Following the Edge: How Inner Disk Edges Shape Ultra-Short-Period Planet Formation

Rocky planets on short-period orbits are the most common planets found around late M dwarfs. Around these small, cool stars, such planets can range from potentially temperate worlds to highly irradiated lava planets with molten surfaces. While many formation pathways have been proposed for such close-in rocky planets, the origin of the most extreme cases, ultra-short-period (USP) planets with orbital periods shorter than one day, is still not fully understood. In this talk, I will discuss whether the formation and survival of USP planets can be linked to the location of the inner edge of the protoplanetary disk. To test this, we performed N-body simulations that include planet-disk interactions, star-planet tidal interactions, and relativistic corrections. The simulations start from lunar-mass planetary seeds growing by pebble accretion in a low-viscosity disk. We compared three different prescriptions for the inner disk edge: a fixed close-in edge, an outward-moving edge set by the magnetospheric truncation radius, and an inward-moving edge associated with the corotation radius. The results show that the formation of USP planets appears to be strongly controlled by the location of the disk’s inner edge. USP planets are produced only when the disk edge either remains close to the star or evolves inward, because migrating planets tend to follow the motion of the disk edge. These findings suggest that USP planet formation around late M dwarfs is favored when the inner edge of the disk stays near the corotation radius of a rapidly rotating star.