Radial–velocity (RV) planet searches are often polluted by signals caused by gas motion at the star’s surface. Stellar activity can mimic or mask changes in the RVs caused by orbiting planets, resulting in false positives or missed detections. Here we use Gaussian process regression to disentangle the contradictory reports of planets versus rotation artifacts from Kapteyn’s star. To model rotation, we use joint quasiperiodic kernels for the RV and Hα signals, requiring that their periods and correlation timescales be the same. We find that the rotation period of Kapteyn’s star is 125 days, while the characteristic active-region lifetime is 694 days. Adding a planet to the RV model produces a best-fit orbital period of 100 yr, or 10 times the observing time baseline, indicating that the observed RVs are best explained by star rotation only. We also find no significant periodic signals in residual RV data sets constructed by subtracting off realizations of the best-fit rotation model and conclude that both previously reported “planets” are artifacts of the star’s rotation and activity. Our results highlight the pitfalls of using sinusoids to model quasiperiodic rotation signals.

径向速度 (RV) 行星搜索经常受到恒星表面气体运动引起的信号的污染。恒星活动可以模仿或掩盖由绕行行星引起的 RV 变化，从而导致误报或漏检。在这里，我们使用高斯过程回归来解决行星与来自 Kapteyn 恒星的自转伪影的矛盾报告。为了模拟旋转，我们对 RV 和 H α使用联合准周期核信号，要求它们的周期和相关时间尺度相同。我们发现Kapteyn 星的自转周期为125 天，而特征活动区寿命为694 天。将一颗行星添加到 RV 模型会产生 100 年的最佳轨道周期，或观测时间基线的 10 倍，表明观测到的 RV 最好仅用恒星自转来解释。我们还在通过减去最佳拟合旋转模型的实现而构建的残余 RV 数据集中没有发现显着的周期性信号，并得出结论，先前报告的两个“行星”都是恒星旋转和活动的产物。我们的结果突出了使用正弦曲线模拟准周期旋转信号的缺陷。