Date : 7/26 (Wed) [16:00 - ] @ F313
Speaker: Prof. Hiroshi Kobayashi (Nagoya University)
Title : Planet formation via collisions
Abstract: Planets are formed from dust grains in a protoplanetary disk composed of gas and solid (rock and ice). Solid is initially sub-micron sized dust grains. Collisional coagulation is believed to produce kilometer sized or larger planetesimals and then planets. For planetesimal formation via collisional growth of dust grains, the collisional growth of dust grains produce fluffy dust aggregates, and such high porosity aggregates overcome the radial-drift barrier for planetesimal formation. From planetesimals to planets, gravitational interaction between bodies is important. Once Mars-sized or larger protoplanets are formed, collisional velocities are accelerated due to planetary perturbation, and then collisional fragmentation becomes active. Protoplanets grow through collisions with surrounding planetesimals. Collisional fragmentation reduces the surface density of planetesimals, resulting in stalling protoplanet growth. The size of surrounding planetesimals depends on the strength of turbulence. Moderate strength turbulence results in 100km sized planetesimals that protoplanets accrete, which allow to produce massive cores to be gas giant planets via gas accretion. This planetesimal size agrees with the characteristic sizes of the main belt asteroids and Kuiper belt objects. On the other hand, the rapid formation timescale of Mars is estimated from Hf-W chronometry, which is explained by week turbulence in the terrestrial planet formation region.

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