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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They look at all of space and can pick out planets passing in front of a sun or so. Imagine looking out in a field and seeing a big bird (sun) fly in the distance. You can notice it and then get binoculars on it and observe it. But the birds (planet)around you are smaller than the (sun) and unless you at that specific area as it’s there you won’t see it.
Space is huge and looking for a non glowing entity super close to us is hard. Unless you’re looking right at the planet in the vastness around us you won’t see it
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