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

TOI-1634 b: an Ultra-Short Period Keystone Planet Sitting Inside the M Dwarf Radius Valley