Confirmation of TOI-6324 b: Mass Measurement of an Earth-sized USP with the Keck Planet Finder

Earth-sized “ultra-short-period” planets (USPs) comprise a recently growing population of exoplanets. USPs provide the clearest look into terrestrial planet composition, as they are among the only Earth-sized planets with readily detectable radial velocity (RV) semi-amplitudes using the current state-of-the-art instrumentation. Moreover, USPs are so intensely irradiated that any H/He atmospheres should be photoevaporated, lifting the degeneracy caused by thick primordial atmospheres, allowing for direct constraint of rocky core compositions. Here we report the confirmation of TOI-6324 b, an Earth-sized (R = 1.05 R_Earth) USP orbiting a nearby (~20 pc) M dwarf on an orbit of only 6.7 hours, with some of the earliest results from the newly commissioned Keck Planet Finder (KPF). Our intensive radial velocity follow-up with KPF has pinned down the planetary mass to be 1.3 M_Earth to more than 5 σ precision, the lowest among USPs. We assess the planet’s iron-to-rock ratio given our radius and mass measurements and compare to the broader population of USPs and terrestrial planets. The emission spectroscopy metric (ESM) for TOI-6324 b is > 20, ranking the planet among the top 5 highest ESM targets. Additionally, TOI-6324 b and other KPF USPs will serve as ideal targets for phase curve studies, such as with JWST, to probe surface composition.