Formation of planetary cores inside very young proto-planets
Intervenant : Sergei Nayakshin
University of Leicester
Understanding planet formation remains a challenge. However, observations of the last decade made decisive strides towards testing planet formation theories beyond the Solar System. I will overview and present recent theoretical work, and detailed 3D simulations, of the gravitational instability hypothesis for formation of planets. In this scenario, pioneered by Gerard Kuiper in 1951, a few Jupiter mass gas clumps form in the protoplanetary disc. These clumps are the sites where dust grows and sediments to the centre. Previous 1D spherically symmetric models predicted that grain growth and sedimentation process is slow, making formation of solid core planets like the Earth unlikely via this channel. Our recent 3D numerical simulations of the process show that due to dust Rayleigh-Taylor instabilities, core formation via this process is far more rapid and robust than thought before. It is now clear that Kuiper’s 70-year-old hypothesis may be a robust way of forming massive solid cores, especially in the outer cold reaches of protoplanetary discs where the standard scenario faces severe difficulties. Cores formed via gravitational instability may thus be Uranus and Neptule in the Solar System, and the putative massive cores in the now famous HL Tau ALMA-observed disc at distance of ~ 70 AU. I conclude by presenting theoretical predictions with which to distinguish the two planet formation scenarios.
Picture: Zoom-in onto the central ~0.5 AU of 3D simulation of a 3 Jupiter mass gas clump during formation of a core. Left: gas density and velocity map. Right: same but for dust. As dust component collapses, a very dense solid core forms. Gas is heated via aerodynamical friction near the forming core and is expelled via a low density channel.