Sub-Neptune planets are common type of planets in our galaxy. Recent modelling of planet formation has highlighted that sub-Neptune planets formed by pebble accretion are characterized by large quantities of silicate vapor in their gas (hydrogen-helium) envelope. Upon cooling this vapor is expected to condense and rain-out into deeper layers.
In this work we aim to examine the rainout process and its timescale.
We calculate thermal evolution of rocky planets formed by pebble accretion. We model the cooling of the polluted envelopes self-consistently with the consequent rainout (condensation and settling) of rocks, and the mass loss by irradiation from the parent star. We find that cooling leads to a significant core growth by rainout in sub-Neptune planets, where a complete rainout results in a core-envelope structure. However, the timescale of the rainout phase varies between 0.1-10 giga-years, and depends mainly on envelope mass. Planets that experienced mass loss and thus have envelopes of less than 0.5 Earth mass reach a core-envelope structure in up to 1 giga-year, while planets with more massive envelopes can be in the rainout phase for giga years. The energy released by the rainout causes a radius expansion, which is bigger but shorter in low mass envelopes. We conclude that interiors of rock dominated super-Earths have core-envelope structure, while Neptune-like planets have diluted cores in which rock rainout is still ongoing.