Planet Hunters TESS I: TOI 813, a subgiant hosting a transiting Saturn-sized planet on an 84-day orbit

N. L. Eisner , O. Barragán , S. Aigrain , C. Lintott , G. Miller , N. Zicher , T. S. Boyajian , C. Briceño , E. M. Bryant , J. L. Christiansen , A. D. Feinstein , L. M. Flor-Torres , M. Fridlund , D. Gandolfi , J. Gilbert , N. Guerrero , J. M. Jenkins , K. Jones , M. H. Kristiansen , A. Vanderburg , N. Law , A. R. López-Sánchez , A. W. Mann , E. J. Safron , M. E. Schwamb , K. G. Stassun , H. P. Osborn (1) , J. Wang , A. Zic , C. Ziegler , F. Barnet , S. J. Bean , D. M. Bundy , Z. Chetnik , J. L. Dawson , J. Garstone , A. G. Stenner , M. Huten , S. Larish , L. D. Melanson , T. Mitchell , C. Moore , K. Peltsch , D. J. Rogers , C. Schuster , D. S. Smith , D. J. Simister , C. Tanner , I. Terentev , A. Tsymbal

1 LAM - Laboratoire d'Astrophysique de Marseille

We 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_\star - 1.94 R_⊙, M_\star = 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 M_Jup (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_-0.0031^+0.0027 d, a planet radius of 6.71 ± 0.38 R_⊕ and a semimajor axis of 0.423_-0.037^+0.031 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 ∼6 m s^-1, making this a promising system to measure the mass of a relatively long-period transiting planet.

Mots clés: methods: statistical; planets and satellites: detection; stars: fundamental parameters; stars: individual (TIC-55525572; TOI 813); Astrophysics - Earth and Planetary Astrophysics

Domaines: Planète et Univers [physics]; Astrophysique [astro-ph]

N. L. Eisner, O. Barragán, S. Aigrain, C. Lintott, G. Miller, et al.. Planet Hunters TESS I: TOI 813, a subgiant hosting a transiting Saturn-sized planet on an 84-day orbit. Monthly Notices of the Royal Astronomical Society, 2020, 494, pp.750-763. ⟨10.1093/mnras/staa138⟩. ⟨insu-03667101⟩