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?
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If you’re looking for a planet around another star, you already know to point the telescope at that star, and it’s easy to find the star again night after night because it doesn’t move around. With a planet, you need to track it over a period of time to figure out its orbit, and from that and its brightness you can estimate size. Otherwise you might see it one night, never find it again, and assume it was some unknown asteroid.
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