Planet Hunters TESS I: TOI 813, a subgiant hosting a transiting Saturn-sized planet on an 84-day orbit
Aigrain, S.; Fridlund, M.; Barragán, O.; Gandolfi, D.; Vanderburg, A.; Schuster, C.; Jenkins, J. M.; Osborn, H. P.; Zicher, N.; Stassun, K. G.; Wang, J.; Briceño, C.; Guerrero, N.; Boyajian, T. S.; Mann, A. W.; Bryant, E. M.; Christiansen, J. L.; Ziegler, C.; Law, N.; Eisner, N. L.; Lintott, C.; Miller, G.; Feinstein, A. D.; Flor-Torres, L. M.; Gilbert, J.; Jones, K.; Kristiansen, M. H.; López-Sánchez, A. R.; Safron, E. J.; Schwamb, M. E.; Zic, A.; Barnet, F.; Bean, S. J.; Bundy, D. M.; Chetnik, Z.; Dawson, J. L.; Garstone, J.; Stenner, A. G.; Huten, M.; Larish, S.; Melanson, L. D.; Mitchell, T.; Moore, C.; Peltsch, K.; Rogers, D. J.; Smith, D. S.; Simister, D. J.; Tanner, C.; Terentev, I.; Tsymbal, A.
United Kingdom, United States, Chile, Mexico, Sweden, Netherlands, Italy, Australia, Denmark, France, Canada
Abstract
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.