TOI-1634 b: An Ultra-short-period Keystone Planet Sitting inside the M-dwarf Radius Valley
López-Morales, Mercedes; Bieryla, Allyson; Latham, David W.; Quinn, Samuel N.; Esquerdo, Gilbert A.; Tamura, Motohide; Stassun, Keivan G.; Howell, Steve B.; Lissauer, Jack J.; Collins, Karen A.; Vanderspek, Roland; Seager, Sara; Jenkins, Jon M.; Buchhave, Lars A.; Shporer, Avi; Fukui, Akihiko; Narita, Norio; Palle, Enric; Udry, Stéphane; Jensen, Eric L. N.; Huang, Chelsea X.; Matson, Rachel A.; Piotto, Giampaolo; Watson, Christopher A.; Dressing, Courtney D.; Charbonneau, David; Pepe, Francesco; Kusakabe, Nobuhiko; Murgas, Felipe; Christiansen, Jessie L.; Ricker, George; Quintana, Elisa V.; Irwin, Jonathan; Cloutier, Ryan; Lovis, Christophe; Dumusque, Xavier; Sasselov, Dimitar; Sozzetti, Alessandro; Rice, Ken; Mortier, Annelies; Harutyunyan, Avet; Martinez Fiorenzano, Aldo F.; Mayor, Michel; Molinari, Emilio; Parviainen, Hannu; McDermott, Scott; Esparza-Borges, Emma; Savel, Arjun B.; Ségransan, Damien; Micela, Giuseppina; Cameron, Andrew Collier; Guerra, Pere; Fűrész, Gábor; Bosch-Cabot, Pau; Winn, Joshua; Girardin, Eric; Haywood, Raphaëlle D.; Pedani, Marco; Phillips, David; Cecconi, Massimo; Massey, Robert; Giacalone, Steven A.; Rose, Mark; Fabrizio, Luca Di; Boschin, Walter; Andreuzzi, Gloria
United States, Spain, United Kingdom, Switzerland, Japan, Italy, Denmark
Abstract
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, $F=121{F}_{\oplus }$, ${r}_{p}={1.790}_{-0.081}^{+0.080}$ R⊕) orbiting a nearby M2 dwarf (Ks = 8.7, Rs = 0.450 R⊙, Ms = 0.502 M⊙) 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 ${4.91}_{-0.70}^{+0.68}$ M⊕, which makes TOI-1634 b inconsistent with an Earth-like composition at $5.9\sigma $ and thus requires either an extended gaseous envelope, a large volatile-rich layer, or a rocky composition 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 Earth supports the gas-depleted formation mechanism to explain the emergence of the radius valley around M dwarfs with ${M}_{s}\lesssim 0.5$ M⊙.