A planetary system with two transiting mini-Neptunes near the radius valley transition around the bright M dwarf TOI-776
Creators: name: Madhusudhan, N.
name: Molaverdikhani, K.
name: Van Eylen, V.
name: Schlieder, J.E.
name: Matthews, E.C.
name: Goffo, E.
name: Persson, C.M.
name: Seager, S.
name: Nowak, G.
name: Cabrera, J.
name: Jenkins, J.M.
name: Georgieva, I.
name: Rodler, F.
name: Mann, A.W.
name: Grziwa, S.
name: Ricker, G.R.
name: Kábath, P.
name: Knudstrup, E.
name: Winn, J.N.
name: Korth, J.
name: Paegert, M.
name: Howell, S.B.
name: Livingston, J.H.
name: Ting, E.B.
name: Gandolfi, D.
name: Guenther, E.W.
name: Redfield, S.
name: Latham, D.W.
name: Luque, R.
name: Cochran, W.D.
name: Goeke, R.F.
name: Dai, F.
name: Narita, N.
name: Csizmadia, S.
name: Hirano, T.
name: Charbonneau, D.
name: Pallé, E.
name: Collins, K.I.
name: Hatzes, A.P.
name: Lissauer, J.J.
name: Serrano, L.M.
name: Mireles, I.
name: Briceño, C.
name: Glidden, A.
name: Chaturvedi, P.
name: Crossfield, I.J.M.
name: Barragán, O.
name: Collins, K.A.
name: Vanderspek, R.K.
name: Kidwell, R.C.
name: Lam, K.W.F.
name: Esposito, M.
name: Irwin, J.
name: Jensen, E.L.N.
name: Nixon, M.C.
name: Deeg, H.J.
name: Scott, N.J.
name: Fridlund, M.
name: Colón, K.D.
name: Albrecht, S.
name: Cloutier, R.
name: Kielkopf, J.F.
name: Henze, C.E.
name: Tan, T.G.
name: Šubjak, J.
name: Ziegler, C.
Publisher: name: EDP Sciences
Year: 2021

description: We report the discovery and characterization of two transiting planets around the bright M1 V star LP 961-53 (TOI-776, J = 8.5 mag, M = 0.54 ± 0.03 M⊙) detected during Sector 10 observations of the Transiting Exoplanet Survey Satellite (TESS). Combining the TESS photometry with HARPS radial velocities, as well as ground-based follow-up transit observations from the MEarth and LCOGT telescopes, for the inner planet, TOI-776 b, we measured a period of Pb = 8.25 d, a radius of Rb = 1.85 ± 0.13 R⊙, and a mass of Mb = 4.0 ± 0.9 M⊙; and for the outer planet, TOI-776 c, a period of Pc = 15.66 d, a radius of Rc = 2.02 ± 0.14 R⊙, and a mass of Mc = 5.3 ± 1.8 M⊙. The Doppler data shows one additional signal, with a period of ~34 d, associated with the rotational period of the star. The analysis of fifteen years of ground-based photometric monitoring data and the inspection of different spectral line indicators confirm this assumption. The bulk densities of TOI-776 b and c allow for a wide range of possible interior and atmospheric compositions. However, both planets have retained a significant atmosphere, with slightly different envelope mass fractions. Thanks to their location near the radius gap for M dwarfs, we can start to explore the mechanism(s) responsible for the radius valley emergence around low-mass stars as compared to solar-like stars. While a larger sample of well-characterized planets in this parameter space is still needed to draw firm conclusions, we tentatively estimate that the stellar mass below which thermally-driven mass loss is no longer the main formation pathway for sculpting the radius valley is between 0.63 and 0.54 M⊙. Due to the brightness of the star, the TOI-776 system is also an excellent target for the James Webb Space Telescope, providing a remarkable laboratory in which to break the degeneracy in planetary interior models and to test formation and evolution theories of small planets around low-mass stars.