TOI-6324 b An Earth-mass Ultra-short-period Planet Transiting a Nearby M Dwarf

We report the confirmation of TOI-6324 b, an Earth-sized (1.059 ± 0.041 R_⊕) ultra-short-period (USP) planet orbiting a nearby (∼20 pc) M dwarf. Using the newly commissioned Keck Planet Finder spectrograph, we have measured the mass of TOI-6324 b 1.17 ± 0.22 M_⊕. Because of its extremely short orbit of just ∼6.7 hr, TOI-6324 b is intensely irradiated by its M dwarf host and is expected to be stripped of any thick H/He envelope. We were able to constrain its interior composition and found an iron-core mass fraction (CMF = 27% ± 37%) consistent with that of Earth (∼33%) and other confirmed USPs. TOI-6324 b is the closest to an Earth-sized USP confirmed to date. TOI-6324 b is a promising target for JWST phase-curve and secondary eclipse observations (emission spectroscopy metric = 25), which may reveal its surface mineralogy, day-night temperature contrast, and possible tidal deformation. From seven sectors of TESS data, we report a tentative detection of the optical phase-curve variation with an amplitude of 42 ± 28 ppm.