Formation of planetary systems from pebble accretion and migration I: the growth dichotomy between close-in rocky super-Earth systems and terrestrial planets
Résumé
Exoplanet surveys have discovered that a large fraction of planetary systems (perhaps, a third around Sun-like stars) possess super-Earth planets on orbits tighter than Earth's. These super-Earths with masses between that of Earth and Neptune are not found in the Solar System, however it has been proposed that they formed in a similar way to our terrestrial planets. Instead, we find that these two inner planetary system architectures correspond to two clearly defined planet formation pathways regulated by the pebble mass-flux. Radio observations of protoplanetary disks reveal large reservoirs of "pebbles" (approximately cm-sized objects), which spiral inwards towards the central star through the disk due to drag with the nebular gas. Protoplanets embedded in the disk accrete a portion of these pebbles as they drift by. Pebble accretion can be the most efficient growth process of solid material. We modeled the growth of a system of rocky protoplanets embedded in disks of varying pebble mass-fluxes, and we find that a change of less than a factor of two in the pebble mass-flux significantly alters the architecture of the final planetary system. When the pebble mass-flux is low, protoplanets grow slowly and remain small. Their low-mass strongly limits their migration and so they are located near where they originally grew in the disk. When the gas disappears, they naturally become dynamically unstable, collide with one another, and a smaller number of larger planets remain, i.e. the terrestrial planet formation process. We find the largest final planets are at most a few Earth masses, which grow from embryos that are at most a third of an Earth mass when the gas disappears and pebble accretion ceases. If the pebble mass-flux is higher, protoplanets grow faster and become more massive while the protoplanetary disk still possesses nebular gas. These massive protoplanets interact with the gas disk and migrate inward leading to stronger planet-planet interactions, mutual merging, and even faster growth. As the nebular gas dissipates, it leaves behind a system of close-in super-Earths, which eventually undergo dynamical instabilities responsible for further growth and the removal of orbital resonances.