Andrius Popovas

A thesis for the degree of Doctor of Philosophy defended April 2018.

The PhD School of Science, Faculty of Science,  Astrophysics and Planetary Science, Niels Bohr Institute, University of Copenhagen

Supervisor: 
Prof. Åke Nordlund

Planet formation. The roles of pebble accretion, radiative and convective energy transport

This thesis deals with the early stages of rocky planet formation, when nascent planets are still embedded in a protoplanetary disk, which consisting mostly of hydrogen, helium gas and dust grains. Hydrostatic equilibrium between the gravitating planetary embryo and the surrounding gas forms an envelope. This envelope acts as a buffer between the embryo and the disk. We conduct high resolution nested-grid hydrodynamic simulations to investigate how the envelope affects the accretion of dust and pebbles, the main building blocks of rocky planets. Only a small fraction of mm size pebbles that cross into the planet’s region of gravitational influence -- the Hill sphere -- are accreted. The pebble accretion rates scale linearly with the size of the pebbles and are, due to cancellation effects, nearly independent of disk surface density, if the dust-to-gas surface density ratio is constant. With the measured accretion rates, we estimate accurate growth times for specified particle sizes. For 0.3--1 mm (“chondrule size”) particles, the growth time from a small seed is ∼1.5 million years for an Earth mass planet at 1 AU and ~1 million years for a Mars mass planet at 1.5 AU.

Accretion of solids onto the embryo releases a lot of heat via the friction force. This heat drives convective motions, which significantly alter the gas dynamics inside ~40 radii of an Earth size embryo. Convective motions do not, however, result in a significant change in the net accretion of mass, and the systematic inward drift of already gravitationally-bound pebbles continues as in the non-convective case. Although the envelopes are generally opaque, they are locally optically thin and thus radiative heat transport has significant effects on the near-planet gas thermodynamics. The convective motions are restricted to a smaller distance from the surface of the planet relative to simulations without radiative transfer, but their velocity dispersion is increased. Considering the ongoing efforts to understand planet formation, and the importance of realistically treating all of the relevant physical mechanisms, this thesis provides a good start and a significant stepping stone to build future research upon.