The absence of super-Earth-mass planets on short-period orbits in the solar system is particularly noteworthy given that such planets are common elsewhere. This unresolved problem is made more compelling by the fact that the solar system differs from most known exoplanet systems. While this emphasis on exoplanets is understandable, it is worth bearing in mind that we still lack a detailed model for how our own planetary system formed.
The recent discovery of several thousand extrasolar planetary systems has spurred great interest in how these systems formed. This delays the formation of embryos and stunts their growth, so that only low-mass planets can form here. In the region occupied by Mercury, pebble Stokes numbers are small. Adding planetesimal accretion allows Mars-sized objects to form inside the ice line, and allows giant-planet cores to form over a wider region beyond the ice line. When only pebble accretion is considered, embryos typically remain near their initial mass or grow to the pebble-isolation mass. The terrestrial planets are prevented from accreting much water ice because embryos beyond the ice line reach the pebble-isolation mass before the ice line enters the terrestrial-planet region. Pebble accretion inside the ice line is slowed by higher temperatures, partial removal of inflowing pebbles by planetesimal formation and pebble accretion further out in the disk, and increased radial velocities due to gas advection. Planetesimal accretion is more important inside the ice line. Planetary growth beyond the ice line is dominated by pebble accretion. The model produces a good fit to these characteristics for a narrow range of parameter space. The aim is to constrain aspects of planet formation that have large uncertainties by matching key characteristics of the solar system. We model the early stages of planet formation in the solar system, including continual planetesimal formation, and planetesimal and pebble accretion onto planetary embryos in an evolving disk driven by a disk wind.
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