Kepler and the Behemoth: Three Mini-Neptunes in a 40 Million Year Old Association

Abstract

Stellar positions and velocities from Gaia are yielding a new view of open cluster dispersal. Here we present an analysis of a group of stars spanning Cepheus (l = 100°) to Hercules (l = 40°), hereafter the Cep-Her complex. The group includes four Kepler objects of interest: Kepler-1643 b (Rₚ = 2.32 ± 0.13 R_⊕, P = 5.3 days), KOI-7368 b (Rₚ = 2.22 ± 0.12 R_⊕, P = 6.8 days), KOI-7913 Ab (Rₚ = 2.34 ± 0.18 R_⊕, P = 24.2 days), and Kepler-1627 Ab (Rₚ = 3.85 ± 0.11 R_⊕, P = 7.2 days). The latter Neptune-sized planet is in part of the Cep-Her complex called the δ Lyr cluster. Here we focus on the former three systems, which are in other regions of the association. Based on kinematic evidence from Gaia, stellar rotation periods from TESS, and spectroscopy, these three objects are also ≈40 million years (Myr) old. More specifically, we find that Kepler-1643 is 46⁺⁹₋₇ Myr old, based on its membership in a dense subcluster of the complex called RSG-5. KOI-7368 and KOI-7913 are 36⁺¹⁰₋₈ Myr old, and are in a diffuse region that we call CH-2. Based on the transit shapes and high-resolution imaging, all three objects are most likely planets, with false-positive probabilities of 6 × 10⁻⁹, 4 × 10⁻³, and 1 × 10⁻⁴ for Kepler-1643, KOI-7368, and KOI-7913, respectively. These planets demonstrate that mini-Neptunes with sizes of ≈2 Earth radii exist at ages of 40 Myr.