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

## TL;DR

This paper confirms Kepler's first exoplanet candidate, KOI 4.01 (Kepler-1658), as a massive hot Jupiter orbiting an evolved subgiant star, providing valuable data for understanding tidal interactions and hot Jupiter formation.

## Contribution

It presents the first confirmation and detailed characterization of Kepler-1658, the earliest discovered Kepler planet candidate, using asteroseismology and radial velocity measurements.

## Key findings

- Kepler-1658 is a massive, evolved subgiant star hosting a hot Jupiter.
- The planet orbits every 3.85 days with a mass of about 5.88 MJ.
- Orbital decay rate constrained to Pdot <= -0.42 s/yr, informing tidal dissipation models.

## 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 (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 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 4 years of Kepler data, we constrain the orbital decay rate to be Pdot <= -0.42 s/yr, corresponding to a strong observational limit of Qstar >= 4.826 x 10^3 for the tidal quality factor in evolved stars. With an effective temperature Teff ~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.