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. VanderburgN. Law, A. R. López-Sánchez, A. W. Mann, E. J. Safron, M. E. Schwamb, K. G. Stassun, H. P. Osborn, 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

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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 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 83.8911-0.0031+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.
Original languageEnglish
Pages (from-to)750-763
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
Early online date23 Jan 2020
Publication statusPublished - 01 May 2020


  • methods: statistical
  • planets and satellites: detection
  • stars: fundamental parameters
  • stars: individual (TIC-55525572
  • TOI 813)


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