TOI-1634 b: An Ultra-short-period Keystone Planet Sitting inside the M-dwarf Radius Valley

Studies of close-in planets orbiting M dwarfs have suggested that the dwarf radius valley may be well-explained by distinct formation timescales between enveloped terrestrials, and rocky form at late times in a gas-depleted environment. This scenario is odds with picture primordial gaseous envelope subsequently stripped away some thermally-driven mass loss process. These two physical scenarios make unique predictions rocky/enveloped transition's dependence on orbital separation such studying compositions within able to establish dominant physics. Here, we present discovery one keystone planet: ultra-short period planet TOI-1634 b ($P=0.989$ days, $F=121 F_{\oplus}$, $r_p = 1.790^{+0.080}_{-0.081} R_{\oplus}$) nearby M2 ($K_s=8.7$, $R_s=0.45 R_{\odot}$, $M_s=0.50 M_{\odot}$) whose size sit valley. We confirm TESS-discovered candidate using extensive ground-based follow-up campaigns, including set 32 precise radial velocity measurements from HARPS-N. measure planetary $4.91^{+0.68}_{-0.70} M_{\oplus}$, which makes inconsistent an Earth-like composition $5.9\sigma$ thus requires either extended envelope, large volatile-rich layer, or portion not dominated iron silicates explain its radius. The bulk Earth favors mechanism emergence around $M_s\lesssim 0.5 M_{\odot}$.