Migrating Planets into Ultra-Short-Period Orbits during Episodic Accretion Events

TL;DR

This paper proposes a new mechanism involving magnetically-driven sub-Keplerian gas flow and FU Ori outbursts that can migrate small planets into ultra-short-period orbits, explaining their observed locations.

Contribution

It introduces a novel migration process driven by sub-Keplerian gas flow and episodic outbursts, expanding understanding of planet migration near disk truncation zones.

Findings

Sub-Keplerian gas flow can reverse Type I migration effects.

FU Ori outbursts can rapidly move planets inward.

Low-mass planets are most affected, matching observations.

Abstract

Ultra-short-period (USP) planets reside inside the expected truncation radius for typical T Tauri disks. As a result, their current orbital locations require an explanation beyond standard disk migration or in situ formation. Modern theories of planet-disk interactions indicate that once a planet migrates close to the disk's inner truncation radius, Type I torques vanish or switch direction, depending on the stellar and disk conditions, so that the planet is expected to stop its orbital decay and become trapped. In this work, we show that that magnetically-driven sub-Keplerian gas flow in the inner disk can naturally counteract these effects and produce systems with USP planets at their observed orbital radii. The sub-Keplerian gas flow provides a headwind to small planets, and the resulting torque can overcome the effects of outward Type I migration near the co-rotation radius. For suitable disk and planet parameters, the torques due to the sub-Keplerian gas flow lead to inward migration on a rapid timescale. Over the time span of an FU Ori outburst, which moves the disk truncation radius inward, the rapid headwind migration can place planets in USP orbits. The combination of headwind migration and FU Ori outbursts thus provides a plausible mechanism to move small planets from $a=0.05-0.1$ AU down to $a=0.01-0.02$ AU. This effect is amplified for low-mass planets, consistent with existing observations.