Planet Hunters TESS. V. A Planetary System Around a Binary Star, Including a Mini-Neptune in the Habitable Zone

We report on the discovery and validation of a transiting long-periodmini-Neptune orbiting a bright (V = 9.0 mag) G dwarf (TOI 4633; R = 1.05 RSun,M = 1.10 MSun). The planet was identified in data from the Transiting ExoplanetSurvey Satellite by citizen scientists taking part in the Planet Hunters TESSproject. Modeling of the transit events yields an orbital period of 271.9445+/- 0.0040 days and radius of 3.2 +/- 0.20 REarth. The Earth-like orbitalperiod and an incident flux of 1.56 +/- 0.2 places it in the optimistichabitable zone around the star. Doppler spectroscopy of the system allowed usto place an upper mass limit on the transiting planet and revealed anon-transiting planet candidate in the system with a period of 34.15 +/- 0.15days. Furthermore, the combination of archival data dating back to 1905 withnew high angular resolution imaging revealed a stellar companion orbiting theprimary star with an orbital period of around 230 years and an eccentricity ofabout 0.9. The long period of the transiting planet, combined with the higheccentricity and close approach of the companion star makes this a valuablesystem for testing the formation and stability of planets in binary systems.