Photoevaporation Does Not Create a Pileup of Giant Planets at 1 AU

TL;DR

Photoevaporation in protoplanetary disks has minimal impact on the final positions of giant exoplanets, making it unlikely to explain the observed pileup near 1 AU.

Contribution

This study combines analytic models and hydrodynamic simulations to show that photoevaporation does not significantly influence giant planet migration or their final orbital distribution.

Findings

Photoevaporation alters giant planet migration by at most 5%.

Migration stalls when disk surface density drops below a threshold.

Photoevaporation is unlikely to create the 1 AU pileup of giant planets.

Abstract

The semimajor axis distribution of giant exoplanets appears to have a pileup near 1 AU. Photoevaporation opens a gap in the inner few AU of gaseous disks before dissipating them. Here we investigate whether photoevaporation can significantly affect the final distribution of giant planets by modifying gas surface density and hence Type II migration rates near the photoevaporation gap. We first use an analytic disk model to demonstrate that newly-formed giant planets have a long migration epoch before photoevaporation can significantly alter their migration rates. Next we present new 2-D hydrodynamic simulations of planets migrating in photoevaporating disks, each paired with a control simulation of migration in an otherwise identical disk without photoevaporation. We show that in disks with surface densities near the minimum threshold for forming giant planets, photoevaporation alters the final semimajor axis of a migrating gas giant by at most 5% over the course of 0.1 Myr of migration. Once the disk mass is low enough for photoevaporation to carve a sharp gap, migration has almost completely stalled due to the low surface density of gas at the Lindblad resonances. We find that photoevaporation modifies migration rates so little that it is unlikely to leave a significant signature on the distribution of giant exoplanets.