The Possible Tidal Demise of Kepler's First Planetary System

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 $\dot{P}={131}_{-22}^{+20}$ ms yr^-1, corresponding to an infall timescale $P/\dot{P}\approx 2.5\,\mathrm{Myr}$ . 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 ${Q}_{\star }^{{\prime} }={2.50}_{-0.62}^{+0.85}\times {10}^{4}$ , 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.