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

• Main Authors: Eisner, N. L, Barragán, O, Aigrain, S, Lintott, C, Miller, G, Zicher, N, Boyajian, T. S, Briceño, C, Bryant, E. M, Christiansen, J. L, Feinstein, A. D, Flor-Torres, L. M, Fridlund, M, Gandolfi, D, Gilbert, J, Guerrero, N, Jenkins, J. M, Jones, K, Kristiansen, M. H, Vanderburg, A, Law, N, López-Sánchez, A. R, Mann, A. W, Safron, E. J, Schwamb, M. E, Stassun, K. G, Osborn, H. P, Wang, J, Zic, A, Ziegler, C, 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, Schuster, C, Smith, D. S, Simister, D. J, Tanner, C, Terentev, I, Tsymbal, A
• Format: Journal Article
• Language: English
• Published: 13-01-2020
• Subjects: Physics - Earth and Planetary Astrophysics
• DOI: 10.48550/arxiv.1909.09094

We report on the discovery and validation of TOI 813b (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 ($R_\star=1.94\,R_\odot$, $M_\star=1.32\,M_\odot$). 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 % confidence) on the mass of the companion, and to statistically
validate its planetary nature. Detailed modelling of the transits yields a
period of $83.8911_{ - 0.0031 } ^ { + 0.0027 }$ days, a planet radius of $6.71
\pm 0.38$ $R_{\oplus}$, and a semi major 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 under-explored region of parameter
space. We estimate that TOI-813b induces a reflex motion in its host star with
a semi-amplitude of $\sim6$ ms$^{-1}$, making this system a promising target to
measure the mass of a relatively long-period transiting planet.