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 only see distant planets when they, by some chance, either block their star’s light or cause it to wiggle.

This means that most distant planets, we have not seen. The vast majority of them. We just get lucky and see a few.

We haven’t happened to see planet 9.