Zodiacal Exoplanets in Time (ZEIT). VIII. A Two-planet System in Praesepe from K2 Campaign 16

Young planets offer a direct view of the formation and evolution processes that produced the diverse population of mature exoplanet systems known today. The repurposed Kepler mission K2 is providing the first sample of young transiting planets by observing populations of stars in nearby, young clusters and stellar associations. We report the detection and confirmation of two planets transiting K2-264, an M2.5 dwarf in the 650 Myr old Praesepe open cluster. Using our notch-filter search method on the K2 light curve, we identify planets with periods of 5.84 and 19.66 days. This is currently the second known multi-transit system in open clusters younger than 1 Gyr. The inner planet has a radius of 2.27(-0.16)(+0.20) R-circle plus and the outer planet has a radius of 2.77(-0.18)(+0.20) R-circle plus. Both planets are likely mini-Neptunes. These planets are expected to produce radial velocity signals of 3.4 and 2.7 m s(-1), respectively, which is smaller than the expected stellar variability in the optical (similar or equal to 30 m s(-1)), making mass measurements unlikely in the optical but possible with future near-infrared spectrographs. We use an injection-recovery test to place robust limits on additional planets in the system and find that planets larger than 2 R-circle plus with periods of 1-20 days are unlikely.