Eisner, N LBarragan, OAigrain, SuzanneLintott, Chris J.Miller, GZicher, NBoyajian, TabethaBriceno, CBryant, E MChristiansen, Jessie L.Gilbert, James2023-03-222023-03-220035-8711http://hdl.handle.net/1885/287269We report on the discovery and validation of TOI 813 b (TIC 55525572 b), a transiting exoplanet identified by citizen scientists in data from NASA’s Transiting Exoplanet Survey Satellite (TESS) and the first planet discovered by the Planet Hunters TESS project. The host star is a bright (V = 10.3 mag) subgiant (R = 1.94 R, M = 1.32 M). It was observed almost continuously by TESS during its first year of operations, during which time four individual transit events were detected. The candidate passed all the standard light curve-based vetting checks, and ground-based follow-up spectroscopy and speckle imaging enabled us to place an upper limit of 2 MJup (99 per cent confidence) on the mass of the companion, and to statistically validate its planetary nature. Detailed modelling of the transits yields a period of 83.8911+0.0027 −0.0031 d, a planet radius of 6.71 ± 0.38 R⊕ and a semimajor axis of 0.423+0.031 −0.037 AU. The planet’s orbital period combined with the evolved nature of the host star places this object in a relatively underexplored region of parameter space. We estimate that TOI 813 b induces a reflex motion in its host star with a semi-amplitude of ∼6ms−1, making this a promising system to measure the mass of a relatively long-period transiting planetapplication/pdfen-AU© 2020 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Societymethods: statisticalplanets and satellites: detectionstars: fundamental parametersstars: individual (TIC-55525572 – TOI 813)202010.1093/mnras/staa1382022-01-16