A planetary system with two transiting mini-Neptunes near the radius valley transition around the bright M dwarf TOI-776★
R. Luque, L. M. Serrano, Karan Molaverdikhani, Matthew C. Nixon, John H. Livingston, E. W. Guenther, Ε. Πάλλη, Nikku Madhusudhan, G. Nowak, J. Korth, William D. Cochran, Teruyuki Hirano, P. Chaturvedi, Elisa Goffo, Simon Albrecht, Oscar Barragán, César Briceño, J. Cabrera, David Charbonneau, Ryan Cloutier, Karen A. Collins, Kevin I. Collins, Knicole D. Colón, Crossfield, IJM, Sz. Csizmadia, F. Dai, H. J. Deeg, M. Esposito, M. Fridlund, D. Gandolfi, Iskra Georgieva, Ana Glidden, Robert F. Goeke, S. Grziwa, A. P. Hatzes, Christopher E. Henze, Steve B. Howell, Jonathan Irwin, Jon M. Jenkins, Jensen, ELN, P. Kábath, Richard C. Kidwell, John F. Kielkopf, Emil Knudstrup, Lam, KWF, David W. Latham, Jack J. Lissauer, Andrew W. Mann, Elisabeth C. Matthews, Ismael Mireles, Norio Narita, Martin Paegert, Carina M. Persson, Seth Redfield, G. Ricker, F. Rodler, Joshua E. Schlieder, Nicholas J. Scott, Sara Seager, J. Šubjak, Thiam-Guan Tan, Eric B. Ting, R. Vanderspek, Vincent Van Eylen, Joshua N. Winn, Carl Ziegler
Abstract
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 P b = 8.25 d, a radius of R b = 1.85 ± 0.13 R ⊕ , and a mass of M b = 4.0 ± 0.9 M ⊕ ; and for the outer planet, TOI-776 c, a period of P c = 15.66 d, a radius of R c = 2.02 ± 0.14 R ⊕ , and a mass of M c = 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.