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

The discovery of thousands of planetary systemsby Keplerhas demonstrated that planets are ubiquitous. However, a major challenge has been the confirmation of Keplerplanet 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 asteroseismologyand radial velocities. Kepler-1658 is a massive, evolved subgiant(Mstar = 1.45 +/- 0.06 Msun, Rstar = 2.89 +/- 0.12 Rsun) hosting a massive (Mp = 5.88 +/- 0.47 MJ, Rp = 1.07 +/- 0.05 RJ) hot Jupiterthat orbits every 3.85 days. Kepler-1658 joins a small population of evolved hosts with short-period ( = 4.826 x 10^3 for the tidal quality factor in evolved stars. With an effective temperatureTeff ~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 Jupiterformation and migration.