In section 10. Planets and planetary systems
Gravitational fragmentation and formation of giant protoplanets on orbits of tens of au
The origin of systems of giant planets orbiting at tens of au from their parent star, such as the iconic HR 8799, remains a mystery. Here, the authors have used high resolution hydrodynamic simulations of protostellar disks to show that gravitationally unstable clumps formed in the outer parts of these disks can indeed eventually lead to the formation of such planets. They show that the clumps initially migrate very rapidly (on 1000 to 10,000 years timescales) towards the central star but that this migration may be halted by Roche lobe overflow at distances of tens of au. If at this point the clumps are warmer than 2000K, a second collapse may occur to lead to the formation of a gas giant protoplanet. While their subsequent evolution (and migration) over millions of years remain to be studied, this seems to be a promising route for the formation of these systems.