HATS-18b: An Extreme Short-period Massive Transiting Planet Spinning Up Its Star

Abstract

We report the discovery by the HATSouth network of HATS-18b: a 1.980 0.077  MJ, - + 1.337 0.049 0.102 RJ planet in a 0.8378 day orbit, around a solar analog star (mass 1.037 0.047  M and radius - + 1.020 0.031 0.057 R) with V =  14.067 0.040 mag. The high planet mass, combined with its short orbital period, implies strong tidal coupling between the planetary orbit and the star. In fact, given its inferred age, HATS-18 shows evidence of significant tidal spin up, which together with WASP-19 (a very similar system) allows us to constrain the tidal quality factor for Sun-like stars to be in the range of 6.5 log 7   Q k * 10 ( )2 even after allowing for extremely pessimistic model uncertainties. In addition, the HATS-18 system is among the best systems (and often the best system) for testing a multitude of star–planet interactions, be they gravitational, magnetic, or radiative, as well as planet formation and migration theories.