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

August 2021 • 2021AJ....162...79C

Authors • 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

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.


IPAC Authors


Jessie Christiansen

Associate Scientist