K2 greatly extended Kepler’s ability to find new planets, but it was typically limited to identifying transiting planets with orbital periods below 40 days. While analyzing K2 data through the Exoplanet Explorers project, citizen scientists helped discover one super-Earth and four sub-Neptune sized planets in the relatively bright (V = 12.21, K = 10.3) K2-138 system, all which orbit near 3:2 mean-motion resonances. The K2 light curve showed two additional transit events consistent with a sixth planet. Using Spitzer photometry, we validate the sixth planet’s orbital period of 41.966 0.006 days and measure a radius of , solidifying K2-138 as the K2 system with the most currently known planets. There is a sizeable gap between the outer two planets, since the fifth planet in the system, K2-138 f, orbits at 12.76 days. We explore the possibility of additional nontransiting planets in the gap between f and g. Due to the relative brightness of the K2-138 host star, and the near resonance of the inner planets, K2-138 could be a key benchmark system for both radial velocity and transit-timing variation mass measurements, and indeed radial velocity masses for the inner four planets have already been obtained. With its five sub-Neptunes and one super-Earth, the K2-138 system provides a unique test bed for comparative atmospheric studies of warm to temperate planets of similar size, dynamical studies of near-resonant planets, and models of planet formation and migration.

K2 极大地扩展了开普勒寻找新行星的能力，但它通常仅限于识别轨道周期低于 40 天的凌日行星。在通过 Exoplanet Explorers 项目分析 K2 数据时，公民科学家帮助在相对明亮 ( V = 12.21, K = 10.3) K2-138 系统中发现了一颗超级地球和四颗亚海王星大小的行星，所有这些行星的轨道都接近 3:2平均运动共振。K2 光变曲线显示了与第六颗行星一致的两个额外的凌日事件。使用斯皮策光度法，我们验证了第六颗行星的轨道周期为 41.966 0.006 天，并测量了半径为，将 K2-138 固化为 K2 系统，其中包含目前已知的最多的行星。外两颗行星之间存在相当大的差距，因为系统中的第五颗行星 K2-138 f 的轨道运行周期为 12.76 天。我们探索在 f 和 g 之间的间隙中存在其他非凌日行星的可能性。由于 K2-138 主星的相对亮度和内行星的近共振，K2-138 可能是径向速度和凌日时间变化质量测量的关键基准系统，实际上是内四颗行星已经获得。K2-138 系统拥有五个亚海王星和一个超级地球，为类似大小的暖温带行星的大气比较研究、近共振行星的动力学研究以及行星形成和迁移模型提供了一个独特的试验台.