The Youngest Planets from the Prime Kepler Mission

Many processes in planet evolution happen during the first few million years after disk dispersal. However, most known exoplanets are billions of years old. I will discuss how new data from Gaia and TESS are helping to remedy this situation, by enabling the discoveries of the youngest planets from the prime Kepler mission. Our teams have identified at least three new open clusters in the Kepler field, with ages spanning 35 to 350 million years. These clusters were previously overlooked due to insufficiently precise stellar kinematics and parallaxes; their ages are now confirmed through isochrones, gyrochronology, and spectroscopy. At least twelve Kepler planets reside in the new clusters. Four are mini-Neptunes and are 38±6 million years old — over ten times younger than previously known Kepler planets with precise ages. The sample enables intriguing follow-up studies, including of early atmospheric outflows and the mechanisms that might drive them. The younger planets are also the first to confirm the theoretical expectation that close-in mini-Neptunes with sizes of 2 Earth radii exist at ages of ≈40 million years. These planets are a step toward observing the early size evolution of small planets, and point to promising future possibilities in the exoplanet age-dating census.