Author(s): K. Hardegree-Ullman, J. Christiansen, Caltech/IPAC-NExScI, Pasadena, California, UNITED STATES|
Institution(s): 1. Caltech/IPAC-NExScI, Pasadena, CA, United States.
Contributing team(s): K2 Spitzer Follow-Up Team
K2-138 is a moderately bright (V=12.2) early-type K dwarf observed in Campaign 12 of the NASA K2 mission. Citizen scientists participating in the Exoplanet Explorers project on the Zooniverse platform helped discover five sub-Neptune planets orbiting K2-138 which form a near 3:2 mean motion resonance chain. A potential sixth planet with a 42 day period was identified in the K2 data, but the 80 day observing campaign only showed two transit events. We confirm the presence and refine the ephemeris of K2-138 g with follow-up observations from the Spitzer Space Telescope. This planet continues the near 3:2 resonant chain with two gaps between the fifth and sixth planets, suggesting K2-138 might host additional smaller or non-transiting planets. The near resonance of this system makes it amenable to TTV measurements, and if combined with precision radial velocity measurements, could make K2-138 a benchmark system for TTV mass calculations.