Many lines of reasoning suggest that external galaxies should host planetary systems, but detecting them by methods typically used in our own Galaxy is not possible. An alternative approach is to study the temporal behaviour of X-rays emitted by bright extragalactic X-ray sources, where an orbiting planet would temporarily block the X-rays and cause a brief eclipse. We report on such a potential event in the X-ray binary M51-ULS-1 in the galaxy M51. We examined a range of explanations for the observed X-ray dip, including a variety of transiting objects and enhancements in the density of gas and dust. The latter are ruled out by the absence of changes in X-ray colours, save any with sharp density gradients that cannot be probed with our data. Instead, the data are well fit by a planet transit model in which the eclipser is most likely to be the size of Saturn. We also find that the locations of possible orbits are consistent with the survival of a planet bound to a mass-transfer binary.

许多推理表明外部星系应该拥有行星系统，但是通过我们自己的星系中通常使用的方法来检测它们是不可能的。另一种方法是研究明亮的河外 X 射线源发出的 X 射线的时间行为，其中一颗轨道行星会暂时阻挡 X 射线并导致短暂的日食。我们在 M51 星系的 X 射线双星 M51-ULS-1 中报告了这样一个潜在事件。我们研究了对观测到的 X 射线倾角的一系列解释，包括各种凌日物体以及气体和尘埃密度的增强。后者因 X 射线颜色没有变化而被排除在外，保存任何无法用我们的数据探测的尖锐密度梯度。反而，这些数据非常适合行星凌日模型，其中日食者最有可能是土星的大小。我们还发现，可能轨道的位置与绑定到质量转移双星的行星的生存是一致的。