Diversity in Planetary Architectures from Pebble Accretion: Water Delivery to the Habitable Zone with Pebble Snow

Abstract “Pebble snow” describes a planet formation mechanism where icy pebbles in the outer disk reach inner planet embryos as the water-ice line evolves inward. We model the effects pebble snow has on sculpting planetary system architectures by developing the “PPOLs model.” The model is capable of growing any number of protoplanet seed masses by pebble accretion simultaneously, and accounts for differences in rocky and icy pebble composition, the filtering of pebbles by other protoplanets, the pebble isolation mass, and a self-consistently evolving snow line. The growth and bulk composition are recorded across a grid of protoplanetary disks with stellar masses ranging from 0.125 to 2.0 M ⊙ (M to A stars) and disk masses ranging from 1% to 40% of the stellar mass. Three system architectures emerge following a low, medium, and high disk mass fraction that remains consistent across stellar mass. The low-mass architecture is the only one to yield short-period Mars- to Earth-mass cores with bulk water content spanning orders of magnitude and may be a prelude to observed “peas in pod” systems. The high-mass architecture produces proto–gas giant cores in the outer disk. The medium-mass architecture produces a bimodal peak in mass within a system, with the outer protoplanet mass at the snow line growing to an order of magnitude larger, resembling the solar system. Solar system–like architectures appear for a small range of initial disk masses around F and G stars, but are not a common feature around K and M stars.