A low-eccentricity migration pathway for a 13-h-period Earth analogue in a four-planet system

Abstract

It is commonly accepted that exoplanets with orbital periods shorter than 1 day, also known as ultra-short period (USP) planets, formed further out within their natal protoplanetary disk, before migrating to their current-day orbits via dynamical interactions. One of the most accepted theories suggests a violent scenario involving high-eccentricity migration followed by tidal circularization. Here, we present the discovery of a four planet system orbiting the bright (V=10.5) K6 dwarf star TOI-500. The innermost planet is a transiting, Earth-sized USP planet with an orbital period of ∼13 hours, a mass of 1.42 ± 0.18 M ⊕ , a radius of 1.166 +0.061 −0.058 R ⊕ , and a mean density of 4.89 +1.03 −0.88 g cm −3 . Via Doppler spectroscopy, we discovered that the system hosts three outer planets on nearly circular orbits with periods of 6.6, 26.2, and 61.3 d and minimum masses of 5.03 ± 0.41 M ⊕ , 33.12 ± 0.88 M ⊕ , and 15.05 +1.12 −1.11 M ⊕ , respectively. The presence of both a USP planet and a low-mass object on a 6.6-day orbit indicates that the architecture of this system can be explained via a scenario in which the planets started on low-eccentricity orbits, then moved inwards through a quasi-static secular migration. Our numerical simulations show that this migration channel can bring TOI-500 b to its current location in 2 Gyrs, starting from an initial orbit of 0.02 au. TOI-500 is the first four planet system known to host a USP Earth analog whose current architecture can be explained via a non-violent migration scenario.

HD 20329b: An ultra-short-period planet around a solar-type star found by TESS

“…At the end of this process, the innermost planet becomes a USP planet, while the second planet, an Earth-or super-Earth-sized planet, stabilizes itself on a 10 day orbit. TOI-500 is the first four-planet system for which this mechanism has been proven to work (Serrano et al 2022). Millholland & Spalding (2020) proposed an obliquity-driven tidal migration mechanism.…”