2021MNRAS.505.1827E

We report on the discovery and validation of a two-planet system around a bright (V = 8.85 mag) early G dwarf (1.43 R_☉, 1.15 M_☉, TOI 2319) using data from NASA's Transiting Exoplanet Survey Satellite (TESS). Three transit events from two planets were detected by citizen scientists in the month-long TESS light curve (sector 25), as part of the Planet Hunters TESS project. Modelling of the transits yields an orbital period of 11.6264_- 0.0025_ ^+ 0.0022^ d and radius of 3.41_- 0.12_^+ 0.14^ R_⊕ for the inner planet, and a period in the range 19.26-35 d and a radius of 5.83_–0.14^+ 0.14^ R_⊕ for the outer planet, which was only seen to transit once. Each signal was independently statistically validated, taking into consideration the TESS light curve as well as the ground-based spectroscopic follow-up observations. Radial velocities from HARPS-N and EXPRES yield a tentative detection of planet b, whose mass we estimate to be 11.56_–6.14^+6.58 M_⊕, and allow us to place an upper limit of 27.5 M_⊕ (99 per cent confidence) on the mass of planet c. Due to the brightness of the host star and the strong likelihood of an extended H/He atmosphere on both planets, this system offers excellent prospects for atmospheric characterization and comparative planetology.