K2-138 g：斯皮策为公民科学系统发现第六颗行星
K2-138 g: Spitzer Spots a Sixth Planet for the Citizen Science System
摘要：
$K2$极大地扩展了$Kepler$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。在通过系外行星探索者项目分析$K2$数据的同时，公民科学家帮助在相对明亮的K2-138系统（$V=12.21$，$K=10.3$）中发现了一颗超级地球和四颗次海王星大小的行星，所有这些行星的轨道都接近3：2平均运动共振。$K2$的光变曲线显示了另外两个与第六颗行星一致的凌日事件。使用$Spitzer$测光，我们验证了第六颗行星的轨道周期$41.966\pm0.006$天，并测量了$3.44^{+0.32}_{-0.31}\,R_{\oplus}$的半径，巩固了K2-138作为目前已知行星最多的$K2$系统。由于该系统的第五颗行星K2-138 f的轨道周期为12.76天，因此外两颗行星之间存在相当大的差距。我们探讨了在f和g之间的间隙中存在额外的非凌日行星的可能性。由于K2-138恒星的相对亮度，以及内行星的近共振，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，实际上内行星的径向速度质量已经获得。K2-138系统有五个次海王星和一个超级地球，为类似大小的暖温带行星的比较大气研究、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平台。
英文摘要：
$K2$ 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 $41.966\pm0.006$ days and measure a radius of $3.44^{+0.32}_{-0.31}\,R_{\oplus}$, 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 non-transiting 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.
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