Planet X is a term used to describe a hypothetical major planet in the outer solar system, beyond Neptune, whose existence was initially inferred from unexplained gravitational perturbations of Uranus and Neptune. Although the search was largely satisfied with the discovery of Pluto, the concept of a distant, undiscovered world has seen a resurgence following new astronomical observations.
We now have direct images of two supermassive black holes: M87* and Sag A*. The fact that we can capture such images is remarkable, but they might be the only black holes we can observe. That is, unless we take radio astronomy to a whole new level.
