The TESS-Keck Survey. VIII. Confirmation of a Transiting Giant Planet on an Eccentric 261 Day Orbit with the Automated Planet Finder Telescope*

Abstract

We report the discovery of TOI-2180 b, a 2.8 M J giant planet orbiting a slightly evolved G5 host star. This planet transited only once in Cycle 2 of the primary Transiting Exoplanet Survey Satellite (TESS) mission. Citizen scientists identified the 24 hr single-transit event shortly after the data were released, allowing a Doppler monitoring campaign with the Automated Planet Finder telescope at Lick Observatory to begin promptly. The radial velocity observations refined the orbital period of TOI-2180 b to be 260.8 ± 0.6 days, revealed an orbital eccentricity of 0.368 ± 0.007, and discovered long-term acceleration from a more distant massive companion. We conducted ground-based photometry from 14 sites spread around the globe in an attempt to detect another transit. Although we did not make a clear transit detection, the nondetections improved the precision of the orbital period. We predict that TESS will likely detect another transit of TOI-2180 b in Sector 48 of its extended mission. We use giant planet structure models to retrieve the bulk heavy-element content of TOI-2180 b. When considered alongside other giant planets with orbital periods over 100 days, we find tentative evidence that the correlation between planet mass and metal enrichment relative to stellar is dependent on orbital properties. Single-transit discoveries like TOI-2180 b highlight the exciting potential of the TESS mission to find planets with long orbital periods and low irradiation fluxes despite the selection biases associated with the transit method.

TESS Observations of Kepler Systems with Transit Timing Variations

“…The benefits of PLD extend to planets discovered by TESS as well. Transits of those with orbital periods greater than the observational baseline will not show the periodicity that is usually necessary to confidently claim a planet candidate detection (Díaz et al 2020;Dalba et al 2022). For faint stars and/or shallow transits (i.e., small planets), these planets may go undetected or at least not be validated.…”

TESS Observations of Kepler systems with Transit Timing Variations

“…The benefits of PLD extend to planets discovered by TESS as well. Transits of those with orbital periods greater than the observational baseline will not show the periodicity that is usually necessary to confidently claim a planet candidate detection (Díaz et al 2020;Dalba et al 2022). For faint stars and/or shallow transits (i.e., small planets), these planets may go undetected or at least not validated.…”

Sonification of TESS Data Validation Timeseries Files

“…Combining space-based TESS photometry, ground-based photometry, and ground-based radial velocity measurements, TOI2184 b has been fnally reported as an orbiting planet, demonstrating the feasibility of detecting planets around faint post-main sequence stars (TESS magnitude greater than 12 with TESS bandpass = 600 -1000 nm, centered on 786.5 nm, the traditional Cousins I-band central wavelength) [53]. TOI-2180 b is a 2.8 MJ giant planet orbiting a slightly evolved G5 host star (TIC 298663873) [54]. TIC 237913194 b, with a mass of MP= 1.942 MJ and a radius of RP= 1.117 RJ, implying a bulk density similar to Neptunes, orbits a G-type star (M= 1.026 Msun,V= 12.1 mag) with a period of 15.17 days on one of the most eccentric orbits of all known warm giants (e0.58) [55].…”