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 Ṗ = 131⁺²⁰₋₂₂ ms yr⁻¹, corresponding to an infall timescale P/Ṗ ≈ 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⁺⁰·⁸⁵₋₀.₆₂ × 10⁴, 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.