How Two Old and Unrelated Exoplanets Turned Out to be 400-Myr-Old Siblings: The Tale of Theia 520, Home to Keplers 52 and 968

Over 4000 planet candidates were discovered by the Kepler mission. The sizes, atmospheres, and orbits of exoplanets evolve over billions of years, and precise ages are needed for understanding the evolutionary processes governing these changes. However, most exoplanets have ages that are quite uncertain, particularly those hosted by low-mass K and M dwarfs. For example, consider Keplers 52 and 968: these mid-K dwarfs each host multiple small planets, and each have isochronal ages between 3 and 16 Gyr spanning nearly the entire history of the Universe, offering us little insight, until now. We recognized that these stars are associated together with 395 other stars in the Theia 520 group, which was identified by Kounkel & Covey (2019) in the Gaia DR2 astrometric data set. First, we established that Theia 520 is indeed a coeval population by completing a stellar rotation census of its candidate members using light curves from Kepler, TESS, and ZTF. We found a tight sequence of slowly rotating stars consistent with a 400-Myr-old empirical gyrochronology relation defined by the open clusters M39 and M48. Next, we gathered spectroscopic metallicities from APOGEE and LAMOST to determine that Theia 520 has a solar chemical composition, then combined the spectroscopic temperatures with Gaia photometric colors to calculate the foreground interstellar reddening. With these critical ingredients in hand, we fit PARSEC isochrones to the Gaia color-magnitude diagram (CMD) and found an age of 380 ± 50 Myr in line with our empirical gyrochronology result. Given that Keplers 52 and 968 have 3D kinematics (from Gaia astrometry and Keck/HIRES radial velocities) consistent with membership and rotation periods placing them on Theia 520’s fully converged, slowly rotating sequence, we consider their membership to be confirmed. As their multiple planets have already been validated, we can now apply Theia 520’s precise and accurate 400 Myr age to them, and begin using them as benchmarks for studying how exoplanets evolve. Our approach has remarkably shown that two old Kepler hosts are actually related to each other, and that they are quite young. This is only the beginning. Gaia has shown that the greater solar neighborhood within 1 kpc is teeming with coeval populations, some stretching hundreds of parsecs in space like the Pisces-Eridanus stellar stream, others arranged in more amorphous distributions, and some that are dense clusters with halos and tails. Like Theia 520, some of these groupings are home to already known and validated planets, as well as planet candidates, with more waiting to be discovered with the ongoing TESS mission.