The Edges of Planetary Systems: Falling Off the Kuiper Cliff in a Dissipating Gas Disk

Abstract

Probably the last planetesimals to have formed from dust in the solar nebula are Cold Classical Kuiper belt objects (CCKBOs). To the extent that they are isolated and unchanged since birth, CCKBOs offer direct insights into nebular processes. Their population density drops abruptly beyond a heliocentric radius of $\sim$47 au, a feature known as the "Kuiper Cliff". We show with global, 1D (radial), time-dependent models how gaseous protoplanetary disks that disperse from magnetic and photoevaporative winds leave behind planetesimal disks with Cliff-like outer edges. The gas disperses from the inside out, creating transitional disks whose inner cavities expand from $\lesssim$ 1 au to $\gtrsim$ 100 au. Gas at the cavity boundary presents a pressure maximum toward which dust particles drift, triggering the streaming instability which clumps dust into planetesimals massive enough to decouple from gas. The receding cavity wall thus paves a disk of planetesimals which truncates when dust and gas are spent. With no fine-tuning, we show how a generic gas disk clearing from the inside out reproduces the Kuiper Cliff and the CCKB surface density. Connecting these global 1D results with published local 3D simulations of dust and gas, we see how many properties of the CCKB -- its radial extent, total mass, individual object sizes, and binary statistics -- follow from the streaming instability at work in a late-stage transition disk.