The Not-so Dramatic Effect of Advective Flows on Gas Accretion

Abstract

Super-Earths and mini-Neptunes are the most common types of exoplanets discovered, yet the physics of their formation are still debated. Standard core accretion models in gas-rich environments find that typical mini-Neptune mass planets would blow up into Jupiters before the underlying disk gas dissipates away. The injection of entropy from the protoplanetary disk into forming gaseous envelopes has recently been put forward as a mechanism to delay this runaway accretion, specifically at short orbital distances. Here, we reevaluate this line of reasoning by incorporating recycling flows of gas into a numerical one-dimensional thermodynamic model with a more realistic equation of state and opacities and the thermal state of the advective flow. At 0.1 au, we find that advective flows are only able to produce mini-Neptunes if they can penetrate below ∼0.25 of the planet’s gravitational sphere of influence. Otherwise, the gas-to-core mass ratio (GCR) reaches above ∼10%, which is too large to explain the measured properties of mini-Neptunes, necessitating other gas-limiting processes such as late-time core assembly. The effect of entropy advection on gas accretion weakens even further beyond 0.1 au. We present an updated scaling relation between GCR and the penetration depth of the advective flows, which varies nontrivially with orbital distances, core masses, and dusty versus dust-free opacity. We further demonstrate how measurements of planet mass distribution beyond ∼1 au using future instruments such as the Nancy Grace Roman Space Telescope could be used to disambiguate between different formation conditions of gas-poor planets.