Silicate Rainout in Planets Formed via Pebble Accretion

Planet formation models of rocky sub-Neptunes suggest that most of the accreted silicates remain in vapor form by the end of the formation phase. As the planet cools, this silicate vapor condenses and settles—a process known as rainout. We incorporate silicate rainout into a planetary thermal evolution model and find that the process occurs over a timescale of approximately a gigayear (Gyr), with shorter durations for smaller planets. Our results show that silicate rainout naturally leads to a range of interior structures, consistent with those inferred from solar system measurements. This mechanism provides a physical link between the composition gradients predicted in massive planets and the distinct core-envelope structures observed in smaller ones. Upcoming precise age estimates from the PLATO mission, along with atmospheric composition measurements from JWST and ARIEL, will enable the identification of planets formed with initially polluted envelopes.