Forming wide-orbit planets via pebble accretion

Sammanfattning: The possibility that detected substructures in protoplanetary disks, such as gaps, rings and cavities, are created by distant protoplanets has prompted an inquiry into the mechanism responsible for their formation. These wide-orbit young bodies are likely to form relatively rapidly in the outer regions of circumstellar disks. Pebble accretion is considered a promising candidate to explain their formation, as distant reservoirs of pebbles (mm-cm sized particles) have been confirmed, and the mechanism can enhance the growth rate of forming planets. However, the inwards migration of planets and the slower growth in the outer regions still challenge wide-orbit planet formation via pebble accretion. Therefore, the disk must harbour the appropriate characteristics for the fast growth of the body. With the aim of constraining such features, we utilised existing models, which describe the growth and migration of distant protoplanets until the dispersal of the disk. We also presented a novel analytical model to explain the depletion of pebbles over the evolving disk. This model facilitates the simulation of more realistic formation scenarios when pebbles drift fast across the disk generating a strong but short-lasting pebble flux. We found that a Moon-sized protoplanetary embryo formed at very early stages and located beyond 50 AU can grow up to become the core of a gas giant (and therefore open a gap) at 20-50 AU within less than 1 Myr. The outermost cores form when there is a strong but short-lasting pebble flux with a Stokes number of St \gtrsim 0.03. A metallicity of Z_0~0.01-0.02 and low turbulence of \alpha_t ~10^(-4) also enhances the formation of distant cores. As these cores form very early, they undergo a fast inwards migration while they accrete gas, and by the end of the disk lifetime (3 Myr), they become giant planets orbiting at <10 AU. In view of these results, we proposed a new mechanism for forming wide-orbit gas giants, and we showed that it might be possible to form gas giants at 10-50 AU at the end of the disk lifetime. According to our model, these planets are rare, in agreement with the low occurrence of gas giants in the outer regions from direct imaging surveys. Overall, protoplanets migrate several AU before they become gas giants. That suggests that giant planets in our Solar System, such as Jupiter, might have started forming from protoplanetary embryos at distant locations. If the giant planets accrete most of their solid material in the outermost regions, this can have a significant impact on their ultimate composition.