The transiting planetary system WASP-86/KELT-12: TESS provides the casting vote

Abstract

A transiting planetary system was discovered independently by two groups, under the names WASP-86 (Faedi et al. 2016) and KELT-12 (Stevens et al. 2017). The properties of the system determined in these works were very different, most tellingly a variation of a factor of three in the measured radius of the planet. We suggest that the system be named WASP-86/KELT-12 to better apportion the credit for discovery between the two groups. We analyse the light curve of this system from the Transiting Exoplanet Survey Satellite, which observed it in two sectors, following the Homogeneous Studies approach. We find properties intermediate between the two previous studies: the star has a mass of 1.278 +/- 0.039 Msun and a radius of 2.02 +/- 0.12 Rsun, and the planet has a mass of 0.833 +/- 0.049 Mjup and a radius of 1.382 +/- 0.089 Rjup. The discrepancy in the two previous sets of measured properties of the system arises from a disagreement over the transit depth and duration, caused by the transit being long and shallow so not well suited to follow-up photometry from ground-based telescopes. We also update the orbital ephemeris to aid future work on this system, which is a good candidate for characterising the atmosphere of a planet through transmission spectroscopy.