Inner Super-Earths, Outer Gas Giants: How Pebble Isolation and Migration Feedback Keep Jupiters Cold

Abstract

The majority of gas giants (planets of masses $\gtrsim10^2 M_\oplus$) are found to reside at distances beyond $\sim1$ au from their host stars. Within 1 au, the planetary population is dominated by super-Earths of $2-20 M_\oplus$. We show that this dichotomy between inner super-Earths and outer gas giants can be naturally explained should they form in nearly inviscid disks. In laminar disks, a planet can more easily repel disk gas away from its orbit. The feedback torque from the pile-up of gas inside the planet's orbit slows down and eventually halts migration. A pressure bump outside the planet's orbit traps pebbles and solids, starving the core. Gas giants are born cold and stay cold: more massive cores are preferentially formed at larger distances, and they barely migrate under disk feedback. We demonstrate this using 2D hydrodynamical simulations of disk-planet interaction lasting up to $10^5$ years: we track planet migration and pebble accretion until both come to an end by disk feedback. Whether cores undergo runaway gas accretion to become gas giants or not is determined by computing 1D gas accretion models. Our simulations show that in an inviscid minimum mass solar nebula, gas giants do not form inside $\sim$0.5 au, nor can they migrate there while the disk is present. We also explore the dependence on disk mass, and find that gas giants form further out in less massive disks.