The Possible Tidal Demise of Kepler’s First Planetary System

Abstract

Abstract We present evidence of tidally-driven inspiral in the Kepler-1658 (KOI-4) system, which consists of a giant planet (1.1 R J , 5.9 M J ) orbiting an evolved host star (2.9 R ⊙ , 1.5 M ⊙ ). 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mi>P</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> <mml:mo>=</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>131</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>22</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>20</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> ms yr −1 , corresponding to an infall timescale <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>P</mml:mi> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mover accent="true"> <mml:mrow> <mml:mi>P</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> <mml:mo>≈</mml:mo> <mml:mn>2.5</mml:mn> <mml:mspace width="0.25em"/> <mml:mi>Myr</mml:mi> </mml:math> . 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi>Q</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⋆</mml:mo> </mml:mrow> <mml:mrow> <mml:mo accent="true">′</mml:mo> </mml:mrow> </mml:msubsup> <mml:mo>=</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>2.50</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.62</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.85</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>×</mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> </mml:msup> </mml:math> , 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.