The Possible Tidal Demise of Kepler's First Planetary System

TL;DR

This paper presents evidence of a giant planet orbiting an evolved star spiraling inward due to tidal forces, providing new insights into tidal dissipation and planetary inspiral in late-stage planetary systems.

Contribution

It reports the first detection of tidal inspiral in an evolved planetary system, measuring the orbital decay rate and constraining the stellar tidal quality factor.

Findings

Orbital period decreasing at 131 ms/yr

Tidal quality factor Q' estimated at ~2.5×10^4

Supports theories of inertial wave dissipation in subgiant stars

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 $P/\dot{P}\approx2.5$~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 $Q_\star' = 2.50_{-0.62}^{+0.85}\times10^4$, 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.