Planet Formation via Gravitational Instability

Schib, Oliver (2021). Planet Formation via Gravitational Instability (Unpublished). (Dissertation, University of Bern, Faculty of Science, Physics Institute)

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Some observed exoplanets cannot be explained in the standard core accretion paradigm. An alternative formation model, planet formation via gravitational instability (GI), has been proposed as a formation mechanism instead. In GI, a region of a protoplanetary disc collapses under its own gravity to form bound objects. The subsequent evolution of these ‘clumps’, including the interaction with the disc and other clumps, may lead to the formation of gas giant planets. Under rare circumstances, such clumps may arrive at wide orbital separations. GI may thus provide an explanation for the formation of planets like the four gas giants observed around the star HR 8799. Giant planets around M-dwarfs, like the one detected around the star GJ 3512, may also have formed by GI.
We developed a framework for GI planet population synthesis. In the first publication, we applied it to the formation and evolution of protoplanetary discs. The model includes infall from the parent molecular cloud core and studies a parameter range of two orders of magnitude in stellar mass. The population of stars at the end of our simulations agrees with the observed initial mass function. We found that magnetic fields have an important impact on disc formation: including magnetic fields in the cloud collapse may prevent fragmentation completely. If, on the other hand, magnetic fields are weak, fragmentation is ubiquitous. This leads to the formation of numerous bound clumps, some of which may evolve to become giant planets. The properties of the fragments depend sensitively on stellar mass and on the temperature in the disc. We also found that protoplanetary discs may well be more massive and live longer than is often assumed.
During the PhD, it was not possible to calculate the final population of GI-planets. However, another important step has been done. In the second paper we developed a prescription for migration and accretion of planets. This will be a central part for the complete population synthesis that we are going to conduct in the near future.
Our work supports the idea, that some observed exoplanets formed via GI. Future theoretical work, in tandem with more and more advanced observations, will help understanding the important topic of planet formation further.

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