The Possible Tidal Demise of Kepler’s First Planetary System

Abstract We present evidence of tidally-driven inspiral in the Kepler-1658 (KOI-4) system, which consists of a giant planet (1.1R J, 5.9M J) orbiting an evolved host star (2.9R ⊙, 1.5M ⊙). Using transit timing measurements from Kepler, Palomar/WIRC, and TESS, we show that the orbital period of Kepler-1658b appears to be decreasing at a rate P ̇ = 131 − 22 + 20 ms yr−1, corresponding to an infall timescale P / P ̇ ≈ 2.5 Myr . We consider other explanations for the data including line-of-sight acceleration and orbital precession, but find them to be implausible. The observed period derivative implies a tidal quality factor Q ⋆ ′ = 2.50 − 0.62 + 0.85 × 10 4 , in good agreement with theoretical predictions for inertial wave dissipation in subgiant stars. Additionally, while it probably cannot explain the entire inspiral rate, a small amount of planetary dissipation could naturally explain the deep optical eclipse observed for the planet via enhanced thermal emission. As the first evolved system with detected inspiral, Kepler-1658 is a new benchmark for understanding tidal physics at the end of the planetary life cycle.