A planetary system with two transiting mini-Neptunes near the radius valley transition around the bright M dwarf TOI-776
Fridlund, M.; Pallé, E.; Guenther, E. W.; Barragán, O.; Dai, F.; Gandolfi, D.; Hirano, T.; Korth, J.; Grziwa, S.; Albrecht, S.; Hatzes, A. P.; Csizmadia, Sz.; Cabrera, J.; Narita, N.; Cochran, W. D.; Persson, C. M.; Van Eylen, V.; Nowak, G.; Winn, J. N.; Knudstrup, E.; Serrano, L. M.; Collins, K. A.; Collins, K. I.; Deeg, H. J.; Esposito, M.; Goffo, E.; Jenkins, J. M.; Lam, K. W. F.; Luque, R.; Redfield, S.; Ricker, G. R.; Rodler, F.; Seager, S.; Molaverdikhani, K.; Latham, D. W.; Vanderspek, R.; Crossfield, I. J. M.; Madhusudhan, N.; Irwin, J.; Charbonneau, D.; Chaturvedi, P.; Jensen, E. L. N.; Kielkopf, J. F.; Briceño, C.; Howell, S. B.; Matthews, E. C.; Mann, A. W.; Cloutier, R.; Schlieder, J. E.; Georgieva, I.; Ting, E. B.; Henze, C. E.; Kidwell, R. C.; Tan, T. G.; Ziegler, C.; Scott, N. J.; Lissauer, J. J.; Šubjak, J.; Livingston, J. H.; Paegert, M.; Goeke, R. F.; Mireles, I.; Kábath, P.; Colón, K. D.; Nixon, M. C.; Glidden, A.
Spain, Italy, Germany, United Kingdom, Japan, United States, Denmark, Chile, Netherlands, Sweden, Czech Republic, Australia, Canada
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.
Based on observations made with ESO Telescopes at the La Silla Observatory under programs ID 1102.C-0923 and 60.A-9709.