The Curious Case of KOI 4: Confirming Kepler’s First Exoplanet Detection

Abstract

The discovery of thousands of planetary systems by Kepler has demonstrated that planets are ubiquitous. However, a major challenge has been the confirmation of Kepler planet candidates, many of which still await confirmation. One of the most enigmatic examples is KOI 4.01, Kepler ’ s first discovered planet candidate detection (as KOI 1.01, 2.01, and 3.01 were known prior to launch). Here we present the confirmation and characterization of KOI 4.01 (now Kepler-1658), using a combination of asteroseismology and radial velocities. Kepler-1658 is a massive, evolved subgiant ( M ⋆  = 1.45 ± 0.06 M ⊙ , R ⋆  = 2.89 ± 0.12 R ⊙ ) hosting a massive (  = 5.88 ± 0.47 ,  = 1.07 ± 0.05 ) hot Jupiter that orbits every 3.85 days. Kepler-1658 joins a small population of evolved hosts with short-period (≤100 days) planets and is now the closest known planet in terms of orbital period to an evolved star. Because of its uniqueness and short orbital period, Kepler-1658 is a new benchmark system for testing tidal dissipation and hot Jupiter formation theories. Using all four years of the Kepler data, we constrain the orbital decay rate to be  ≤ −0.42 s yr −1 , corresponding to a strong observational limit of  ≥ 4.826 ×  for the tidal quality factor in evolved stars. With an effective temperature of  ∼ 6200 K, Kepler-1658 sits close to the spin–orbit misalignment boundary at ∼6250 K, making it a prime target for follow-up observations to better constrain its obliquity and to provide insight into theories for hot Jupiter formation and migration.