How can astronomers observe planets thousands of lightyears away in other solar systems, but yet can’t be sure that there isn’t an undiscovered “planet 9” in our own solar system?

Question

How can astronomers observe planets thousands of lightyears away in other solar systems, but yet can’t be sure that there isn’t an undiscovered “planet 9” in our own solar system?

In: 1

Answer 1

We can’t see every planet around every star. We can see a few planets around a few stars if they happen to pass between the star and us. If planet X passed between us and the Sun, we’d certainly see it. But, however, it’s out with Pluto so it’s probably not a Planet under the current “rules for planets”.