dust-Gas Dynamics and Planetesimal Formation
Dust particles feel aerodynamic drag forces in the gaseous protoplanetary disks, similar to a moving golf ball in the air. This drag force will lead to the dust’s radial and azimuthal drift in the disk. When particles have the right size, so that their stopping time due to the drag force equals the disk’s orbital time, they will become extremely mobile, drifting toward either the central star or some disk sub-structures.
When a significant amount of dust is concentrated in some disk regions, the dust can push the gas, altering the gas motion. A gas-dust instability, called the streaming instability, then operates and concentrates dust particles enough to be gravitationally bound. The resulting planetesimals can be kilometers in size.