Fredagskollokvium: Planetary formation
Henrik Eklund, ITA
Planetary systems forms in disks of dust and gas surrounding their parent star. Solid particles in the disk clumps together and becomes ever-larger until they become planetary embryos which are decoupled from the gas and will due to the tidal forces experience an inwards radial drift. The timescale of the radial drift is orders of magnitude shorter than the life timespan of the disk.
New research shows that when solid particles are accreted to the planetary embryos, a temperature asymmetry will arise around the planet which in turn gives rise to a force directed outwards counteracting the inwards migration. This effect is referred to as the 'heating torque'. If the mass accretion is large enough so that the heating torque is larger than the tidal damping torques, the planet will experience an outwards migration.
The mass accretion also affects the evolution of the orbital eccentricity and inclination of the planetary embryos. It has been shown that in typical protoplanetary disks, for low mass planets (a fraction of, to several Earth masses) with initially circular coplanar orbits, there is a growth of the orbital eccentricity and inclination up to values similar to the disk aspect ratio. The eccentricity has a growth rate approximately three times larger than the inclination. The eccentricity and inclination are shown to be coupled and when the eccentricity reaches a certain critical value, the inclination growth is truncated.
If a planetary embryo have a highly eccentric or inclined orbit, for instance after a collision with a heavy object, the eccentricity or inclination is be damped towards their respective critical values. In the case of a planetary embryo in a circular, highly inclined orbit the eccentricity would over time be excited by the inclination.
This results in eccentric and non-inclined orbits, which favours further collisions between the planetary embryos and formation of massive planets. The combined features rising from the heating torque gives an explanation to how giant planets actually can form and survive in the outskirts of a solar system.