TOI-1634 b : an ultra-short-period keystone planet sitting inside the M-dwarf radius valley
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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, F = 121F⊕, rp = 1.790 +0.080 -0.081 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 +0.68-0.70 M⊕, which makes TOI-1634 b inconsistent with an Earth-like composition at 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 Ms ∼< M⊙.
Cloutier , R , Charbonneau , D , Stassun , K G , Murgas , F , Mortier , A , Massey , R , Lissauer , J J , Latham , D W , Irwin , J , Haywood , R D , Guerra , P , Girardin , E , Giacalone , S A , Bosch-Cabot , P , Bieryla , A , Winn , J , Watson , C A , Vanderspek , R , Udry , S , Tamura , M , Sozzetti , A , Shporer , A , Ségransan , D , Seager , S , Savel , A B , Sasselov , D , Rose , M , Ricker , G , Rice , K , Quintana , E V , Quinn , S N , Piotto , G , Phillips , D , Pepe , F , Pedani , M , Parviainen , H , Palle , E , Narita , N , Molinari , E , Micela , G , McDermott , S , Mayor , M , Matson , R A , Martinez Fiorenzano , A F , Lovis , C , López-Morales , M , Kusakabe , N , Jensen , E L N , Jenkins , J M , Huang , C X , Howell , S B , Harutyunyan , A , Fűrész , G , Fukui , A , Esquerdo , G A , Esparza-Borges , E , Dumusque , X , Dressing , C D , Fabrizio , L D , Collins , K A , Cameron , A C , Christiansen , J L , Cecconi , M , Buchhave , L A , Boschin , W & Andreuzzi , G 2021 , ' TOI-1634 b : an ultra-short-period keystone planet sitting inside the M-dwarf radius valley ' , Astronomical Journal , vol. 162 , no. 2 , 79 . https://doi.org/10.3847/1538-3881/ac0157
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DescriptionFunding The HARPS-N project has been funded by the Prodex Program of the Swiss Space Office (SSO), the Harvard University Origins of Life Initiative (HUOLI), the Scottish Universities Physics Alliance (SUPA), the University of Geneva, the Smithsonian Astrophysical Observatory (SAO), the Italian National Astrophysical Institute (INAF), the University of St Andrews, Queens University Belfast, and the University of Edinburgh.
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