Planet formation and evolution: key processes to understand the diversity of planetary systems

The discovery of a large number of extrasolar planets has demonstrated that our own system is not “typical”. Exo-planetary systems can be very different from our own, and diverse from each other. Understanding this diversity is a major goal of modern planetary science.

The formation of planetary systems is not fully understood, but major advances have been obtained in the last 10 years. New concepts have been proposed, such as the streaming instability for the formation of planetesimals and pebble accretion for the formation of protoplanets. It is also now clear that planets forming in the proto-planetary disks have to migrate during their accretion, if their mass exceeds a few times the mass of Mars. Accretion and dynamical evolution are therefore very coupled processes. This leads to complex evolutions, very sensitive to initial conditions and fortuitous events, that are the key to understand the observed diversity of planetary systems. The early formation of Jupiter and its limited migration due to the formation of Saturn are two fundamental ingredients that determined the basic structure of the Solar System. The lack of early formation of giant planets typically leads to the formation of super-Earth planets on short period orbits. There is also evidence that the vast majority of planetary systems become unstable after the removal of the protoplanetary disk. The effects of this instability are very different depending on the masses of the planets involved. Our Solar System also experienced a global instability, but fortuitously our giant planets did not develop large orbital eccentricities.