Presentation #302.11 in the session “Atmospheric Characterization of TESS Exoplanets”.
In this talk, I present the discovery and characterization of two transiting planets around the bright M1 V star TOI-776 (J=8.5 mag), detected during Sector 10 observations of the Transiting Exoplanet Survey Satellite (TESS). Combining them with HARPS radial velocities, as well as ground-based follow-up transit observations from MEarth and LCOGT telescopes, we measured for the inner planet, TOI-776 b, a period of 8.25 d, a radius of 1.83+-0.11 REarth, and a mass of 4.66+-0.97 MEarth; and for the outer planet, TOI-776 c, a period of 15.66 d, a radius of 2.06+-0.13 REarth, and a mass of 6.1+-1.5 MEarth. The bulk densities of TOI-776 b and c allow for a wide range of possible interior and atmospheric compositions. However, both planets must 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. Due to the brightness of the star, the TOI-776 system is an excellent target for the JWST, providing a remarkable laboratory to break the degeneracy in planetary interior models and to test formation and evolution theories of small planets around low-mass stars.