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

Cloutier, Ryan; Charbonneau, David; Stassun, Keivan G.; Murgas, Felipe; Mortier, Annelies; Massey, Robert; Lissauer, Jack J.; Latham, David W.; Irwin, Jonathan; Haywood, Raphaëlle D.; Guerra, Pere; Girardin, Eric; Giacalone, Steven A.; Bosch-Cabot, Pau; Bieryla, Allyson; Winn, Joshua; Watson, Christopher A.; Vanderspek, Roland; Udry, Stéphane; Tamura, Motohide; Sozzetti, Alessandro; Shporer, Avi; Ségransan, Damien; Seager, Sara; Savel, Arjun B.; Sasselov, Dimitar; Rose, Mark; Ricker, George; Rice, Ken; Quintana, Elisa V.; Quinn, Samuel N.; Piotto, Giampaolo; Phillips, David; Pepe, Francesco; Pedani, Marco; Parviainen, Hannu; Palle, Enric; Narita, Norio; Molinari, Emilio; Micela, Giuseppina; McDermott, Scott; Mayor, Michel; Matson, Rachel A.; Martinez Fiorenzano, Aldo F.; Lovis, Christophe; López-Morales, Mercedes; Kusakabe, Nobuhiko; Jensen, Eric L. N.; Jenkins, Jon M.; Huang, Chelsea X.; Howell, Steve B.; Harutyunyan, Avet; Fűrész, Gábor; Fukui, Akihiko; Esquerdo, Gilbert A.; Esparza-Borges, Emma; Dumusque, Xavier; Dressing, Courtney D.; Fabrizio, Luca Di; Collins, Karen A.; Cameron, Andrew Collier; Christiansen, Jessie L.; Cecconi, Massimo; Buchhave, Lars A.; Boschin, Walter; Andreuzzi, Gloria
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The Astronomical Journal

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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, <?CDATA $F=121{F}_{\oplus }$?> , <?CDATA ${r}_{p}={1.790}_{-0.081}^{+0.080}$?> 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 <?CDATA ${4.91}_{-0.70}^{+0.68}$?> M⊕, which makes TOI-1634 b inconsistent with an Earth-like composition at <?CDATA $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 <?CDATA ${M}_{s}\lesssim 0.5$?> M⊙.
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