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

Abstract

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 ($R_{\star }=1.94R_{\odot }$, $M_{\star }=1.32M_{\odot }$). 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 $2M_{Jup}$ (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 $\sim 6$ ms${}^{-1}$, making this system a promising target to measure the mass of a relatively long-period transiting planet.

Accepted for publication in MNRAS (16 pages, 10 figures, 3 tables)