TOI-1634 b: an Ultra-Short Period Keystone Planet Sitting Inside the M Dwarf Radius Valley
Creators: name: Cloutier, R.
name: Charbonneau, D.
name: Stassun, K. G.
name: Murgas, F.
name: Mortier, A.
name: Massey, R.
name: Lissauer, J. J.
name: Latham, D. W.
name: Irwin, J.
name: Haywood, R. D.
name: Guerra, P.
name: Girardin, E.
name: Giacalone, S. A.
name: Bosch-Cabot, P.
name: Bieryla, A.
name: Winn, J.
name: Watson, C. A.
name: Vanderspek, R.
name: Udry, S.
name: Tamura, M.
name: Sozzetti, A.
name: Shporer, A.
name: Ségransan, D.
name: Seager, S.
name: Savel, A. B.
name: Sasselov, D.
name: Rose, M.
name: Ricker, G.
name: Rice, K.
name: Quintana, E. V.
name: Quinn, S. N.
name: Piotto, G.
name: Phillips, D.
name: Pepe, F.
name: Pedani, M.
name: Parviainen, H.
name: Palle, E.
name: Narita, N.
name: Molinari, E.
name: Micela, G.
name: McDermott, S.
name: Mayor, M.
name: Matson, R. A.
name: Fiorenzano, A. F. Martinez
name: Lovis, C.
name: López-Morales, M.
name: Kusakabe, N.
name: Jensen, E. L. N.
name: Jenkins, J. M.
name: Huang, C. X.
name: Howell, S. B.
name: Harutyunyan, A.
name: Fűrész, G.
name: Fukui, A.
name: Esquerdo, G. A.
name: Esparza-Borges, E.
name: Dumusque, X.
name: Dressing, C. D.
name: Di Fabrizio, L.
name: Collins, K. A.
name: Cameron, A. Collier
name: Christiansen, J. L.
name: Cecconi, M.
name: Buchhave, L. A.
name: Boschin, W.
name: Andreuzzi, G.
Publisher: name: arXiv
Year: 2021
Subjects: subject: Earth and Planetary Astrophysics (astro-ph.EP)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: Studies of close-in planets orbiting M dwarfs have suggested that the M dwarf radius valley may be well-explained by distinct formation timescales between enveloped terrestrials, and rocky planets that form at late times in a gas-depleted environment. This scenario is at odds with the picture that close-in rocky planets form with a primordial gaseous envelope that is subsequently stripped away by some thermally-driven mass loss process. These two physical scenarios make unique predictions of the rocky/enveloped transition's dependence on orbital separation such that studying the compositions of planets within the M dwarf radius valley may be able to establish the dominant physics. Here, we present the discovery of one such keystone 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 a nearby M2 dwarf ($K_s=8.7$, $R_s=0.45 R_{\odot}$, $M_s=0.50 M_{\odot}$) and whose size and orbital period sit within the M dwarf radius valley. We confirm the TESS-discovered planet candidate using extensive ground-based follow-up campaigns, including a set of 32 precise radial velocity measurements from HARPS-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 $5.9σ$ and thus requires either an extended gaseous envelope, a large volatile-rich layer, or a rocky portion that is not dominated by iron and silicates to explain its mass and radius. The discovery that the bulk composition of TOI-1634 b is inconsistent with that of the Earth favors the gas-depleted formation mechanism to explain the emergence of the radius valley around M dwarfs with $M_s\lesssim 0.5 M_{\odot}$.
description: 27 pages, 13 figures, accepted to AAS journals. Our time series are included as a csv file in the arXiv source files