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

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

Research output: Contribution to journalArticlepeer-review

22 Scopus citations


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 (K s = 8.7, R s = 0.450 R o˙, M s = 0.502 M o˙) 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 o˙.

Original languageEnglish (US)
Article number79
JournalAstronomical Journal
Issue number2
StatePublished - Aug 2021

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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