Dynamical Upheaval in Ice Giant Formation
A protoplanetary disk dynamical instability may have played a crucial role in determining the atmospheric size of the solar system’s ice giants. In contrast to the gas giants, the intermediate-size ice giants never underwent runaway gas accretion in a full gas disk. However, their substantial core masses would have allowed them to go runaway, given enough time. In the standard scenario, the ice giants stay at roughly their current size for most of the disk lifetime, undergoing period of slow gas accretion onto ~full-sized cores that formed early-on. The gas disk dissipates before the ice giants accumulate too much gas. Pebble accretion makes this scenario newly fine-tuned: the cores cannot stay at roughly their current masses for very long. Assisted by gas drag, the pebbles cause rapid core growth, allowing the ice giants to quickly surpass critical core mass and go runaway. To resolve this problem, we propose that Uranus and Neptune stayed small for the bulk of the disk lifetime, only finishing their core growth and accreting most of their gas from a disk depleted to ~1% of its original mass. The ice giants have atmospheric mass fractions comparable to the disk gas-to-solid ratio of this depleted disk. Orbit damping by gas in such a disk is inefficient, triggering dynamical instability. We propose that the cores started growing closer-in, where they were kept small by proximity to Jupiter and Saturn. The cores were kicked out to their current orbits during the late-stage dynamical upheaval, where they completed their growth.