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 (1.1$R_{\mathrm{J}}$, 5.9$M_{\mathrm{J}}$) orbiting an evolved host star (2.9$R_{\odot }$, 1.5$M_{\odot }$). 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 $P/\dot{P}\approx 2.5$~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.

9 pages, 3 figures, 2 tables, published in Astrophysical Journal Letters

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