Planet Birth at the Disk's Edge express
Rocky planets orbit dim red stars. Some of them have a year shorter than an Earth day: these worlds are called ultra-short-period planets. Their surfaces are scorching, and their skies are forever bathed in crimson light. How did they end up so close to their star?
New research tests a simple idea: it's all about the edge of the gas-and-dust disk from which planets are born. This disk resembles a whirlpool of gas and dust around a young star. Its inner edge is the boundary beyond which matter can no longer orbit: the star heats and pushes it away. The authors ran computer simulations with three scenarios for this edge: stationary, moving outward, and moving inward toward the star.
Only two scenarios produced ultra-short-period planets. In the first, the disk's edge remained frozen close to the star, and planets, like pebbles at the edge of a table, piled up right there. In the second, the edge slowly crept toward the star, and the planets followed it, like a wake trailing a boat. The third scenario, where the edge receded, left the planets no chance: they formed far out and didn't have time to migrate.
So the fate of an entire class of planets is decided at the very boundary — where the disk meets the star. A fine-tuning, but it's precisely this that creates the worlds we later find around distant red dwarfs.
Original abstract
as written by the authors arXiv:2603.26204
Close-in rocky planets are the most common type of exoplanets around late M dwarfs, ranging from more temperate worlds to highly irradiated lava planets with molten surfaces, and many theoretical studies have attempted to explain their formation. However, the origin of rocky planets with orbital periods shorter than one day, known as ultra-short-period (USP) planets, remains uncertain. We aim to investigate whether the formation and survival of USP planets is connected to the location of the inner edge of the protoplanetary disk, considering different disk edge prescriptions. We use N-body simulations that include planet-disk interactions, star-planet tidal interactions, and relativistic corrections, applied to a sample of lunar-mass planetary seeds growing via pebble accretion in a low-viscosity disk ($$\alpha_t = 10^{-4}$$). The inner edge of the disk is modeled in three ways: as a fixed close-in edge, as an outward-evolving edge set by the magnetospheric truncation radius, and as an inward-evolving edge defined by the corotation radius. USP planet formation appears to be tightly controlled by the location of the disk's inner edge. Our simulations show that only the close-in-fixed-edge Scenario and the inward-evolving-edge Scenario are capable of producing USP planets, as planets tend to follow the movement of the disk's inner edge. This suggests that USP planet formation is favored when the inner edge remains close to the corotation radius of a rapidly rotating star.
Ultra-short-period planets are the fastest known: their year lasts from 4 to 24 hours, and surface temperatures can exceed 2000 degrees Celsius.