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The crucial role of metallicity for planetesimal formation

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Dynamics of Discs and Planets

The probability of finding exoplanets around a main sequence star rises sharply for metallicities around solar or higher. I present computer simulations of particle clumping and planetesimal formation in protoplanetary discs with varying amounts of solid material. The sedimentary mid-plane layer of pebbles is unstable to both Kelvin-Helmholtz and streaming instabilities. For metallicities below the solar value the equilibrium mid-plane layer is thick and displays no clumping. However, already at slightly super-solar metallicities, strong clumping occurs in the mid-plane layer. These particle clumps can locally obtain more than a hundred times the gas density. We interpret the onset of clumping as an effect of a curious trait of the streaming instability: the strength of the turbulence increases with a decreasing solids-to-gas mass ratio. Particles inside dense clumps have collision speeds of a few meters per second, and the clumps readily contract gravitationally into a number of interacting 100-km-size planetesimals. Our results show that the metallicity dependence of exoplanets may have been imprinted already in the early stages of planet formation, or during the dispersal of the gaseous part of the disc.

This talk is part of the Isaac Newton Institute Seminar Series series.

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