Effect of entropy advection in planetary growth

Vincent Savignac ( Université McGill )

Super-Earths and mini-Neptunes are the most common types of exoplanets discovered, yet the physical scenarios behind their formation are still up to debate. Indeed, existing numerical models describe an accretion of gas from the surrounding gaseous disk that is too efficient, so much so that typical mini-Neptune mass planets would blow up into Jupiters before the underlying disk gas dissipates away. Gas accretion is governed by the rate at which the gas cools. In this work, we investigate whether entropy advection can significantly slow down the cooling of the envelope by incorporating recycling flows of gas into a numerical 1D thermodynamic model with realistic equation of state and opacities. We find that entropy advection can delay significantly the cooling of the gaseous envelope, and therefore the runaway accretion, especially at short orbital distances. The relative delay of runaway times is similar for 5M⊕ and 10M⊕ cores, considerably stronger in dust-free atmospheres and slightly stronger for planets embedded in depleting gas disk. Although entropy advection can successfully delay runaway, it still produces planets that are too gas-heavy compared to the observed mini-Neptunes.