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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I’m not an expert but my basic understanding is we detect exoplanets by seeing how they change the light of their star as the planet passes in front of the star. The star also gives us a specific target to look at.
A 9th planet in our system will never pass in front of our star from our perspective. So instead we need to look around in the dark over a huge area looking for something minuscule compared to the sky and hope to see it reflecting some light back at us.
Imagine trying to decide if someone was in a dark room by watching for them to walk in front of a flashlight pointing at you vs trying to see them by noticing the flashlight behind you reflected off their shirt.
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