TOI-1235 b: A Keystone Super-Earth for Testing Radius Valley Emergence Models around Early M Dwarfs
Authors: Cloutier Ryan , Rodriguez Joseph E. , Irwin Jonathan , Charbonneau David , Stassun Keivan G. , Mortier Annelies , Latham David W. , Isaacson Howard , Howard Andrew W. , Udry Stéphane , Wilson Thomas G. , Watson Christopher A. , Pinamonti Matteo , Lienhard Florian , Giacobbe Paolo , Guerra Pere , Collins Karen A. , Beiryla Allyson , Esquerdo Gilbert A. , Matthews Elisabeth , Matson Rachel A. , Howell Steve B. , Furlan Elise , Crossfield Ian J. M. , Winters Jennifer G. , Nava Chantanelle , Ment Kristo , Lopez Eric D. , Ricker George , Vanderspek Roland , Seager Sara , Jenkins Jon M. , Ting Eric B. , Tenenbaum Peter , Sozzetti Alessandro , Sha Lizhou , Ségransan Damien , Schlieder Joshua E. , Sasselov Dimitar , Roy Arpita , Robertson Paul , Rice Ken , Poretti Ennio , Piotto Giampaolo , Phillips David , Pepper Joshua , Pepe Francesco , Molinari Emilio , Mocnik Teo , Micela Giuseppina , Mayor Michel , Martinez Fiorenzano Aldo F. , Mallia Franco , Lubin Jack , Lovis Christophe , López-Morales Mercedes , Kosiarek Molly R. , Kielkopf John F. , Kane Stephen R. , Jensen Eric L. N. , Isopi Giovanni , Huber Daniel , Hill Michelle L. , Harutyunyan Avet , Gonzales Erica , Giacalone Steven , Ghedina Adriano , Ercolino Andrea , Dumusque Xavier , Dressing Courtney D. , Damasso Mario , Dalba Paul A. , Cosentino Rosario , Conti Dennis M. , Colón Knicole D. , Collins Kevin I. , Cameron Andrew Collier , Ciardi David , Christiansen Jessie , Chontos Ashley , Cecconi Massimo , Caldwell Douglas A. , Burke Christopher , Buchhave Lars , Beichman Charles , Behmard Aida , Beard Corey , Akana Murphy Joseph M. 
Published: June 2020

Small planets on close-in orbits tend to exhibit envelope mass fractions of either effectively zero or up to a few percent depending on their size and orbital period. Models of thermally driven atmospheric mass loss and of terrestrial planet formation in a gas-poor environment make distinct predictions regarding the location of this rocky/nonrocky transition in period–radius space. Here we present the confirmation of TOI-1235 b (P = 3.44 days,

), a planet whose size and period are intermediate between the competing model predictions, thus making the system an important test case for emergence models of the rocky/nonrocky transition around early M dwarfs (R
s = 0.630 ± 0.015

, M
s = 0.640 ± 0.016

). We confirm the TESS planet discovery using reconnaissance spectroscopy, ground-based photometry, high-resolution imaging, and a set of 38 precise radial velocities (RVs) from HARPS-N and HIRES. We measure a planet mass of

, which implies an iron core mass fraction of

% in the absence of a gaseous envelope. The bulk composition of TOI-1235 b is therefore consistent with being Earth-like, and we constrain an H/He envelope mass fraction to be <0.5% at 90% confidence. Our results are consistent with model predictions from thermally driven atmospheric mass loss but not with gas-poor formation, suggesting that the former class of processes remains efficient at sculpting close-in planets around early M dwarfs. Our RV analysis also reveals a strong periodicity close to the first harmonic of the photometrically determined stellar rotation period that we treat as stellar activity, despite other lines of evidence favoring a planetary origin (

days,

) that cannot be firmly ruled out by our data.